Aspartate kinase regulation in maize: Evidence for co-purification of threonine-sensitive aspartate kinase and homoserine dehydrogenase.

The threonine-sensitive and resistant forms of homoserine dehydrogenase activities were assayed in all fractions obtained during the purification of three aspartate kinase isoenzymes (threonine, lysine and lysine plus S-adenosylmethionine-sensitive) from maize cultures. The homoserine dehydrogenase isoenzyme resistant to inhibition by threonine, has a molecular weight of 70 000 and could be easily separated from the three aspartate kinase isoenzymes by ion-exchange chromatography and gel filtration; similarly the two lysine-sensitive forms of aspartate kinase could be separated from both forms of homoserine dehydrogenase. The homoserine dehydrogenase sensitive to threonine inhibition has a molecular weight of 180 000 and co-purified with the threonine-sensitive aspartate kinase isoenzyme. The hypothesis of the presence of a bifunctional protein containing activities of aspartate kinase-homoserine dehydrogenase is discussed