Purification and characterization of beta-cyanoalanine synthase and cysteine synthases from potato tubers: Are beta-cyanoalanine synthase and mitochondrial cysteine synthase same enzyme?

beta-Cyanoalanine synthase (CAS; EC 4.4.1.9) and two kinds of cysteine synt hases (CS; EC 4.2.99.8) have been purified from the particulate fraction of potato tubers. By DEAE Sephacel and Resource PHE chromatography, CAS activ ity was separated from two CS activities, designated as CS-1 and CS-2. The molecular masses of GAS, CS-1 and CS-2 were estimated to be 37, 39 and 34 k Da, respectively, by SDS-PAGE analysis. The purified CAS had CS activity, a nd both CS-1 and CS-2 had CAS activity. However, CAS and CSs had significan t differences in kinetic characters. The antibody raised against purified C AS discriminated CAS from CSs, whereas the antibody raised against purified CS-2 recognized CS-1 and CS-2 but not GAS. The molecular mass and the part ial amino acid sequence of CS-2 were similar to those of the cytosolic CS o f potato, whereas the molecular mass of CS-1 was similar to that of the pla stidic CS. The partial amino acid sequence of CAS was similar to those of C S isozymes, especially the mitochondrial CS isolated from spinach.