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.|Second period.|Third period.|Fourth period. | Purin-free | | | Purin-free | diet. | | | 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. FATE OF THE UNEXCRETED PURIN