Analyses of cystine in human hair: its level in women's hair of former times.

An appropriate procedure for analysis of cystine in the hair has been established, and this procedure was applied to the hair of Japanese women. The recovery of authentic cystine added to a hair sample was 85-90% in 6N HCl hydrolysis, and 96% in acid hydrolysis after performic acid oxidation. The 18-hr acid hydrolysis, but not 4-hr one, was sufficient to digest the hair. In colorimetry by using phosphotungstate, Brown's reagent gave a stable color development. The cystine content by amino acid autoanalysis was significantly correlated with that by the phosphotungstate colorimetry by the modified method of Kassel et al. or of Shinohara. The cystine content in women's hair, which was collected from specimens of different individuals cut during the period from 1910s to 1980, indicated a wide variation ranging from 0.654 to 1.607 mmol half-cystine per g of hair after washing with 0.5% sodium laurylsulfate.