Exogenous Purins
The foodstuffs that cause an increase in purin excretion are divisible into three groups:—
- (a) Amino-purins.
- (b) Oxy-purins.
- (c) Methyl-purins.
Amino-purins.—In man the taking of food rich in nucleated cells and therefore in nucleo-protein and nucleins, increases the quantity of uric acid in the urine. Thymus gland, pig’s pancreas, and herring roe, containing the characteristic conjugated proteins of nuclei, or Liebig’s meat extract, rich in purin bases, when ingested, lead to a distinct increase in purin excretion.
The researches of Kossel and Horbaczewski showed that such augmentation was mainly due to the production of uric acid from the nuclein substances of the food; in other words, it was due to the katabolism of nuclein, the cleavage products of which comprise adenine derived from thymus, and guanine from the pancreas, both of these bodies being amino-purins. According to Burian and Schur, of the amino-purins ingested, a fourth is excreted as purin.
Oxy-purins.—To this group belong xanthine and hypoxanthine. These substances occur in muscle, and in great abundance in meat extract, and Minkowski noted that the ingestion of xanthine bases markedly augmented the amount of uric acid excreted. In man, given ingestion of hypoxanthine as such, only one half thereof appears as uric acid in the urine. It may here be mentioned that not all the purin bases ingested exist bound up in the nuclein substances. An appreciable amount is present in the tissues in a free state, e.g., hypoxanthine in the muscles; consequently, a moiety of the intake of purin bodies, especially in the animal constituents of the food, is to hand ready formed, and does not require the disruption of nucleic acid for its liberation.
Methyl-purins.—The nuclei of vegetable cells also contain nucleo-protein, and, therefore, can add their quota to the purin intake. The most important are caffeine, theobromine, and theophyllin, the active principles of tea, coffee, and cocoa. It may here be recalled that of the purins administered in food, not all are excreted as uric acid, but some as purins. Now it is doubtful whether the methyl-purins lead to the formation of uric acid in the organism, or whether they are excreted as purin bases in the urine. According to Stewart, a fractional part of the purin bases in the urine is composed of heteroxanthine, 1-methyl-xanthine, and paraxanthine derived from the active principles of coffee, tea, and cocoa when consumed as beverages. As stated by Burian and Schur, one third of the methyl-purins ingested is excreted as purin.
From the foregoing data it will be obvious that the exogenous urinary purins are derived from nuclein and certain free xanthine bases, and that the influence of other nitrogenous foodstuffs in this direction is practically negligible.