As to what becomes of that portion of the ingested purin that, so to speak, disappears in the body, is largely a matter of speculation. As MacCallum states, “the liberation of guanine and adenine is well in the line of uric acid formation,” but “the fate of the pyrimidin groups, thymine and cytosine, is still uncertain.” According to this observer, Levene has hitherto been unable to find an enzyme which will decompose the nucleoside in which they occur, and that since they cannot form uric acid, they are possibly excreted as urea or in other forms. He adds that only 50 per cent. of the nucleic acid nitrogen can be counted on for the production of uric acid, viz., that in the guanine and adenine groups.
MacLeod, discussing this same point, suggests that some of the unrecovered purin may undergo decomposition in the intestine, but why so much should, after absorption of the blood, disappear is, as he remarks, difficult of explanation; for while uricase, which can decompose uric acid, exists in the tissues of the lower animals, no such ferment is found in man, and uric acid is excreted as such. According to MacLeod, too, “the destroyed purins cannot be shown to influence any of the other well-known nitrogenous metabolites of the urine.”
Lastly, Stewart, discussing the ultimate destiny of the absorbed products of nucleic acid digestion, suggests that, when undergoing further cleavages, “they may be in part utilised for the synthesis of nucleo-proteins, replacing those destroyed in the process of cell metabolism;” or, that it is “possible that they may be wholly disrupted into their components, and these again re-synthesised.”... “Finally, and this fate is probably not long delayed in the case of the surplus of purin compounds contained in ordinary dietaries, both the purins of the food and the purins arising from the waste of the tissues, are for the most part converted into uric acid and excreted in the urine.”
Also, it should be recollected that the purin bases normally found in human fæces are in part of exogenous origin, and are increased in amount after the ingestion of meat extract or thymus.
Endogenous Purins
Even if we entirely eliminate all purin substances, by restricting the diet to purin-free foodstuffs (bread, milk, cheese, eggs and butter), purin in the form of uric acid is still excreted in the urine.
To this moiety the term endogenous purin is applied; for the continued excretion of purin on such a diet is explicable only on the view that they are derived from the waste of the tissues, the daily “wear and tear” of cells. In other words, it is the outcome of the katabolism of the nucleo-protein of the body tissues.
Source of Endogenous Purins
Is the nuclear destruction of localised or generalised distribution?
Mares (and subsequently many other observers), having noted that, following the ingestion of purin-free protein food, a marked increase in endogenous uric acid excretion ensued, suggested that the said augmentation was the outcome of the “wear and tear” entailed upon the nuclear material of the secretory glands of the gastro-intestinal tract, following such intake.