Colorimetric determination of cystine (disulfide bond) in hair using dithiothreitol

We developed a simple method for the quantification of cystine (disulfide bond) content in hair by measuring the amount of oxidized dithiothreitol (lambda max 283 nm) derived from dithiothreitol (DTT) with cystine. Because the cystine content in hair is almost fixed for each animal species, it can be used as a reliable indicator of hair weight. The absorbance (A280, y) of the supernatant of the reaction mixture correlated well with hair weight (mg, x) (y = 0.10x + 0.06, r = 1.00, n = 10). Within-run and between-day reproducibilities (C.Vs., %) for the assay were 3.1 and 2.8 (n = 5 each), respectively. Hair cystine content (nmol/mg hair, mean +/- S.D.) in normal volunteers was 903 +/- 50.6 (n = 10) by the present method and 755 +/- 24.9 (n = 10) when an amino acid analyzer was used. After assay by our method, the hair sample can be washed, then used repeatedly to assay other analytes. The present method should be useful for assays of analytes present only in small amounts (2-20 mg), without the need for precise weighing of the hair samples.