The iodimetric titration of cysteine and allied substances.

THE increasing interest of biochemists in sulphydryl compounds has led to the necessity of finding some method for the accurate determination of these substances. That the problem is complex may be judged from the number of papers proposing new methods or modifications of old ones. The original procedure of Rosenheim and Davidsohn [1904] for the volumetric estimation of thiol acids by the use of iodine in the presence of sodium bicarbonate was found by Klason and Carlson [1906] to give too high values. However, in neutral or acid solutions the reaction between sulphydryl compounds and iodine was shown to be quantitative. Nearly all subsequent iodimetric procedures are based on this observation. The sulphydryl compounds of especial biochemical interest, cysteine, glutathione and ergothioneine, appear to be peculiarly sensitive to the acidity and temperature of the reaction mixture in which they are titrated. It has been known in a general way that cysteine is oxidised to cystine by iodine and to cysteic acid by bromine. The bromate and iodate methods for cysteine described by Okuda [1925] are based on these observations. Okuda showed that on adding standard bromate to an acid solution of cysteine containing excess of bromide, quantitative oxidation to the sulphonic acid occurred. The concentration of the bromide had no effect as long as it was in excess; the concentration of acid affected only the rate, but not the nature of the reaction, and temperature had almost no effect. However, in spite of the simple course of the reaction, the procedure is not suitable for use on protein hydrolysates, since tyrosine, histidine and tryptophan all react with bromine. Since cystine alone of the amino-acids reacts rapidly with iodine in acid solutions, Okuda studied a similar iodate method. The reaction is somewhat complex, a mixture of disulphide and sulphonic acid (and possibly other products) being formed. The temperature and concentration of acid in the mixture have a pronounced effect on the course of the reaction, which is influenced to a less degree by the concentration of iodide [Okuda, 1924] and other substances present. That this is also true of glutathione has been shown by King and Lucas [1931], who found the amount of iodine required by glutathione to be influenced by both temperature and acidity, and by Perlzweig and Delrue [1927], Gavrilescu [1931]