_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. EXOGENOUS URIC ACID EXCRETION As to the amount of exogenous purins that, when administered orally, can be recovered from the urine, it would appear that a certain rough parallelism obtains between the purin content of the food and that of the urine. The amount of the exogenous urinary purin differs for different forms of food, a variation well illustrated by the following table, giving the results of Burian and Schur’s researches. ---------------+-------------------+---------------- | Total percentage | Percentage of Diet. | of purin | exogenous |substances in diet.| urinary purin. ---------------+-------------------+---------------- Beef | 0·06 | 0·030 Coffee | 0·20 | 0·075 Calf’s liver | 0·12 | 0·060 Calf’s spleen | 0·16 | 0·080 Calf’s thymus | 0·40 | 0·100 ---------------+-------------------+---------------- Walker Hall, experimenting with various purin-containing foods, found that (1) with chicken 54·4 per cent., (2) with plaice 58·7 per cent., (3) with beef 47·4 per cent., (4) with haricot beans 55 per cent. of the food purin appears in the urine as exogenous purin. These findings of Walker Hall’s, like Burian’s and Schur’s, reveal that, roughly speaking, 50 per cent. of the purin content in food is excreted in the urine.[12] These figures must be taken as a broad average relating only to healthy individuals upon diets capable of perfect assimilation.