Isolation and characterization of two homoserine dehydrogenases from maize suspension cultures

Homoserine dehydrogenase in unpurified extracts of maize (Zea mays L.) cell suspensions is inhibited 73% by the feedback regulator threonine; the remaining 27% of the total activity is not affected even by high concentrations of threonine. The threonine-resistant and threonine-sensitive homoserine dehydrogenase activities were separated by affinity chromatography on Blue Sepharose columns, and the two distinct homoserine dehydrogenases were purified. The threonine-resistant enzyme is an Mr = 70,000 dimer of two Mr = 38,000 subunits and the threonine-sensitive enzyme is an Mr = 190,000 dimer containing two apparently different subunits with molecular weights of 89,000 and 93,000. The threonine-resistant enzyme exhibits normal Michaelis-Menten kinetics and its activity is not affected by any of the amino acid end products of the aspartate pathway. The threonine-sensitive enzyme exhibits positive cooperative kinetics with respect to NADPH and is inhibited by threonine and stimulated by isoleucine. All attempts to affect interconversion of the two purified enzymes have been unsuccessful. Because the purified enzymes correspond to activities present in crude extracts of various maize tissues, it is concluded that the two types of homoserine dehydrogenase are natural in vivo constituents of maize.