STUDIES ON CATALYTIC AND REGULATORY PROPERTIES OF HOMOSERINE DEHYDROGENASE OF ZEA MAYS ROOTS

1. 1. Homoserine dehydrogenase has been isolated and partially purified from the apical centimeter of Zea mays roots. It appears to be a single enzyme possessing dual coenzyme specificity. The apparent Km for NADP+ was significanty lower than that for NAD+, although the υmax with NAD+ was twice that observed with NADP+. Two apparent Km values for homoserine were calculated from double-reciporcal plots of initial velocity and homoserine concentration in the presence of excess NADP+. Enzyme-catalyzed reduction of aspartic semialdehyde could be inhibited by either of the reduced coenzymes. When noninhibitory levels of coenzymes were employed, reaction rates were always greater with NADPH. 2. 2. The enzyme was inhibited by the pathway product threonine, and by three other natural amino acids: serine, aspartic acid and cysteine. The NADP+- and NAD+-linked enzyme activities proved to be differentially sensitive to these amino acids. The results of kinetic and desensitization experiments suggest that the mechanism of action or the enzyme binding site of each inhibitory amino acid may be different. 3. 3. Properties of the enzyme are discussed in relation to properties of microbial homoserine dehydrogenases and physiological conditions in developing roots. © 1969.