The C-S lyases of higher plants: homogeneous beta-cystathionase of spinach leaves.

S-Substituted cysteines and their derivatives are prominent secondary amino acids in a number of plant families. The substituents are often specific and unique to each family. Cystathionine, however, is an ubiquitous S-substituted cysteine found in all autotrophic plants since it is an intermediate in the biosynthesis of methionine. beta-Cystathionase will produce homocysteine and pyruvate from cystathionine by a beta-elimination reaction. The present report describes the purification of this enzyme to homogeneity from spinach leaves and some of its properties. The enzyme has a molecular weight of 210,000 and consists of four identical subunits of Mr 53,000. It has a pH optimum for activity of 8.6-8.7 and utilizes pyridoxal-5'-phosphate as a cofactor. Its specificity is limited to L-cystathionine, L-djenkolate, and L-cystine as substrates with a relative activity of 100:126:17, respectively. It is not a glycoprotein unlike a number of previously described plant C-S lyases.