DETERMINATION OF THE NUMBER OF THIOETHER-LINKED CYSTEINE RESIDUES IN CYTOCHROMES-C AND PHYCOBILIPROTEINS

Hydrolysis of cytochromes c and phycobiliproteins in 6 n HCl containing 0.21 m dimethylsulfoxide at 110°C for 22 h results in the quantitative conversion of cysteinyl, cystinyl, and thioether-linked cysteinyl residues to cysteic acid. Methionine is converted to homocysteic acid in a 10% yield. The number of thioether-linked cysteinyl residues can be calculated, if the number of cysteinyl and cystinyl residues is determined separately as S-carboxymethylcysteine in a 6 n HCl hydrolyzate of reduced, carboxymethylated protein. Data are presented on the number of thioether-linked cysteinyl residues present in R-, B-, and C-phycoerythrins, C- and R-phycocyanins, allophycocyanins, and the subunits of these proteins. © 1979.