Birds eliminate most of their nitrogen in the form of uric acid, and, undoubtedly, in their instance synthetic formation of uric acid in the liver takes place on a large scale. Thus, when blood containing ammonium lactate is perfused through the liver of the goose, an increase in the uric acid content of the blood occurs. Also the ingestion of lactic, pyruvic and other organic acids leads to augmented output of uric acid; in short, it is generally agreed that in birds synthesis is the chief mode of formation of uric acid, homologous with the formation of urea in the liver of mammals.
If this be true of birds, on the other hand, splitting and oxidation of nucleins is in mammals the most important source of uric acid, but there is evidence that it cannot all be accounted for in this way. As before remarked, the old belief that purin excretion remains almost constant on a purin-free diet, despite great variations in the amount of the ingests, is not strictly true. Thus, using swifter and more reliable methods for the estimation of nitrogenous metabolites, Folin noted, on an absolutely purin-free diet, that an increase in purin excretion ensued, given marked variations in the intake of food. Again, the Dalmatian dog, as we have seen, excretes uric acid in his urine. S. R. Benedict was therefore able to demonstrate that a very distinct increase in his uric acid output ensued in sequence to increase in the amount of his non-purin food; moreover, that even when such non-purin foods were continued for a year, “the total amount of uric acid excreted was at least ten times greater than could have come from the traces unavoidably included in the food” (MacLeod).
Also Ascoli and Izar, experimenting with dog livers, noted on incubation thereof and passage through the same of oxygen that the uric acid disappeared; but on the substitution of carbon dioxide an accumulation thereof ensued. Wells, however, was unable to confirm this re-synthesis of uric acid by dog livers, and Spiers also failed to corroborate their findings.
On the other hand, there is evidence pointing to the fact that a certain small percentage of synthetic formation does take place in the organism. Thus certain chemical substances, and these not purin, do cause an appreciable though slight increase in the purin excretion of mammals, and a very marked augmentation of the same in birds, viz., lactic, tartronic and B-oxybutyric acids.
But, as MacLeod, discussing these experimental and clinical findings, observes, there are to hand even more direct proofs that purin synthesis occurs in mammals. Thus, as McCallum has pointed out, we cannot escape the admission that young mammals are able to synthetise the purins essential for their growth, and this from food containing no purin, e.g., milk. Again, prior to incubation, a hen’s egg contains practically no nucleic acid, whereas after development its content in the same increases by great strides. The eggs of insects, too, with the progress of development, amass purin very rapidly.
Again, Miescher noted long since that salmon, on leaving the sea to ascend rivers for the object of spawning, have at that time well-developed muscles; but on arriving at the upper reaches, marked muscular wasting ensues, while the testes undergo enormous enlargement. MacLeod, reflecting on these observations, argues that, “as the fish takes no food during the migration, there must be conversion of the protein of the muscles into the cellular tissue of the sexual glands, and nucleic acid must be produced.” MacLeod’s conclusion is that “Purin synthesis undoubtedly occurs in the mammalian body, but it is difficult to recognise in metabolism investigation, because it is a slow continuous process ... whether or not changes in the activity of purin synthesis occur in conditions of disease, is a question which awaits investigation.” Lastly, the opinion of most authorities is that, while they concede the possibility of synthetic formation, the amount of uric acid produced in this manner is negligible, and that by far the most important mode of formation in mammals is by the splitting and oxidation of nucleins; in other words, that uric acid in the main is derived from the amino-purins by deaminisation and subsequent oxidation, and from the oxy-purins directly by oxidation.
CHAPTER VIII
FORMATION AND DESTRUCTION OF URIC ACID
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.