The Determination of Cystine in Urine

At various times methods have been devised for the determination of cystine. Aside from the direct isolation of cystine, all the methods proposed for cystine entail the reduction of cystine to cysteine and determination of the latter. Most of the proposed methods are dependent on the reactive -SH group of the cysteine and accordingly give positive results with reducible disulfides other than cystine. The Sullivan method (l-3), on the other hand, requires that the three groups of cysteine be unsubstituted and in the naturally occurring order -SH,-NH2,---COOH. The reaction covering cystine is negative with many other disulfides and even with isocystine, homocystine, dithioethylamine (cystine amine), and glutathione, all of which react like cystine in other cystine methods. A positive reaction is naturally given by complexes which readily yield cystine as, for example, cystine dimethyl ester.’ In quantitative work with proteins, Sullivan (4) and Sullivan and Hess (5,6) showed that most protein hydrolysates require the addition of strong sodium hydroxide before the addition of the final reducing agent. With this modification the Sullivan method has been applied to the quantitative estimation of cystine in the hydrochloric acid hydrolysates of various purified foodstuffs, and of wool, hair, nails, etc. The findings have been checked by the Okuda (7,s) iodometric method (which is satisfactory in most hydrochloric acid hydrolysates). Both methods gave results of the same order of magnitude.