The chemical structure and sources of uric acid having been dealt with, we are now in a position to resume our narrative, and to take up the thread at the point when Horbaczewski revealed the derivation of uric acid from nucleic acid. It now devolves upon us to scrutinise more narrowly the process by which the formation of uric acid from nucleic acid is achieved. Incidentally, it will not be unprofitable to note, if only briefly, the steps by which the necessary expansion of our chemical and physiological knowledge of nucleic acids has been acquired.

As may be imagined, the primary difficulty was to prepare nucleic acids of such purity as admitted of their elementary chemical analysis. The necessary researches were to a large extent confined to two types of nucleic acid, one derived from yeast, and the other from the thymus gland; in other words, to representatives of the only two nucleic acids in nature, one derived from the nuclei of animal cells, the other from the nuclei of vegetable cells.

A feature common to nucleic acids of animal and vegetable origin is that, on hydrolysis with boiling mineral acid, they yield two purin derivatives, guanine and adenine, and a pyrimidin derivative, cytosine. From thence as regards their remaining constituent elements they display distinctions. Thus animal nucleic acids yield thymine, and contain a hexose group in their molecule. On the other hand, vegetable nucleic acids give forth uracil and possess a hexose group.

To sum up, nucleic acid is a chemical complex, made up of phosphoric acid with purin bases, pyrimidin bases and carbohydrate radicles. Moreover, nucleic acids, whatever their source, show a striking similarity in structure, containing always two amino-purins (adenine and guanine), two pyrimidines (either cytosine and uracil, or cytosine and thymine), and a carbohydrate. Now, while purin bases are always present, yet, in respect of their carbohydrate group, nucleic acids display variations; this, according as they are of animal or vegetable origin. If the former, the carbohydrate group is a hexose (contains six carbon atoms) with thymine. If the latter, it contains pentose (five carbon atoms) with uracil.

The constancy in the content of the various nucleic acids is such that Levene and Jacobs have felt justified in putting forward the following provisional formula as to the constitution of a nucleic acid of animal origin.

Distribution of the Enzymes

The enzymes responsible for the disruption of the nucleic acid complex are not to be found in all the body tissues. Moreover, the distribution of the enzymes in the various organs and tissues varies in different species of animals. Of the various organs the liver, spleen, thymus, and pancreas more particularly contain enzymes in abundance. As to their varied location in different animals, it may be noted that the enzyme responsible for the oxidation of xanthine into uric acid, viz., xanthine-oxidase, is found in man only in the liver. In other animals, also, it is of localised distribution, being as a rule only found in the liver or in the liver and kidney. The dog, however, appears to be an exception, xanthine-oxidase being found in a variety of its tissues.

Adenase, the deaminising enzyme, is not to be found in any organs in man. Neither does it exist in any of the tissues of the rat. Consequently, if adenine be injected subcutaneously in rats, it undergoes oxidation, without abstraction of its amino group.

On the other hand, guanase, also a deaminising enzyme, is in man to be detected in the kidney, lung, and liver, but not in the pancreas or spleen. In the pig, however, guanase is lacking, and its absence no doubt explains why deposits of guanine may occur in the muscles constituting the so-called guanine gout met with in swine. It is worthy of note also that in pigs’ urine the content of purin bases exceeds that of uric acid.

To sum up, in man the enzyme, xanthine-oxidase, which forms uric acid from xanthine, is located chiefly or exclusively in the liver. This, of course, represents the final stage of purin metabolism, but the antecedent chemical processes involved in the disruption of nucleic acids are initiated by the action of enzymes in the intestinal juices and wall, and to a consideration seriatim of these changes we now proceed.

Stages in Disruption of Nucleic Acid

As might be expected from the complex structure of the nucleic acid molecule, a number of ferments are concerned in its disruption. The gastric and pancreatic juices contain not a trace of any enzymes. Thus, when nucleo-protein is subjected to the gastric juice a moiety of protein is readily split off and hydrolysed to peptone and other products of proteolysis.

But the nuclein element remains unacted upon until it comes under the action of the pancreatic juice. Hydrolysis then ensues, and the ingested nuclein is broken down into nucleic acid and protein. The nucleic acid remains unaffected by the pancreatic juice, but, coming in contact with the succus entericus, it undergoes partial decomposition through the action of a ferment called nuclease or nucleic-acidase. Under its disruptive effect the nucleic acids or poly-nucleotides are further split up into groups known as nucleotides. The two pyrimidine nucleotides split off and undergo no further change. But, through the action of another ferment, nucleotidase, the purin nucleotides are further decomposed to yield nucleosides (substances of the glucoside class made up of a combination of a purin base with a carbohydrate group of the nucleic acid with which also phosphoric acid is linked).

No further stage in hydrolysis of nucleic acid occurs in the intestine, but the nucleosides are again in turn split up after reaching the tissues, particularly in the spleen, liver, and thymus. This, under the action of specific enzymes, nucleosidases, which succeed in breaking the nucleosides down into the so-called “building stones” of the nucleic acid molecule, phosphoric acid group, carbohydrate group, pyrimidine and purin bases, especially adenine and guanine. The adenine and guanine thus formed are, by the action of the ferments adenase and guanase, converted and, by the removal of their amino group, transformed, adenine into hypoxanthine, and guanine into xanthine, thus:—

By the action of oxidases also present in the tissues hypoxanthine is changed into xanthine and xanthine into uric acid (trioxy-purine), this by a specific ferment xanthine oxidase.

It will be seen that the disintegration of nucleic acid involves many stages, and its complexity is such that we make no apology for drawing upon the masterly monograph of Walter Jones for further elucidation of this intricate question. In relating the history of nucleic acid in the animal body Jones has found it convenient to introduce certain terms wherewith to designate the various elements of the nucleic acid molecule. Thus, the molecule in its entirety is termed a tetra-nucleotide. The cleavage of this complex is initiated by the action of two specific enzymes. Through their agency the tetra-nucleotide is first cloven into two di-nucleotides, which immediately divide up into four mono-nucleotides. These ferments are:—

(1) Phospho-nuclease (which splits off the phosphoric acid radicle, leaving a nucleoside, guanosine or adenosine).

(2) Purin-nuclease (which splits off the purin radicle, viz., separates out both phosphoric acid and carbohydrate groups, leaving free purin bases).

Now, in sequence to either of the foregoing cleavages by the phospho- or purin-nucleases another set of enzymes come into the field. Under their deaminising effect the amino group is abstracted, with the formation of either free oxy-purins or oxy-purins still bound in glucoside-like combination with sugar.

If the oxy-purins are free, the following is the reaction:—

Should, however, the guanine glucoside be present:—

In the latter instance a hydrolysing enzyme, xanthosine-hydrolase, by its action, splits off xanthine. We see, therefore, that by either route the end-product is the same. Following a like series of changes, the adenine radicle is transmuted into hypoxanthine. This either directly by the action of adenase:—

or indirectly through the agency of adenosine-deaminase, the hypoxanthine-glucoside (inosine) is formed, and subsequently the hypoxanthine is split off.

Xanthine and hypoxanthine are, therefore, now to hand, and given the presence of oxygen, their oxidation to uric acid ensues:—

Now, in man and the anthropoid apes, uric acid is the end-product of purin catabolism. In contrast therewith in most mammals only a minimal amount of the exogenous or endogenous purins escapes in the urine as uric acid. Most of it undergoes further oxidation into allantoin,[15] this change taking place in most mammals chiefly in the liver. According to Schittenhelm, if nucleic acid be given to dogs, pigs or rabbits, from 93-95 per cent. thereof appears in the urine as allantoin, and only 3-6 per cent. as uric acid, and 1-2 per cent. as purin bases.

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.”

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.