400 THE METABOLISM OF THE PURIN BODIES 



explain why some observers believed the purin excretion to depend 

 on the diet whilst others denied any connection between the two. 



Our next question, what proportion of ingested purin re- 

 appears as such in the urine ? cannot be answered until we have 

 become acquainted with a method for estimating the endogenous 

 moiety (that portion of the purins derived from the tissues), and 

 it is with this problem that we will concern ourselves now. 



THE DETERMINATION AND FUNDAMENTAL CHARACTERISTICS 

 OF THE ENDOGENOUS PURIN EXCRETION 



If the daily purin excretion be determined during starvation, 

 it will be found to fall gradually during the first few days, and 

 then to gain a level at which it remains practically constant. The 

 gradually lessening excretion during the first few days is no doubt 

 due to exogenous purins being drained out of the system, and the 

 constant level afterwards attained must mean that all this store 

 of exogenous purins is exhausted, and that the tissues are them- 

 selves furnishing purins. Is this starvation excretion, then, not 

 an accurate measure of endogenous purin ? Probably not, for 

 starvation is no normal state, there being, during it, a greater 

 break-down of the tissues, amongst others of muscle, than in health, 

 and, consequently, an abnormal liberation of purins. To measure 

 the endogenous moiety, therefore, we must disturb the general 

 metabolism as little as possible ; we must estimate the purin excre- 

 tion whilst the organism is living on a diet containing an adequate 

 amount of nitrogen and a sufficient number of calories to maintain 

 the tissues, but no purin bodies. Is such a diet obtainable ? Con- 

 sidering analytical data alone, the only food- stuffs which contain 

 absolutely no trace of purins are carbohydrates, pure fats and eggs. 

 There are other food-stuffs, however, which contain only minute 

 traces of purin bodies (milk, 0-004--006 per cent. ; potatoes, 

 0-0005--0006 per cent. ; white bread, minutest traces), and since 

 Burian and Schur ( 9 ) have shown that these latter foods do not 

 have any appreciable influence on the urinary purins, we may, for 

 all practical purposes, consider them also as purin-free food- 

 stuffs. 



The following experiment by Burian and Schur ( 9 ) will make 

 clear how the endogenous moiety of purin excretion may le measured. 

 The experiment was divided into four periods. During these 



