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.