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.
| Diet. | Total percentage of purin substances in diet. | Percentage of exogenous 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.
More recently, Mendel and Lyman found that about 60 per cent. of injected hypoxanthine, 50 per cent. of xanthine, 19-30 per cent. of guanosine, and 30-37 per cent. of adenine were excreted in the form of uric acid. While this is true of free purins, on the other hand, when bound purins, i.e., nucleins are administered, only a small proportion thereof appears as uric acid in the urine. But before proceeding to canvass the fate of the missing purin, it will, we think, be helpful if we interpolate here a table (Taylor and Rose), illustrative of the variations in uric acid excretion that attend a purin as opposed to a purin-free diet.
The subject of the experiment was, for three days, fed on a purin-free diet of milk, eggs, starch and sugar. At the end of this period a portion of the total nitrogen (3 grams) was administered in the form of sweetbreads, thymus gland, etc., with a high percentage content (0·482) of purin nitrogen. During the succeeding four days still more (6 grams) of the total nitrogen was replaced by sweetbread nitrogen. Subsequently the person was placed on the original purin-free diet.
| First period. Purin-free diet. | Second period. | Third period. | Fourth period. Purin-free diet. | |
|---|---|---|---|---|
| Total urinary N | 8·9 | 8·7 | 9·1 | 8·8 |
| Urea N and NH₂ | 7·3 | 7·1 | 7·1 | 7·05 |
| Creatine | 0·58 | 0·55 | 0·56 | 0·47 |
| Purin N (total) | 0·11 | 0·17 | 0·26 | 0·10 |
| Uric acid N | 0·09 | 0·14 | 0·24 | 0·07 |
| Remainder N | 0·91 | 0·88 | 0·18 | 1·18 |
From a study of the table it will be noted that, following the introduction of sweetbreads rich in nucleins, the uric acid content of the urine markedly increased, to sink again when a purin-free diet was substituted. But it will be seen also, as MacLeod points out, that “the increase of uric acid accounted for less than half of the purin nitrogen ingested. This appeared as uric acid, the excretion of purin bases being practically unchanged.” In other words, a moiety of the bound purins, i.e., nucleins ingested, appears as uric acid in the urine.