Vitamin B6 課程大綱 - TMU

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2009/2/18 Pyridoxine 1

Vitamin B6

PyridoxinePyridoxalPyridoxamine

Shih-Yi Huang, PhDChair & ProfessorSchool of Nutrition and Health SciencesTaipei Medical [email protected]~6552

營養生化學 Nutritional Biochemistry


課程大綱

IntroductionMetabolismFunctionsNutritional statusDeficiencyDRI

ToxicitySupplementSummary


學習目標

Students should understand followingtopics such as: The history of selected vitamin Learn and understand the pyridoxal

involved mechanism in protein metabolism The application of selected vitamin


IntroductionDr. Gyorgy identified in 1934 a family of chemically-related compounds including

pyridoxamine (PM) & pyridoxal (PL) and pyridoxine (PN).

The form most commonly form is pyridoxine HCl.

Phosphorylation of 5’position of B6 (PM, PL, & PN),which make PMP, PLP, & PNP. In animal, predominant of PMP & PLP;

in plant foods, a glucoside form in which glucose (5’-O-[-glucopyranosyl] pyridoxine) may play as storage formof vitamin.

2009/2/18 Pyridoxine 5 2009/2/18 Pyridoxine 6

bacterial alanine racemase

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Introduction Vitamin B6 is one of the most versatile enzyme

(>100 enzymes) cofactors.

Pyridoxal is the predominant biologically activeform (PLP). (More inborn error???)

Pyridoxal phosphate (PLP) is a cofactor1) in the metabolism of amino acids and neurotransmitters;2) in the breakdown of glycogen;3) bind to steroid hormone receptors and may have a role

in regulating steroid hormone action;4) in the immune system


ChemistryName: pyridoxine, pyridoxal & pyridoxamine,vitamin B6

Structure: 2-methyl-3-hydroxy5-hydroxy methylpyridines.Chemistry & physical property property: Vitamin B6s are readily soluble in water, stable in acid, unstable in alkali, is fairly easily destroyed with UV light, e.g. sunlight.

Stability --- pyridoxine > pyridoxal or pyridoxamine


Bioavailability

Complexed forms B6 poorly digested

B6 can be condensed with peptide lysyl or cysteinylresidues during food processing (e.g. cooking)thus, reduced the utilization of B6.

Plants contain complexed forms bioavailabilityof B6 in animal products tends to be greater thanin the plant materials.


Absorption

Vitamin B6 is readily absorbed in the smallintestine. Pyridoxine uptake is by passivediffusion rather than by active transport, mainlyin jejunum & ileum.Driving force for B6 depends on the de- &rephosphorylation & protein binding. De- & Rephosphorylation of PLP, PNP is catalyzed byalkaline phosphatase & pyridoxal kinase,respectively.


Figure 1: B6 metabolism and transport2009/2/18 Pyridoxine 12

Absorption

Excess vitamin B6 is excreted in theurine so adequate daily intake isessential.Phosphorylated pyridoxine andpyridoxamine are then oxidized to thecommon forms.

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Transportation

Once absorbed, it (PLP and small amount of freepyridoxal) is carried in the blood tightly bound toproteins [the erythrocytes (hemoglobin) &albumin in plasma] to all body cells.

B6 binds via the amino group of the N-terminalvaline residue of the hemoglobin chain.(Schiff-Base)


TransportationPyridoxal more readily crosses cell membranesthan pyridoxal phosphate.

After being taken into the cell, the B6 vitamersare converted via a saturable two-step process(phosphorylation) to pyridoxal phosphate (PLP).

The reactions are catalyzed by a pyridoxal kinase(an enzyme present in the cytoplasm of mucosalcell.)


70-80% of the vitamin B6 in the body is located inmuscle (glycogen phosphorylase); 10% of B6 inthe liver; the remainder is distributed among theother tissues.

Small amount of vitamin B6 stored in body (40-150 mg; 20-75 days needed to deplete), mainlyas PLP & PNP.

Distribution


Conversion

When the vitamins are phosphorylated,transmural absorption decreases whereas uptakeis unaffected.Phosphorylation thus serves as a means ofcontrol of the cellular PMP, PLP, & PNP levels.This called “metabolic trapping”, because non-phosphorylated vitamers cross membranes moreeasily their phosphorylated analogues.


Why phosphated form of B6?Reason: to protect it from hydrolysis and providestorage of the vitaminPrinciple:

1. phosphorylation/dephosphorylation;2. oxidation/ reduction;3. amination/deaminationThe involved hepatic enzymes: pyridoxal kinase (Zn depended) produced phosphated

vitamers alkaline phosphatase produced dephosphated vitamers pyridoxal phosphate oxidase (FMN depended) e.g. PL

PLP

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CatabolismThe binding of PLP to albumin protects the coenzyme fromdegradation in the blood.In the liver, dephosphorylated & oxidized by pyridoxaloxidase (FAD, FMN) & aldehyde dehydrogenase (NAD),respectively.PMP & PNP are oxidized by a FMN-dependent oxidase toform PLP.The forms of vitamin B6 found in food are converted toactive forms in the liver. Zinc, riboflavin, and niacin arenecessary for this process.Fasting and reducing diets usually deplete the vitamin B6supply unless it is supplemented. Usually within 8 hours,much of the excess is excreted through the urine.


ExcretionBound B6 vitamers are catabolized by FAD-dependentaldehyde (pyridoxal) oxidase as well as NAD-dependentaldehyde dehydrogenase to produce 4-pyridoxic.4-Pyridoxic acid (PA) is the major excretory product, 50%in urine. It is also produced by the intestinal bacteria.Alcohol and its degradation product, acetaldehyde, displacePLP from proteins resulting in enhanced catabolism of thecoenzyme.Amphetamines, chropromazine, oral contraceptives, andreserpine all increase B6 loss. Oral contraceptives had beenfound to increase tryptophan use and thus increased B6 use.


Functions

Helping amino acid and protein metabolismEnabling the breakdown of glycogen to glucoseConverting tryptophan to niacin (amino acid &vitamin interaction)Enabling red blood cell metabolismHelping the nervous system function efficientlyHelping the immune system function efficiently


FunctionsPLP is involved in over 100 enzymaticreactions. Some of the major functionsare as follows: Aminotransferase reactionsDecarboxylation reactions Site-chain cleavage reactionsDehydratase reactionsOther functions


Mechanism of ActionMetabolically active form of B6 (PLP) serves as a coenzymeof numerous enzymes most of which are involved especiallyin amino acid metabolism and in neurotransmitters ofcentral nervous systemCofactor of Many Enzymes: The largest group of vitamin B6-dependent enzymes is transaminase,

which use -ketoglutarate as amino group acceptor.

Other PLP-dependent enzymes include decarboxylases, racemases &enzymes that catalyze amino acid side-chain alterations.

PLP also serves as a coenzyme for the phosphorylase & as amodulator of protein structure (e.g. steroid hormone receptor,hemoglobin & other enzymes).


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Metabolic Roles1. PLP involved in both amino acids biosynthesis &

catabolism (amino acid transamination) The response of erythrocyte aspartate transaminatase

(EAST) to in vitro additions of PLP has been used as abiomarker of B6 status

Transsulfuration of methionine to cysteine needs PLP(cystathionine synthase & cystathionase), thus B6

deprivation, individual shows homocysteinuria &cystathionuria.


Binding of substrate to a PLP-dependent enzyme


Mechanism of transaminases


(continued)


Serine, Glycine and Cysteine

Serine reacts with homocysteine,derived from methionine, to formcystathione. This reaction iscatalyzed by cystathione -synthase (CS).

Cystathione is cleaved by cysta-thionase to yield cysteine and-ketobutryate which goes onto form succinyl CoA, a TCAcycle intermediate.

Both enzymes are homologousto aminotransferases and requirePLP (vitamin B6).


Metabolic Roles2. PLP involved in:

Lipid metabolism: Sphingolipid biosynthesis (by serinepalmitoyltransferase) & arachidonic acid to prostaglandinE2 & carnitine synthesis

Gluconeogenesis: the release of glycogen (glycogenphosphorylase) from the liver and muscles.

Hemoglobin synthesis: helps red blood cell production(the synthesis of porphyrin precursors to heme) [-aminolevulenic acid synthase]

Niacin formation: the conversion of tryptophan to niacin(kynureninase)

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Glycogen Phosphorylase catalyzes phosphorolytic cleavage ofthe (14) glycosidic linkages of glycogen, releasing glucose-1-phosphate as reaction product.

glycogen(n residues) + Pi glycogen (n–1 residues) + glucose-1-phosphate

This phosphorolysis may be compared to hydrolysis:Hydrolysis: R-O-R' + HOH R-OH + R'-OH

Phosphorolysis: R-O-R' + HO-PO32- R-OH + R'-O-PO3

2-

glucose-1-phosphate

H O

OH

H

OHH

OH

CH2OH

H

OPO32

HGlycogen catabolism(breakdown):


Metabolic Roles3. PLP involved in Neurotransmitters synthesis:

Conversion of tryptophan to serotonin (tryptophandecarboxylase needs PLP)

Conversion of tyrosine to norepinephrine (tyrosinedecarboxylase) and acetylcholine that depend on PLPtheir metabolism.

-Aminobutyric acid (GABA) synthesis for brain as energysource

Synthesis of vasodilator and gastric secretagoguehistamine (histidine decarboxylase).


Biosynthesis of Serotonin

Tryptophan hydroxylase

5-HTP decarboxylase


Biosynthesis of the catecholamines

Tyrosine hydroxylase

DOPA decarboxylase

Dopamine -hydroxylaseCu+


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QuickTime?and aGIF decompressor

are needed to see this picture.

Homocysteine Metabolism and Vascular Dysfunction

Hajjar KA, J Clin Invest 107:663, 2001


Metabolic RolesPLP involved in Nucleic acids & immunesystem: The synthesis of nucleic acids process modulates via

1-carbon metabolism, e.g. serine hydroxymethyl-transferase (SHMT);

PLP Inhibits thymidylate synthease through thisprocess B6 could impair DNA synthesis.


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Deficiency

Primary deficiency of vitamin B6 is rare,secondary deficiency may result in certainsituations, including malabsorption, alcoholism,some medications, cigarette, and smoking.Frank deficiencies are rare, but subclinicaldeficiencies may exist, especially in women,heavy smoker, alcoholics and the elderly.


Deficiency

Symptoms of vitamin B6 deficiency include: Skin inflammation and irritation, glossitis (sore

or inflamed tongue), cheilosis (cracking andscaling of the lips), angular stomatitis, extensivedandruff and anemia.

Confusion, depression, irritability andnervousness, fatigue and sleepiness,convulsions, water retention.


Assessment

4-Pyridoxic acid (4-PA): main biologically inactivemetabolite of B6 in urine, but not in blood &tissue.

Plasma PLP: the predominant vitamer in plasma& the most frequently used index of B6 status inhuman, decreases with age, exhibiting highestconc. in new born & early children.


AssessmentErythrocyte transaminase index (PLP-dependentenzyme activity): Erythrocyte Asparate aminotransferase

(EAST or GOT index)

Erythrocyte Alanine aminotransferase (EALT or GPTindex)

With or without B6 added if EAST-index > 2.2 deficiency

Normal EAST index < 1.5; EALT index < 1.25


EAST, GOT

EALT, GPT

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Erythrocyte and whole-blood PLP, plasmahomocysteine

Other

Urinary xanthurenate excretion,especially after a tryptophan load test

Tryptophan catabolites

Enzymes for aspartate and alanine reflectPLP levels; large variation in activitycoefficients

Erythrocyte aminotransferaseactivity coefficients

Excretion rate of vitamin and particularly4-pyridoxate reflects intake; 5-pyridoxateappears with excess intake

Urinary vitamin B6 cataboliteexcretion

Major vitamin B6 form in tissue andreflects liver PLP; changes fairly slowly inresponse to vitamin intake

Plasma pyridoxal 5’-phosphate(PLP)

AssessmentIndicator

Some indicators for assessing vitamin B6 status


Methods of analysis

MicrobiologicalEnzymaticHPLC assay for various nutritional &food statusPLP is one of the most measurement.


Requirements

Vitamin B6 intake is determined primarily byprotein intake.The RDA for adults is a minimum of 2 mg of B6per 100 grams of protein consumed. In children,it ranges from 0.6-1.2 mg. per 100 grams ofprotein.


Sources

The richest sources of vitamin B6 arechicken, fish, liver, kidney, pork, eggs, milk,wheat germ and brewer's yeast.Other good sources include brown rice,soybeans, oats, whole wheat products,peanuts and walnuts.


Supplements

Vitamin B6 is available as pyridoxine HCl andpyridoxal-5'-phosphate. The latter is the moreactive form and may be best for those with liverdisease who cannot convert pyridoxine topyridoxal-5'-phosphate.Anyone at risk of deficiency or who is sufferingfrom a condition possibly linked to vitamin B6deficiency is likely to benefit from supplements.


ToxicityThe tolerable upper intake level (UL) for vitamin B6from dietary sources and supplements combined is100 mg per day.Large doses of vitamin B6 (over 2,000 mg) cancause nerve damage. Symptoms of vitamin B6toxicity include: muscle incoordination, numbnessof the hands and feet, impaired reflexes, abnormalplasma amino acid levelsNight restlessness, vivid dreams, sun sensitivity andan acne-like rash may also occur with high doses.

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Therapeutic UseB6 supplements are used to treat deficiencysymptoms. They have also been widely used in thetreatment of a number of other health conditions. Asthma Cardiovascular disease Mood disorders Premenstrual syndrome and estrogen therapy Nausea of pregnancy Carpal tunnel syndrome Convulsions Immune system Other uses


Disease PreventionImmune functionLow vitamin B6 intake and nutritional status have beenassociated with impaired immune function, especially in theelderly. Decreased production of immune system cellsknown as lymphocytes and IL-2 have been reported invitamin B6 deficient individuals.A study found that the amount of vitamin B6 required toreverse these immune system impairments in the elderlywas 2.9 mg/day for men and 1.9 mg/day for women; thesevitamin B6 requirements are higher than the current RDA.

Meydani SN, et al.,Am J Clin Nutr. 1991;53(5):1275-1280.


Homocysteine and cardiovascular diseaseA large prospective study found the risk of heart diseasein women who consumed 4.6 mg of vitamin B6 daily wasonly 67% of the risk in women who consumed an averageof 1.1 mg daily. Moreover, higher plasma levels of PLPwere associated with a decreased risk of CVD independentof homocysteine levels.

Rimm EB et al., 1998;279(5):359-364.Folsom AR et al., 1998;98(3):204-210.

Disease Prevention


Cognitive functionDouble-blind, placebo-controlled studies in healthyelderly men found that vitamin B6 supplementation (75mg/day) improved memory but had no effect on mood ormental performance.

Riggs KM et al., Am J Clin Nutr. 1996;63:306-314.

Deijen JB et al., Psychopharmacology (Berl). 1992;109:489-496.

Bryan J et al, J Nutr. 2002;132(6):1345-1356.

Disease Prevention


Kidney stonesA group of more than 85,000 women without a priorhistory of kidney stones were followed over 14 yearsand those who consumed 40 mg or more of vitamin B6daily had only 2/3 the risk of developing kidney stonescompared with those who consumed 3 mg or less.

Curhan GC et al., J Urol. 1996;155:1847-1851.

However, in a group of more than 45,000 men followedover six years, no association was found betweenvitamin B6 intake and the occurrence of kidney stones.

Bender DA. Br J Nutr. 1999;81:7-20.

Disease Prevention


Disease TreatmentDepressionBecause a key enzyme in the synthesis of theneurotransmitters serotonin and norepinephrine is PLP-dependent, it has been suggested that vitamin B6deficiency may lead to depression.However, clinical trials have not provided convincingevidence that vitamin B6 supplementation is an effectivetreatment for depression, though vitamin B6 may havetherapeutic efficacy in premenopausal women.

Williams AL, et al., 2005;22(5):532-537.

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Disease TreatmentSide effects of oral contraceptivesA placebo-controlled study in women on the lower doseoral contraceptives, which are commonly prescribed today,found that doses up to 150 mg/day of vitamin B6(pyridoxine) had no benefit in preventing side effects,such as nausea, vomiting, dizziness, depression, andirritability.

Villegas-Salas E et al., Contraception. 1997;55(4):245-248.


Disease TreatmentPremenstrual syndrome (PMS)A review of 12 placebo-controlled double-blind trials onvitamin B6 use for PMS treatment concluded that evidencefor a beneficial effect was weak.

Kleijnen J, et al., Br J Obstet Gynaecol. 1990;97(9):847-852

A more recent review of 25 studies suggested thatsupplemental vitamin B6, up to 100 mg/day, may be ofvalue to treat PMS; however, only limited conclusionscould be drawn because most of the studies were of poorquality.

Wyatt KM, et al., Bmj. 1999;318(7195):1375-1381.


Disease TreatmentMorning sicknessThe results of two double-blind, placebo-controlled trialsthat used 25 mg of pyridoxine every 8 hours for 3 days or10 mg of pyridoxine every 8 hours for 5 days suggest thatvitamin B6 may be beneficial in alleviating morningsickness. Each study found a slight but significantreduction in nausea or vomiting in pregnant women.

Vutyavanich T, Am J Obstet Gynecol. 1995;173:881-884.Sahakian V, et al., Obstet Gynecol. 1991;78:33-36.

A recent systematic review of placebo-controlled trials onnausea during early pregnancy found vitamin B6 to besomewhat effective.

Jewell D et al., Database Syst Rev. 2002(1):CD000145.


Disease TreatmentCarpal tunnel syndromeCarpal tunnel syndrome causes numbness, pain, andweakness of the hand and fingers due to compression ofthe median nerve at the wrist.While a few trials have noted some symptomatic relief withvitamin B6 supplementation, double-blind, placebo-controlled trials have not generally found vitamin B6 to beeffective in treating carpal tunnel syndrome.

Bender DA. Br J Nutr. 1999;81(1):7-20Spooner GR, et al., Can Fam Physician. 1993;39:2122-2127.


Interactions with nutrients

Vitamin B6 requires riboflavin, zinc and magnesiumfor its normal function in the body.

Vitamin B6 deficiency may result in low blood levelsof vitamin C, increased excretion of calcium, zincand magnesium, and reduced copper absorption.

More B6 is utilized with an increased intake of theamino acid methionine.


Interactions with drugsDrugs that influence needs for B6 are oralcontraceptives, isoniazid (for tuberculosis), hydralazine(for high blood pressure), amphetamines, reserpine (for

high blood pressure), and some antibiotics.Alcohol increases breakdown of the biologicallyactive form of vitamin B6 and long-term use maycause liver damage, which interferes with theconversion of vitamin B6 to the active form.Levodopa, some antidepressants, andpenicillamine may alter vitamin B6 requirements.

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Cautions

B6 supplements should not be taken by thosetaking anticonvulsants or levodopa for Parkinson'sdisease as they may alter the effectiveness ofthese medications.B6 may lower blood sugar and should be usedwith caution by diabetics.


Summary

Pyridoxal involves of protein and aminoacids metabolism. The requirement dependson the intake of proteins.Lack of pyridoxal may show the skin lesionand mental or neuro-retardation bybiochemical processes.

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Vitamin B6 課程大綱 - TMU