The Photometric Determination of Cystine, Cysteine, Ascorbic Acid, and Related Com- Pounds with Phosphotungstic

Some time ago a brief report (1) was made on the use of the Pulfrich photometer for the determination of sulfhydryl and disulfide compounds by a modified Folin (2) method. Since then the method has been extensively used in this laboratory (3-9). Folin’s method is based upon the observation that cystine plus sulfite gives a blue color with phospho-18-tungstic acid; whereas cystine alone develops no color. The blue color is associated with the formation of phosphotungstic acid in which one of the 18 tungsten atoms is reduced (lo), as indicated in Equation 1.’ Clarke (12) and Lugg (13) have shown that in the action of sulfite upon cystine one-half is converted into S-cysteinesulfonic acid and the other half int,o cysteine (Equation 2). Cysteine is oxidized to cystine according to Equation 3, the phospho-18-tungstic acid (color reagent) acting as a hydrogen acceptor. In the presence of both sulfite and color reagent, the reactions of Equations 2 and 3