Part I 1. It was proved that to oxidize one molecule of cysteine to cysteic acid 6 atoms of bromine are required. 2. A method for determining cysteine by titration with a bromate solution in the presence of acid and bromide was proposed. 3. A single determination of cysteine by this bromine method requires only a few minutes. The concentrations of cysteine and acid in sample-solutions have practically no influence upon this method. Part II A method, called the iodine-method, for the determination of cysteine is described. The principle of the method is to titrate cysteine in a hydrochloric acid solution, in the presence of iodide, with a standard iodate solution. The procedure of this method is not so simple as that of the bromine method, but the method is applicable in the presence of all the cleavage products of proteins. Consequently it is available for the qualitative and quantitative determination of cysteine in amino acids mixture. Part III (1)When treated with nascent hydrogen, cystine and cysteine alone among amino acids react to iodine very actively in acid solution and at room temperature. (2)A method for the determination of cystine is described. The principle is to titrate cysteine produced by the reduction of cystine, with a standard iodate solution in presence of iodide and acid. (3)The quantitative method must be made under strictly comparable conditions of the concentrations of acid and of the temperature of the solution etc. Part IV It was verified that there is no appreciable decomposition of cystine during ordinary acid hydrolysis of proteins, using the bromine-method and iodine method and sometimes FOLIN and LOONEY's colorimetric method for the determination of cystine. Part V (1)The cystine content in some proteins was determined by FOLlX and LOONEY's colorimetric method and by OKUDA's iodine method. (2)The colorimetric method gave too high results. This was ascribed to the fact that there are some substances which are present in the hydrolysate of proteins and give a blue color as cystine does. (3)Cysteine was absent in all of the completely hydrolyzed solutions of the proteins examined. Part VI (1)A method for the determination of cystine in urine was described. It is an application of the iodine-method for cystine, and we designated it, for the sake of convenience "micro iodine method" or "general method." (2)This method, with some modification, may be useful for the determination of cystine in blood and in small quantities of proteins. (3)Special methods for removing thiocyanide were also described, but they are useful only in very rare cases. (4)LOONEY's method for the determination of cystine in urine was studied, but we failed to prove it satisfactory. In conclusion I wish to express my thanks to Messrs. J. MOTOMURA and Y. NISHIJIMA for their help in analytical work.
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