R E S E A R C H ON URIC ACID. 205

R E C E N T RESEARCH ON URIC ACID A N D PURINE METABOLISM.

By WILLIAM ~FEARON,

Trinity College, Dublin.

T I-IE discovery by Hopkins, in 1892, that uric acid is completely precipitated from human urine as ammonimn urate on standing after saturation with

ammonium chloride provided investigators with a simple method of analysis, and stimulated interest in thi~ long- known but little understood purine.

tIowever, in the absence of a reliable method for the estimation of uric acid in the blood, the data from urinary analysis would only afford relative information. The be- haviour of the renal filters was unknown; and changes in the uric acid output might be due to changes in the ~m~onnt formed or in the amount allowed to pass • the kidneys. A similar uncertainty existed in the cases of urea, glucose, ammonia, and creatinine.

I~ is only within recent times that uric acid ha, been posi.tiveiy detected in normal blood; and the method of its estimMfion in blood is still a subject of active research (Folin and Wu, 1921-22). Work is in progress on the elucidation of fhe factors which determine the amount of uri(- a~'i,l in blood, and already a considerable amount of new knowledge has been obtained.

Before discussing the results of recent investigation~ into the subject of purine metabolism, it will be found con- venient to consider briefly the history of uric ~tci(t i~ chemistry and physiology.

Uric Acid.

Uric acid was discovered by Scheeh, (1770) in urinary calculi and also in the precipitate from acidifi~'d urine. Fourcroy examined it (1793) and found it somewh~t re- sembled urea, which led him to call it ourie acid. It waa also termed lithie acid, from its occurrence in calculi.

206 I I t l S H J OUR NAL OF M E D I C A L SCIENCE.

Later on, uric acid was detected in guano (1805); in birds" excrement (1815), and in snakes' excrement. Liebig and W6hler analysed uric acid (1834), and found the empirical formula to be C H,N~O~. I t s constitution was estab-

lished by ]3aeyer, and Fischer, who showed it to be a tri-oxy derivative of a parent compound, which they called purine (C ~H.~N ~).

The Purines.

lk[eanwhile, several substances closely related to uric acid had been discovered. These were: xanthine, in urinary calculi (1817) and meat extracts (1859); hypoxanthine, in many animal and plant tissues; adenine and guanine, in nucleoproteins; caffeine, theophylline, and theobromine, in tea and coffee.

Each of these substances could give rise to uric acid or its direct oxidation derivatives, and was shown to be de- rived from the parent substance, purine.

For this reason, all these compounds were classified as the purines, or members of the purine group.

Most of them are intimately associated with the nucleo- proteins which are found in the chromatin and karyoplasm of cell-nuclei, and in consequence are widely distributed in the cellular structures of plants and animals. The number of purines is continually being increased by chemical inves- tigation.

The Purines oJ the Human Body.

Upwards of a dozen purine derivatives have been detected in the tissues a~d fluids of the human body. H u m a n urine contains on an average 0.4 ~o 0.8 gins. of purine bodies in the litre; or about 0.9 gms. in 24 hours. Of these purines, uric acid is by far the most important , and constitutes over four-fifths of the total purine excretion in man.

Uric Acid Dis- Milligms. per bution. 100 ccs. Percentage.

Normal Blood - 1 to 3 0.001 to 0.003 Normal Urine 50 to 150 0.05 to 0.15

These figures are from the data obtained by Myers and Fine (1915), Folin and Benis (1913-15), Hawke (1921),

R E S E A R C H ON URIC ACID. 207

Folin and Wu (1921), and others. The output of uric acid by the kidneys is between 0.6 to 1.8 gins. in 24 hours, sub- ject to dietetic variations.

The purines other than uric acid are generally termed the purine bases. They constitute a small and variable fraction of the total urinary purines: about 16 to 60 mgms. in 24 hours. The largest~ in amount are paraxanthine, hetero- zanthi~e, and methylxanthine, which are formed from the purines of the food: caffeine, theobromine, theophylline (in tea, coffee, and cocoa).

The Origin of the Purines in the Blood. In common with all the products of animal metabolism,

~wo sources of blood purines may be recognised : - - (a) Exogenous purines: Arise from purines formed out-

side the organism, and depend directly on the nature and quantity of the diet.

(b) Endogenous purines : Arise inside the organism as the result of metabolism of non-purine precursors, and only depend ,indirectly on the nature and quality of the diet.

Endogenous purine metabolism appears to be remarkably constant in the normal adult. Even when the diet is quite free from purines uric acid is excreted in the urine in quan- tities of 0.1 to 0.5 gms. per diem. :Folin, on a purine-free diet of cream and starch, reduced his purine excretion to 0.3 gins. in the day, but was not able to go below this mini- mum; which is generally accepted as the normal value of endogenous metabolism.

This endogenous purine is being continually synthesised in the animal body; its precursors are probably simple pro- ducts of normal metabolism, but they have not yet been identified.

Owing to the existence of purines in the nucleoproteins that go to make up the nuclei of all forms of cells, it is be- lieved tha~ the 'output of endogenous purine represents the normal cell activity and disintegration of the organism. In leukmmias the excretion is extremely high owing to increased nuclear destruction. The excretion of endogenous purine is also increased by exercise and by the ingestion of large

208 I R I S H JOURNAL OF MEDICAL SCIENCE.

quantities of purine-free proteins. This latter observation may be by the action of the food in stimulating the digestive glands to activity (Mares, 1910; Mendel and Stehle, 1915), but it may also be due to fhe presence of non-purine pre- cursors of endogenous purine. This post-prandial increase on a purine-free diet was first noted by Hopkins (1899).

Significance o~ Uric Acid in the Human Body. Uric acid is the waste-product of purine-metabolism in

man. Allantoin is the waste-product of purine-metabolism in all other mammals investigated, except the anthropoid apes and the Dalmatian dog, which resemble man in their ,excretion of uric acid.

As a result of oxidation and deamina~ion brought about by .tissue enzymes the purines of the organism are converted into uric acid. In the human subject purine break-down stops at this stage a n d t h e uric acid is excreted. By injec- :tion experiments, Mendel and Lyman found that 60 per ,cent. of hypoxanthine, 50 per cent, of xanthine, 30 per cent. of adenine and guanine were converted into uric acid and eliminated in the urine.

In the lower mammals, the break-down of purines goes a stage further than uric acid. An enzyme, uricase, or uric ,acid oxidase, is present in the visceral organs and ~issues of most warm-blooded animals, which rapidly converts uric .acid into the much more soluble body, allantoin.

I-IN--C0 (Uricase) NI.I~

f I I CO C - - N t t ~ CO C 0 - - N H ~ J II - - - c o I l ~ c c

HN C - - N H / J HIN- -CH--NH /

Uric Acid A]lantoin (Trioxy-purine) (Glyoxal diureide)

Allantoin may constitute 90 per cent. or more of the total purine output of the lower mammals. I t has been found in small traces in human urine (10 mgm. in 24 hours), where it appears to be entirely exogenous, or may possibly be due to the ingestion of some of the uricase ferment in animal foods. The influence of raw liver on purine-meta- bolism in man does not appear to have been inves{igated.

R E S E A R C H ON U R I C A C I D . 209

~Purine me tabo l i sm in m a n is pecul ia r owing to the com- ple te absence of the enzyme uricase f rom h u m a n t issues. Consequent ly , the h u m a n pur ines are not excre ted in any s impler form than uric acid. Uric acid, af ter inject ion, can be recovered a lmos t quan t i t a t ive ly f rom the urine in the h u m a n subjec t ; bu t if in jec ted into dogs, cats , and rabbi t s

very l i t t le can be recovered unchanged f rom the urine owing to i ts a lmos t comple te conversion into a l lan to in and urea. This f u n d a m e n t a l d is t inc t ion in pu r ine -me tabo l i sm has made the expe r imen ta l inves t iga t ion of pur ine format ion difficult, since the conclusions drawn from exper imen t s on an imals o ther t han an thropoids will not apply to man. The Ur icoly t ic I n d e x of an imal t issues has been examined by Hun te r , who expresses i t as the percentage of uric acid changed to a l lantoin.

Uricolytic Index. Rabbit 95 Dog 98 Sheep 80 Monkey - 89 H or~e 88 Chimpanzee 0 Pig - 98 Man 0.2

Al lan to in is near ly three hundred t imes as soluble as uric acid in cold water , and is much more easi ly e l imina ted f rom the an imal body. Consequent ly , gout and al l ied disorders are probably unknown in the an imal k ingdom below man and the anthropoids .

The Condition of the Uric Acid in the Blood.

Normal h u m a n blood contains f rom 1 to 3 mg. uric acid in 100 ccs., or 0.01 to 0.03 gms. per l i tre. The uric acid found in the c i rcula t ion exists chiefly as mono sodium and po t a s s ium ura tes : C s H ~(Na)N~0z and C 5 I:[~(K)N~O.~.

These sa l ts are more soluble t han uric acid i n wa te r at body t e m p e r a t u r e (37 ~ C.), and even more soluble in blood se rum (Taylor).

Solubility. Water at 37" C.

Uric Acid - 0.77 gnu per litre Mono-Potassium

Urate - 2.70 . . . . . . Mono-Sodium Urate 1.50 Mono-Ammonium '" ' ' "

Urate - 0.74 . . . . . .

Blood Serum at 37 Q C.

4.5 gin. per litre

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From this it will be seen that the normal blood is able to hold in solution about a hundred times more uric acid as urate than it normally does. I t is doubtful if this maximum value is ever reached even in advanced conditions of gout.

In early interstitial nephritis values of 3-10 mg. uric acid per 100 ccs. of blood were noted. Uric acid increases in the blood sooner than creatinine or urea, and its determination is of value in early states of nephritis. In severe nephritis the uric acid content of blood may be as high as 25 rag. per 100 ccs.