Tissue Distribution of β-Cyanoalanine Synthase in Leaves

beta-Cyanoalanine synthase, which catalyzes the reaction between cysteine and HCN to form beta-cyanoalanine and H(2)S, was assayed in leaf tissues from cyanogenic (Sorghum bicolor x Sorghum sudanense [sorghum]) and noncyanogenic (Pisum sativum [pea], Zea mays [maize], and Allium porrum [leek]) plants. The activity in whole leaf extracts ranged from 33 nanomoles per gram fresh weight per minute in leeks, to 1940 nanomoles per gram fresh weight per minute in sorghum. The specific activities of beta-cyanoalanine synthase in epidermal protoplasts from maize and sorghum and in epidermal tissues from peas were in each case greater than the corresponding values for mesophyll protoplasts or tissues, or for strands of bundle sheath cells.THE TISSUE DISTRIBUTIONS FOR THIS ENZYME WERE DETERMINED FOR PEA, LEEK, AND SORGHUM: the mesophyll protoplasts and tissues in these three plants contained 65% to 78% of the enzyme, while epidermal protoplasts and tissues contained 10% to 35% of the total leaf activity. In sorghum, the bundle sheath strands contained 13% of the leaf activity. The presence of beta-cyanoalanine synthase in all tissues and species studied suggests a fundamental role for this enzyme in plant metabolism.