Niacin requirement and inability of tryptophan to act as a precursor of NAD+ in channel catfish, Ictalurus punctatus

8/11/2019 Niacin requirement and inability of tryptophan to act as a precursor of NAD+ in channel catfish, Ictalurus punctatus

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LS NI R Aquaculture 152 (1997) 273-285

Niacin requirement and inability of tryptophan toact as a precursor of NAD+ in channel catfish,

Ictalurus punctatus

Wing-Keong Ng, Giovanni Serrini , Zhan Zhang,Robert P. Wilson *

Deparrment of Biochemi stry and M olecular Biol ogy, Mi ssi ssippi Stat e Uni uersit y, M i ssissippi , M S 39762 USA

Accepted 3 1 October 1996

bstract

Two separate experiments were conducted, firstly to determine the niacin requirement ofchannel catfish, and secondly to evaluate the efficacy of dietary tryptophan as a niacin precursor.In Experiment 1, purified diets containing graded levels of supplemental nicotinic acid (0, 3, 6, 9,12, 15, 18, 21 and 24 mg per kg diet) were fed to channel catfish fingerlings for 12 weeks. Thedietary niacin requirement for rapidly growing channel catfish was estimated to be 7.4 mg per kgdiet. Fish that were fed diets without added niacin demonstrated poor growth, low feed intake,anaemia, lethargy, and mortality when stressed. Liver NAD concentrations increased linearlyr = 0.98) with increasing niacin supplementation in the diet, and did not plateau. In Experiment

2, 10 diets with different supplemental niacin and tryptophan combinations were formulated(niacin:tryptophan), 0:0, 5:0, lO:O, 15:0, 0:250, 0:500, 0:750, 5:250, lo:500 and 15:750 mg per kgdiet, and fed to channel catfish fingerlings for 9 weeks. The addition of excess tryptophan inniacin-deficient diets did not significantly (P < 0.05) improve growth rates, feed efficiencies,haematocrits or liver NAD concentrations in channel catfish. Tryptophan is an inefficientprecursor of dietary niacin for channel catfish. 0 1997 Elsevier Science B.V.

Keyw ords: Niacin; Tryptophan: Requirements; Channel catfish; NAD

* Corresponding author. Present address: Instituto di Zootecnica Veterinaxia, Universita degli Studi di Milano, Via Trentacoste 2,

20134 Milan, Italy.

0044-8486/97/ 17.00 0 1997 Elsevier Science B.V. All rights reserved.PI I SOO44-8486(96)01510-4


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1 Introduction

Niacin is essential for the optimal growth and health of fish. The signs of niacin

deficiency in most fish species are loss of appetite, poor growth and increased mortality,but may also include photosensitivity or sunburn in rainbow trout (Poston and Wolfe,1985), skin and fin lesions and haemorrhages in common carp (Aoe et al., 1967),deformed jaws and anaemia in channel catfish (Andrews and Murai, 1978), deformedsnouts and gill edema in hybrid tilapia (Shiau and Suen, 1992), and abnormal swimmingand ataxia in Japanese eels (Arai et al., 1972). The diverse effects of niacin deficiency infish are not surprising, since niacin or its amide are used to form the pyridinenucleotides, NAD and NADP. These cofactors are well known to be involved innumerous metabolic reactions including critical roles in mitochondrial respiration and inthe metabolism of amino acids, lipids, and carbohydrates. The role of NAD has also

been expanded to include providing substrates for the ADP-ribosylation of proteinsinvolved in DNA repair (Carson et al., 1987) and in the formation of cyclic ADP-ribosewhich is involved in calcium mobilization (Lee and Aarhus, 1991).

Published dietary niacin requirements in fish (on a per kg diet basis) seem to varywidely among species, from 10 mg for rainbow trout (Poston and Wolfe, 19851, 28 mgfor common carp (Aoe et al., 1967), 63-83 mg for gilthead seabream (Morris andDavies, 1995), to 121 mg for hybrid tilapia (Shiau and Suen, 1992). Channel catfishfingerlings have been reported to require about 14 mg of niacin per kg diet (Andrewsand Murai, 1978). Preliminary feeding trials in our laboratory indicated that the niacinrequirement for channel catfish may be much lower than previously reported. Thepresent study was therefore carried out to re-evaluate the niacin requirements forchannel catfish.

Differences in the niacin requirement among land animals can primarily be attributedto the differences between species in their ability to utilize dietary tryptophan as aprecursor of NAD(P) (National Research Council, 1987). Not much is known about theefficacy of tryptophan as a niacin precursor in different fish species. Tryptophan hasbeen reported to be an inefficient precursor of niacin activity in several species of troutand salmon (Poston and DiLorenzo, 1973; Poston and Combs, 1980) based on therelative activities of two liver enzymes, 3-hydroxyanthranilic acid oxygenase (3HAA)and picolinic acid carboxylase (PC), which are involved in the pathway of conversion oftryptophan to NAD(P). Other researchers have suggested that the same conversioninefficiency may exist in channel catfish (Andrews and Murai, 1978) and hybrid tilapia(Shiau and Suen, 1992), based on the fact that niacin deficiency signs were induced infish fed niacin-deficient diets, despite the high protein diets used in their studies. On thebasis of this indirect evidence, fish are generally believed to be incapable of synthesizingNAD(P) from tryptophan. However, more recent work by Chuang (199 1) recorded3HAA/PC ratios which indicated that carp, tilapia, red seabream, black seabream andmilkfish may have the capacity to synthesize NAD(P) from tryptophan. It is currentlyunknown to what extent fish may depend on tryptophan as a source of niacin. Thesecond objective of the present study was to evaluate the efficacy of tryptophan as aNAD(P) precursor, based on growth, absence of deficiency signs and liver NAD levelsin rapidly growing channel catfish fingerlings.


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2 Materials and methods

2 1 Diet preparation

Formulations of the basal diets for the niacin requirement study (Experiment 1) andthe study on the efficacy of tryptophan as a niacin precursor (Experiment 2) are shownin Table 1. All diets were formulated to contain 30% crude protein and 14.39 kJ per gdiet of digestible energy (Garling and Wilson, 1976). Vitamin-free casein and gelatin(U.S. Biochemical, Cleveland, OH) were used as natural protein sources in bothexperiments. Using a microbiological assay (Association of Official Analytical Chemists,1990), no measurable amount of niacin activity was detected (the lower thresholdsensitivity of the assay is about 2 mg kg-). This was further confirmed when weanalysed the basal pelleted diets using the cyanogen bromide-calorimetric method

(Association of Official Analytical Chemists, 1990). The analysis of niacin level invitamin-free casein provided by the supplier (U.S. Biochemical) was 0.3 mg kg-. Thecontribution of niacin from casein in the experimental diets is therefore negligible.

The vitamin premix used was similar to that used by Satoh et al. (19891, except that itdid not contain niacin. In Experiment 1, nicotinic acid (Sigma Chemical, St Louis, MO)was added to the basal diet at 0, 3, 6, 9, 12, 15, 18, 21 and 24 mg per kg diet. Sinceniacin is known to be stable to heat in mineral acids and alkali (Halver, 1989), vitamin

Table 1

Composition of the basal diets

Ingredient a

Casein (vitamin free) bGelatinAmino acid mixture (tryptophan free) White dextrin dCod liver oilCorn oilVitamin premix (niacin free) Mineral premix Calcium phosphate, dibasicCarboxymethyl celluloseCellulose B

Niacin requirement study, Niacin-tryptophan study,Experiment 1 (g per 100 g) Experiment 2 (g per 100 g

25.06 12.656.00 3.51- 15.80

32.44 32.095.00 5.005.00 5.003.00 3.004.00 4.00

1 oo 1 OO2.00 2.00

16.50 15.95

a All ingredients were purchased from US Biochemical (Cleveland, OH) except for the amino acids, vitaminsand minerals which were purchased from Sigma Chemical (St Louis, MO).b Niacin concentration below detectable levels. A value of 0.3 mgkg- was given by the supplier of casein. Casein and gelatin were supplemented with crystalline L-amino acids (with the exception of tryptophan) toprovide the amino acid pattern found in 30% crude protein from whole egg powder (Wilson et al., 1978). Thebasal diet in Experiment 2 was formulated to contain 0.15% tryptophan and the diets were made isonitroge-nous by adjusting the levels of glycine when tryptophan was added. Levels varied to maintain diets isoenergetic in Experiment 2.

According to Satoh et al. (19891, except the niacin was removed. According to Satoh et al. (1989).g Levels varied when a niacin premix (2 mgg- was added.


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losses during diet preparation and storage should be minimal. The diets were preparedby mixing the dry ingredients with cod liver oil, corn oil and water in a Hobart mixerand the resultant moist mash extruded through a 2-mm die. The moist pellets were then

oven-dried with forced air at room temperature to about 10% moisture, ground in a foodblender and stored frozen at -20C until needed. Morris and Davies (1995) reportedminimal niacin losses using a similar diet preparation procedure resulting in dietaryniacin levels which were close to the formulated content.

In Experiment 2, casein and gelatin were supplemented with crystalline L-aminoacids (Sigma Chemical), with the exception of tryptophan, to provide the amino acidpattern found in 30% crude protein from whole egg powder (Wilson et al., 1978). Caseinand gelatin levels were adjusted to provide 0.15% tryptophan in the basal diet. This isequivalent to 0.5% of the dietary protein which is the tryptophan requirement reportedfor fingerling channel catfish (Wilson et al., 1978). Since the tryptophan requirement

was determined in channel catfish that were fed diets containing excess niacin (Wilsonet al., 19781, dietary tryptophan in the basal diet was presumed to be just adequate forprotein accretion and serotonin synthesis only. It is important that any excess tryptophanin the diet be minimized so that maximal growth responses can be obtained from addedNAD(P) precursor compounds in the niacin-deficient basal diet (Oduho and Baker,1993).

Assuming a conversion efficiency (wt/wt) of about 2% (50: 1) of tryptophan to niacinactivity based on studies with chickens (Oduho and Baker, 1993), 10 experimental dietswith different supplemental niacin and tryptophan combinations were formulated(niacin:tryptophan mg per kg diet); O:O, 5:0, lO:O, 15:0, 0:250, 0:500, 0:750, 5:250,lo:500 and 15:750. Diets l-4 had graded niacin levels without supplemental tryptophan;diets 5-7 had no niacin but with added tryptophan equal to the niacin levels in diets 2, 3and 4, respectively, assuming a conversion efficiency of 2%; and diets 8 to 10 had bothadded niacin and tryptophan, to see if there is any feedback regulation of tryptophanconversion to NAD(P). Diets were made isonitrogenous by adjusting the levels ofglycine as tryptophan levels increased. During the mixing of dietary ingredients, 6 NNaOH was added to establish a pH of about 7.0 units (Wilson et al., 1978). Diets werethen prepared and stored as in Experiment 1.

2.2. Experimental procedure

Channel catfish Zctalurus punctatus) fingerlings were obtained from the MississippiAgricultural Experiment Station, MS, and were reared to experimental size in our FishNutrition Laboratory. All fish in the present study were maintained and handledhumanely according to protocols developed in our laboratory. Prior to the start ofExperiment 1, all experimental fish were acclimatized to the basal diet for two weeks.Catfish used in Experiment 2 underwent a shorter l-week conditioning period due totheir larger initial size. The experiments were conducted in 27 (in Experiment 1) and 30(in Experiment 2) 1 lo- 1 fl ow-through aquaria with a flow rate of about 900 ml mitt- .

Water temperature in both experiments was maintained at 28 + 2C. Fluorescent lightingprovided a diurnal 1ight:dark cycle of 14:lO.At the start of the experiment, 20 catfish fingerlings (mean weight 5.6 f 0.1 g) in


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278 W.-K. Ng et al./Aquaculture 152 19971273-285

mortalities during this period. After 5 weeks, growth rates and feed intake in fish fed theunsupplemented basal diet began to drop drastically (data not shown). At this time,mortalites started to occur in these fish and also in fish fed diets with 3 mg niacin per kg

diet, mainly associated with stress after the weekly weighings. After 8 weeks, growthrates and feed intake in fish fed diets supplemented with 3 mg niacin per kg diet becamesignificantly lower (data not shown). Fish in all other dietary treatments continued toconsume their feed vigorously and remained healthy until the end of the experiment.

Fish fed diets without added niacin and with 3 mg niacin per kg diet appearedlethargic and convulsions or muscle spasms usually preceded death or temporary bodyparalysis. No other gross deficiency signs were observed. Significantly higher mortali-ties were observed in fish fed the unsupplemented diet and diets with 3 mg niacin per kgdiet compared to fish fed higher levels of niacin. The addition of 6 mg niacin per kg dietsignificantly increased survival. Very few or no mortalities occured in fish fed dietscontaining 6 mg niacin per kg diet or at higher niacin levels.

Weight gains, specific growth rates, feed efficiency and protein efficiency ratiosresponded significantly (P < 0.05) to niacin additions up to 9 mg per kg diet (Table 2).No further significant growth or feed efficiency response was obtained from higherniacin supplementation in the diet. Fish fed diets containing 9 mg niacin per kg diet andabove showed excellent growth (about 800% average weight gains) and feed efficiency.

Significantly lower haematocrits were observed in fish fed the unsupplemented basaldiet (Table 2). The addition of 3 mg niacin per kg diet significantly increased thehaematocrit levels. No significant differences were observed between the haematocritsof fish fed 3 mg niacin per kg diet or fish fed higher niacin levels.

Liver NAD concentrations in fish fed diets containing 0, 3 and 6 mg niacin per kg

Table 2Growth performance, survival and haematocrit levels of channel catfish fingerlings fed purified dietssupplemented with various niacin levels for 12 weeks in Experiment 1 a

Niacin Final weight(mg per kg diet) (g)

10.83 23.9

6 43.99 50.812 50.915 50.618 50.221 50.424 48.7Pooled s.e.m. 4.6

Weight gain b

(%I

92.24323.1

675.0*799.1811.3796.7790.3806.0770. 1.2

82.1

SGR (8 per day)

0.7g41.72

2.44?2.612.632.612.602.62 2.57,0.20

FERd PER

0.1g40.48

0.891.021.041.031.001.021.010.09

o.5941.613

2983.393.45 3.42 3.353.42 3.370.32

Survival

(%I

80.086.62

96.698.3

100.0100.0loo.0100.0100.0

2.3

Haematocritlevelsf%)

26.4?31.1

31.9233.2,33.8,*32.6,*35.235.733.4,?

0.8

a Values are the mean of triplicate groups of 20 fish. Average initial body weight of individual fish was 5.6 g.Mean values in columns with different superscripts are significantly different (P < 0.05).b Expressed as the percent of initial body weight at the end of 12 weeks.

Specific growth rate = [(ln final weight - In initial weight)/time in days] X 100.d Feed efficiency ratio = wet weight gain (g)/total dry weight of diet fed (8). Protein efficiency ratio = wet weight gain (g)/total protein intake (8).


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0.50 ,I a

=f F 0. 40

5 0. 30w

s

B9 0. 20

z

z3 0. 10

0. 00

0 3 6 9 12 15 16 21 24

dded niacin (mg/kg diet)

Fig. I. Liver NAD concentrations in channel catfish fingerlings fed graded levels of niacin for 12 weeks.Values are means+s.e. of triplicate groups of pooled livers from five fish. Bars with different letters aresignificantly different, P < 0.05.

diet were the lowest and were not significantly different among themselves (Fig. 1). Asignificant increase in liver NAD was observed in fish fed the 9 mg niacin per kg diet.Liver NAD concentrations continued to increase in fish fed increasing levels of dietaryniacin. When regression analysis was performed on the amount of niacin in the diet (mgper kg diet) versus liver NAD concentration (km01 per g wet weight), a good fit resultedr = 0.98). Liver NAD responded linearly to niacin supplementation within the range

tested. The regression equation obtained was y = 0.014~ + 0.104 (where y = NADconcentration in liver and x = niacin concentration in the diet).

Using the one-slope broken line analysis model of Robbins et al. (1979) the dietaryniacin requirement for channel catfish based on percentage weight gains was estimatedto be 7.4 mg niacin per kg diet. This amount of niacin was estimated to be adequate formaximum growth of channel catfish fingerlings.

3.2. Experiment 2

In this 9-week study, growth and feed intake of fish fed diets without niacinsupplementation (diets 1, 5, 6 and 7) were observed to decrease markedly after 7 weeksof feeding. Fish in all other groups consumed their feed vigorously. Fish appearedhealthy and no gross deficiency signs were observed in fish in all dietary groups,including those without niacin supplementation. Mortalities were not significantlydifferent among the 10 dietary groups (Table 3).

In general, fish fed diets without added niacin had the lowest specific growth rate,and feed efficiency ratios which were mostly not significantly lower than those fish fedonly added niacin (diets 2-4) but were significantly lower than fish that were fed both


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280 W .-K. N g et al ./Aguacult ure 172 1997) 273-285

Table 3Growth performance, survival and haematocrit levels of channel catfish fingerlings fed purified dietssupplemented with various niacin and tryptophan levels for 9 weeks in Experiment 2 a

Diet Niacin: tryptophan Final weight SGR b FER PER* Survival Haematoctit(mg per kg diet) (g) (% per day) (%o) levels (%o)

1 0:O 51.53,4 l.793,4 0.773,4 2.583.4 91.6 28.62.32 5:o 54.82,3 1.94,2,3 0 872,3

O:813.42.882.3 100.0 41.2

3 lo:o 54.82,3 1.903 2.772,3,4 95.8 40.94 15:o 54.22,3 1.912.3 0.813,4 2.693,4 95.8 41.05 0:250 47.34 1.773,4 0.724 2.414 93.7 28.42s36 0:500 46.74 1.704 o.744 2.464 97.9 31.327 I750 45.84 l.724 0.724 2.384 93.7 27.338 5:250 61.2~ 2.10 0.98 3.27 97.9 40.49 10:500 62.6 2.09, 0.9332 3.11J 91.6 40.010 15:750 59.0*2 2.09J 0.99 3.29 100.0 41.6Pooled s.e.m. 1.8 0.04 0.03 0.10 0.9 1.8

a Values are the mean of triplicate groups of 16 fish. Average initial body weight of individual fish was 16.1 g.Mean values in columns with different superscripts are significantly different (P < 0.05).b Specific growth rate = [(ln final weight-In initial weight)/time in days] X 100. Feed efficiency ratio = wet weight gain (g)/total dry weight of diet fed (g).* Protein efficiency ratio = wet weight gain (g)/total protein intake (g).

added niacin and tryptophan (diets 8-10). In most cases, fish fed diets with both addedniacin and tryptophan had significantly higher specific growth rates and feed and proteinefficiency ratios than fish fed diets with only added niacin. Niacin supplementation(5-15 mg per kg diet) alone did not significantly affect the growth performance ofcatfish in this study. Tryptophan additions (250-750 mg per kg diet) also did notsignificantly affect weight gain or feed efficiency in fish fed diets with or without niacinsupplementation.

Haematocrits of fish fed diets without niacin supplementation were significantlylower than fish fed diets that had niacin added (Table 3). Addition of niacin at 5 mg perkg diet significantly increased the haematocrit values, but no further significant responsewas obtained with further niacin additions. Addition of tryptophan at all levels tested didnot significantly affect haematocrit values in fish fed diets with or without niacinsupplementation. Haematocrits in fish fed diets with added tryptophan and niacin werenot significantly different from the values obtained in fish that were fed diets with addedniacin only.

Liver NAD levels (Fig. 2) were significantly lower in fish fed diets without addedniacin. Addition of tryptophan alone in diets 5, 6 and I (250, 500 and 750 mg kg-,respectively) did not elicit any significant response in liver NAD concentrations whichwere similar to concentrations found in fish fed the basal niacin and tryptophanunsupplemented diet (diet 1). Addition of 5 mg niacin per kg diet caused a significantincrease in liver NAD concentration. At increasing niacin supplementation, liver NAD

levels continued to increase. Liver NAD levels in fish fed diets with both supplementalniacin and tryptophan (diets S-10) responded in a similar manner to the liver NADlevels in fish fed diets with added niacin alone (diets 2-4). No significant differences


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reported by Andrews and Murai (1978). Markedly different niacin requirement values(ranging from 1 to 150 mg per kg diet) have also been reported for the rainbow trout bydifferent researchers (McLaren et al., 1947; Phillips and Brockway, 1947; Poston and

Wolfe, 1985; Halver, 1989).The niacin requirement of 7.4 mg per kg diet for the adequate growth of channel

catfish was further confirmed by feed and protein efficiency ratios which paralleled thegrowth response of fish to added niacin in Experiment 1. Mortality and lethargy of fishwere prevented by about 6 mg niacin per kg diet. Anaemia was prevented by as low as 3mg niacin per kg diet. All these results would seem to suggest that channel catfishrequire less than 10 mg niacin per kg diet for optimum growth, feed efficiency, survivaland prevention of deficiency signs.

A primary determinant of the variation in the niacin requirements of non-ruminantland vertebrates is in their ability to utilize dietary tryptophan as a precursor of NAD(P)

(National Research Council, 1987). Results reported here in Experiment 2 provideddirect evidence of the inability of dietary tryptophan to act as a precursor of NAD(P) inchannel catfish. The same may be true in certain salmonid fish based on indirect liverenzyme evidence (Poston and DiLorenzo, 1973; Poston and Combs, 1980). Niacindeficiency signs have also been observed to occur relatively quickly in most fish speciesstudied to date despite being fed high protein diets, which indirectly points to their lackof ability to utilize dietary tryptophan as a source of niacin.

The low niacin requirement determined for channel catfish is noteworthy from thestandpoint of fish nutrition, as it brings into question the niacin requirements reportedfor other fish species which are well above this estimated value. Since the currentliterature (National Research Council, 1993) seems to indicate that most if not all fishspecies cannot efficiently utilize tryptophan as a source of NAD(P), another metabolicbasis, if any, needs to be given to explain the wide variations in niacin requirementsreported. Even though there may be some inherent species differences in niacinrequirements, we believe that the wide range of reported niacin values for fish aremainly due to differences in experimental diets and conditions. For example, Shiau andSuen (1992) reported that the dietary niacin requirement for tilapia was 26 mg per kgdiet in fish fed a glucose diet, whereas 121 mg niacin per kg diet was needed in fish feda dextrin diet. A mere change in one dietary ingredient, in this case the type ofcarbohydrate, altered the niacin requirement of tilapia almost five-fold.

Some studies (Phillips and Brockway, 1947; Halver, 1989; Shimeno, 1991) have usedmaximal liver storage to define niacin requirement in fish. From Experiment 1 of thepresent study, liver NAD levels were observed to increase linearly with increasing levelsof dietary niacin (Fig. 1) within the range tested. Work on rats indicates that liver NADlevels do not plateau, but continue to increase even at pharmacological doses of dietaryniacin (Jackson et al., 1995). Liver storage of niacin would therefore not be a good basisfor determining the niacin requirement in fish for optimal growth.

Fish fed the unsupplemented and 3 mg niacin per kg diets in Experiment 1 appearedlethargic and remained stationary at the bottom of the aquarium. No other grossdeficiency signs were observed. In the last few weeks of the 12-week study, feed intakewas drastically reduced and outright feed refusal was observed in fish fed the unsupple-mented basal diet. Body convulsions or muscle spasms preceded by imminent death or


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temporary paralysis often occurred in fish fed these two diets when the fish werestressed. This observation and the general weakness of the fish fed the niacin-deficientdiets is consistent with the critical roles NAD and NADP play in the synthesis of high

energy phosphate bonds which furnish energy for intermediary metabolism and othernormal physiological functions in animals. It is also posssible that some of thedeficiency signs may be due to decreased post-translational modification of proteinswith mono or poly(ADP-ribose) or through changes in the formation of cyclic(ADP-ribose), which all require NAD+ as a substrate.

Dupree (1966) reported tetany, high mortality after stress, lethargy and reducedcoordination as niacin deficiency signs for channel catfish. These signs reported byDupree (1966) were more similar to those observed in the present study than that byAndrews and Murai (1978) who reported anaemia, skin and fin lesions and haemor-rhages, deformed jaws, exophthalmia and extremely high mortality in channel catfish

fed niacin-deficient diets for 12 weeks. Such variation in niacin deficiency signs has alsobeen observed within the salmonid species (McLaren et al., 1947; Phillips and Brock-way, 1947; Halver, 1989) and among other fish species such as common carp (Aoe etal., 19671, Japanese eel (Arai et al., 1972) and hybrid tilapia (Shiau and Suen, 1992).Other researchers have reported no gross deficiency signs other than retarded growth instudies with rainbow trout (Kitarnura et al., 1967) and gilthead seabream (Morris andDavies, 1995). Andrews and Murai (1978) suggested that such discrepancies in defi-ciency signs among fishes may be due to differences in the experimental conditions,species, age of fish, and degree of bacterial synthesis in the gastrointestinal tract.

It was unfortunate that the feeding trial in Experiment 2 had to be stopped after 9weeks, since the carrying capacity of the aquaria was exceeded in some dietary groups.Together with the larger initial fish size and the shorter conditioning period used inExperiment 2, this probably explains why the growth performance of fish fed the basaldiet with no added niacin was much better than that observed in fish from Experiment 1.Other than a slight retardation of growth and the observed slower and incompleteconsumption of the feed offered, all fish fed diets with no added niacin in Experiment 2appeared healthy. However, haematocrit and liver NAD levels responded to changes indietary niacin more rapidly than did the growth rates. Fish fed diets with no added niacinhad significantly lower haematocrit and liver NAD levels. The objectives of Experiment2 were therefore not compromised by the shorter duration of the feeding trial as fish feddiets with no added niacin were clearly in a niacin-deficient state. Rawling et al. (1994)showed that dietary niacin deficiency decreased tissue NAD+ pools in rats even in theabsence of severe clinical signs. They reported that the NAD+ pool most sensitive toniacin deficiency in the rat was in the blood, which would therefore be reflected in thehaematocrits since it is based on packed red blood cell volume.

The basal diet used in Experiment 2 contain synthetic amino acids which is known todecrease the growth performance in most fish (Dupree and Halver, 1970; Ng et al.,1996). This is reflected in the specific growth rates of fish fed niacin-adequate diets inExperiment 2, which were lower compared to the growth rates of fish fed similar niacin

levels in Experiment 1.In conclusion, the results of the present study indicate that the niacin requirement ofchannel catfish fingerlings is 7.4 mg per kg diet, about half the previously accepted


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284 W.-K. Ng et al./Aquaculture 152 1997) 273-285

value of 14 mg per kg diet. Data on growth, feed efficiency, haematocrit and liver NADlevels provided direct evidence that L-tryptophan is an inefficient precursor of NAD(P)for the channel catfish. This study also presents the first reported data on the influence

of dietary niacin and tryptophan on liver NAD levels in fish.

cknowledgements

We would like to thank the Aquaculture Unit at the Delta Research and ExtensionCenter at Stoneville, Mississippi, for determining the niacin content of casein for thisstudy. This research was supported in part by a grant from the Southern RegionalAquaculture Center. Publication No. J-8925 of the Mississippi Agricultural and ForestryExperiment Station, Mississippi State University.

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Carson, D.A., Seto, S. and Wasson, D.B., 1987. Pyridine nucleotide cycling and poly(ADP-ribose) synthesis inresting human lymphocytes. J. Immunol., 138: 1904-1907.Chuang, J.L., 1991. Fish and Shrimp. In: R. Fenster and R.A. Blum (Editors), Niacin in Animal Nutrition.

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Kitamura, S., Suwa, T., Ohara, S. and Nakagawa, K., 1967. Studies on vitamin requirements of rainbow troutII. The deficiency symptoms of fourteen kinds of vitamins. Bull. Jpn. Sot. Sci. Fish., 33: 1120-l 125.

Klingenberg, M., 1974. Nicotinainide-Adenine Dinucleotides (NAD, NADP, NADH, NADPH). Spectrophotemetric and fluorimetric methods. In: H.U. Bergmeyer (Editor), Methods of Enzymatic Analysis, Vol. 4.Verlag Chemie Weinheim and Academic Press, New York and London, pp. 2045-2072.

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Niacin requirement and inability of tryptophan to act as a precursor of NAD+ in channel catfish, Ictalurus punctatus