Gout, with a section on ocular disease in the gouty

by Llewellyn J. Llewellyn

In man, as in most mammals, uric acid is formed chiefly in the liver from purins, and in the preceding table Amberg and Walter Jones schematically represent the various steps by which disruption of the nucleic acid molecule is attained, and uric acid formed.

Destruction of Uric Acid

Uricolysis, or the destruction of uric acid, is, in most mammals, achieved through the agency of the oxidising enzyme uricase, which oxidises uric acid to allantoin. Consequently, in their instance, purin bases, ingested as such or set free in the tissues, appear in the urine, not as uric acid, but in the form of allantoin. On the other hand, both in man and in the anthropoid apes, this particular enzyme uricase is absent. In accordance therewith, only a trace of allantoin is to be found in the urine of man and the higher apes, while in the lower animals, e.g., dogs, pigs, and rabbits, a large proportion of the purin excretion assumes this form.

Now, the absence of uricase, in man, is held to be proved by the fact established by Wiechowski and others, viz., that uric acid, if injected subcutaneously, may be almost wholly recovered in the urine, and moreover, unchanged. On the other hand, the total excretion of uric acid and the other purin bodies by no means tallies exactly with the amount of the uric acid ingested as purin bases in the food and that produced from the tissues; in other words, it has been found that, when given by the mouth, nucleic acid or purins are by no means quantitatively excreted in the urine, even though not only uric acid, but also allantoin and the purin bases, are included within the estimate. According to most experiments, a considerable proportion of the purin-nitrogen intake, about 50 per cent., is excreted as urea.

The question then arises as to what becomes of that moiety of the food purins which fails to appear in the urine as uric acid. Now the amount of allantoin that appears in the urine is negligible. Moreover, Ackroyd, having shown that the organism cannot destroy allantoin, it is possible that the minimal amounts excreted thereof in the urine are all derived from the food.

Accordingly, if, as experimental feeding with purins or nucleic acid appears to indicate, purins are destroyed in the body they “pass through some other route than allantoin, and possibly, that part of the purin which is destroyed does not pass through the stage of uric acid.” Such is Wells’ opinion, and he reminds us that in vitro the destruction of uric acid can be attained by other routes than through allantoin. Thus, it can be broken down into glycocoll, ammonia, and CO₂, or by another method of disintegration it furnishes first alloxan (C₄H₂N₂O₄), then parabanic acid (C₃H₂N₂O₃), which in turn yields oxalic acid and urea.

But while it is probable that there is more than one way in which uric acid can be decomposed in the body, nevertheless there is, according to Wells, no evidence that either of the alternative routes above suggested is ever affected in the animal body. In this impasse Siven suggests the further possibility, viz., that the moiety of the food-purins which fail of recovery from the urine undergo partial destruction in the intestine by bacteria.

Stewart, however, in his “Physiology,” discussing uricolysis, maintains that a considerable destruction of uric acid and other purin bodies goes on in the body and mainly in the liver. He reminds us that when uric acid is heated in a sealed tube with strong hydrochloric acid, it breaks down into glycin, carbon-dioxide and ammonia, and he maintains that “there are grounds for believing that a similar decomposition takes place in the body, and that the products are then transformed into urea in the liver”; this, through the agency of a special ferment called the uricolytic enzyme.

Also, Flack and Hill, discussing the metabolism of nuclein, hold that some of the uric acid thus formed may be transmuted into urea by an uricolytic ferment present in the liver, muscles, and kidneys. This same agent they consider “probably destroys a considerable amount of the uric acid formed in the body. Indeed, uric acid, even when given in the food, owing to the presence of this enzyme, causes no increase in the uric acid output of the body.”