Gout, with a section on ocular disease in the gouty

by Llewellyn J. Llewellyn

Complexity of the Problem

How complex, indeed, the task of the bio-chemist may be gathered from some reflections of Walker Hall. He reminds us that the oxidation and deaminisation of the nuclein derivatives, nucleins, nucleotides and nucleosides, is never complete. For purin bases and pyrimidin bases run side by side in the blood-stream together with uric acid. Also, that the unstable but soluble biurate is constantly changing into a less soluble type, viz., from one isomer to another. Moreover, since the red blood corpuscles abound in potassium, urates of potassium must also occur, and to these may be added, too, ammonium and calcium compounds in small quantities.

But more striking is his inference that the occurrence of isomeric forms of uric acid suggests that isomers of purins and pyrimidins also may occur. For the purin ring or pyrimidin nucleus, with their numerous receptors for the linking up of other substances, offer wide potentialities in the direction of isomerism.[10] Some of these, he hazards, may be born of one type of cell nucleus, some of another, while it is not inherently improbable that, “In response to abnormal stimuli or excessive demand, other isomers may be formed.”

Now, though uric acid and the urates can be extracted from the blood, it does not, as he remarks, necessarily follow that they circulate as such in vivo; for, despite modern achievements, “the best of the existing methods for the determination of uric acid in the blood are nearly barbarous in their crudity and intensity.” The various procedures available for such estimates fall short of distinction between the several tautomeric forms of uric acid, much less do they furnish any information as to the associations or combinations of purins or pyrimidins with other substances.

For himself, recognising the generally more complex nature of biological processes, he considers that “the circulation of the purins as sodium mono-urate and its simple extraction by kidney cells, seems almost too simple to be true.”

As to the solubilities of uric acid and urates in gouty blood he points out that the suspension capability of the blood-stream for uric acid much transcends the highest amount of uric acid as yet found in the gouty subject. Accordingly, to him, therefore, it seems that “neither chemical nor physico-chemical processes suffice to explain the problem. There must be something more, something vital, biological.”

Having ascertained as far as possible the measure of our knowledge in regard to the foregoing points, we shall, in the succeeding chapter, proceed to discuss the sources of uric acid, whether of intrinsic or extrinsic origin.

CHAPTER VII
SOURCES OF URIC ACID

Uric acid, like the “purin bodies” (xanthine, hypoxanthine, guanine, and adenine), is derived from nucleins, i.e., from the breaking down of tissues rich in cells. The end-product of purin or nuclein katabolism uric acid represents but a further stage in the oxidation of the purin bodies. To the serial enzymatic transformations that mark its derivation from nucleic acid we shall allude later, but at this juncture we are concerned not with the mode of formation of uric acid, but with the sources thereof.

In this sphere we are greatly indebted to the pioneer researches of Burian and Schur. These observers noted that on a diet rich in nucleins (sweetbreads, liver, kidneys) the total daily excretion of uric acid was considerably higher than on a milk or purin-free diet. This difference in response to varying dietaries, in respect of the excretion of uric acid, led Burian and Schur to the conclusion that the purins excreted must be partly of exogenous and partly of endogenous origin; in other words, the exogenous purins are derived from the nucleins ingested in the food, whereas the endogenous are the outcome of the breaking down of the cellular tissue of the organism itself.

Here it may be noted that all the ingested purins are not excreted in the urine as uric acid, for some pass away as purins. Moreover, the amount excreted will vary with the kind of purin ingested, and also with the species of the animal that consumes it. Thus, in man “only one half of the hypoxanthine administered as such appears as uric acid in the urine, and but one fourth of the purin in nuclein when that is fed. In the dog, compared with man, about ten times as much purin disappears in its passage through the organism; in the rabbit, about three times” (Flack and Hill).[11]

In amount about 0·4-0·7 gramme of uric acid is excreted in human urine daily, and the purin bodies, hypoxanthine, xanthine, and adenine, in small quantities.

Beyond exogenous and endogenous purins there is yet one other possible source of uric acid, viz., its synthetic formation within the organism. This supposition took origin in Horbaczewski’s discovery that in the laboratory he was able to produce uric acid by the interaction of urea and glycocine, a finding afterwards confirmed by Latham. The theory was then advanced that a similar synthesis might be effected by the kidneys; but it was found that glycocine and urea, even when given in excess to mammals, caused no change in the uric acid excretion.

So much by way of preface to our detailed discussion seriatim of the various sources of uric acid, and to which we now pass on.