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.”

On the other hand, Wells, discussing the destruction of uric acid, observes that repeated investigations show “that the tissues of man have no power whatever to destroy uric acid in vitro; the earlier reports of positive uricolysis undoubtedly being erroneous.” His final conclusion, after weighing all available evidence, is that it is highly probable that in man “most of the purin absorbed from the food, and practically all the purin from cell metabolism, is converted into uric acid and excreted as such.” MacLeod, however, reflecting on the fact that uric acid is not destroyed when extracts of the organs are incubated at body temperature with uric acid or its precursors, bids us bear in mind that, “although the uric acid is thus shown not to be destroyed in vitro, it may nevertheless be destroyed in the living animal.”

We see, therefore, that the question, Whether uric acid can undergo destruction in the human body? is still a matter of dispute, and must, pending further investigation, remain sub judice. Still, despite the conflict of evidence, clinicians have felt justified in assuming that one of the factors in the genesis of gout may be an entire absence or a diminution in the amount or activity of this uricolytic ferment.

But the awkward fact remains that all researches up to date have failed to establish the presence in the human body of any enzyme which can decompose uric acid. Should, therefore, future investigators place beyond the reach of cavil the claim that no uric-acid-destroying enzyme exists in the body, it would seem that, ipso facto, man, through lack of this capacity for rapid oxidation of uric acid, is, by this same disability, rendered a potential victim of uric acid retention and deposition.

Elucidation of this vexed point seems more probable in view of the striking discovery recently made by R. Benedict, viz., that in one particular breed of dog, the Dalmatian, uricase is wholly absent. In respect of this lack of a uric-acid-destroying ferment, the Dalmatian breed of dog has a purin metabolism apparently identical with that of man.[16] Thus, if fed on a purin-free diet, he passes large quantities of uric acid, and if the latter be injected subcutaneously, elimination in quantity as such ensues; this, in striking contrast to what obtains in all other animals in whom, as before noted, uric acid is mostly oxidised to allantoin before excretion. Now, as MacLeod observes, investigation into the metabolism of nucleic acid has, in man, been hampered greatly, in that the absence of uricase from his tissues, prior to Benedict’s discovery, rendered experimental researches on the lower animals valueless. But, in light of the above revelation later by R. Benedict, it may reasonably be hoped that in the near future our knowledge as to the location and nature of the intermediary chemical processes occurring in the metabolism of nucleic acids may be materially clarified.

CHAPTER IX
URIC ACID IN RELATION TO GOUT

It will be recalled that at the close of our chapter on Pathogenesis we referred to the growing scepticism of Garrod’s views as to the pathogeny of gout. Still, if we except Edward Liveing’s pertinent observation that uricæmia was not peculiar to gout, naught, save alternative hypotheses, unsupported by pathological data, was advanced. Consequently, Garrod’s facts never being seriously called in question, his position remained unassailable, until, in the year 1898, his original observations as to the lowered alkalinity of the blood in acute gout, and the increased uric acid content thereof during the same, were definitely contradicted by Magnus Levy.

Working with more modern and more reliable methods of technique, this observer, in a series of seventeen cases of acute gout, found no evidence of any lessening in alkalinity of the blood or of any augmentation of its uric acid content as compared with the inter-paroxysmal period.

Again, as to Garrod’s claim that there was a diminished excretion of uric acid during the attack, this also, while supported by Minkowski, was called in question by Pfeiffer, Levy, and Badt, who found the reverse to be the case, i.e., a notable increase in the excretion of uric acid during the paroxysm.

These results were again in 1900 confirmed by Chalmers Watson. An exhaustive study of a series of cases of acute gouty polyarthritis convinced him that:—

(1) The alkalinity of the blood is not diminished during the attack.

(2) The excretion of uric acid is not lessened during the paroxysm, but the reverse; there is, therefore, no ground for the supposition that there is a temporary diminution in the capacity of the kidneys to excrete uric acid.

(3) The amount of the uric acid in the blood is not greater during the attack than in the intervening period, and if these points be accepted, we must start de novo in search of the cause of the acute paroxysm.

The iconoclastic revelations of the foregoing researches may well form a preface to our discussion of gout from the triple aspect of:—