A new method for the detection of cystine. I. A new color reaction of cystine and its mechanism (author's transl)

Abstract

1. Cystine solution colors brown on addition of nickel ion and sodium dithionite. This coloration reaction occurs in the vicinity of pH7 and the degree of coloration increases according to the concentration of nickel ion, sodium dithionite, and cystine. For the mechanism of this reaction, it was assumed that cystine is reduced to cysteine by sodium dithionite and cysteine forms a complex with nickel ion. Then the complex produces a sulfide on reaction with sodium dithionite, the sulfide binds to the complex, and the resulting complex is brown. 2. Cystine solution also colors brown on addition of potassium borohydride in place of sodium dithionite as a reducting reagent. As alanine and cysteine are detected together in the solution, it may be that a part of cysteine, reductive product of cystine, is transformed to sulfide and alanine by reductive desulfurization in the presence of a nickel ion. The sulfide forms the same brown complex with cysteine and nickel ion as that described above.