Regulation of homoserine dehydrogenase in developing organs of soybean seedlings

Abstract

The regulation of homoserine dehydrogenase (HSD) activity (EC 1.1.1.3) by L-threonine, L-cysteine and K + was examined using extracts of organs of soybean seedlings harvested 3, 6, 11, and 19 days after germination. K + stimulated HSD activity from each source at least 2-fold. HSD activity was completely inhibited by 10 mM L-cysteine while 10 mM D-cysteine was not inhibitory. A progressive decrease in sensitivity of NAD-dependent HSD to inhibition by 10 mM L-threonine occurred in all organs except the leaf during the sampling period. This progressive decrease in sensitivity of the HSD to threonine inhibition was detected only when K + was present in the assay mixtures. Four major molecular forms, including one rapidly migrating form (form I) and three more slowly migrating forms (forms II, III, IV) of HSD, were identified in extracts of soybean organs by polyacrylamide electrophoresis. Chromatographic and electrophoretic data indicate that form I, which was not inhibited by threonine or stimulated by K +, was of lower MW than forms II, III and IV which were of similar MW. These latter 3 forms were inhibited by threonine and stimulated by K +. During soybean seedling development form II increased in amount and forms I and IV decreased in amount. This alteration in the amounts of the forms of HSD occurred during the same period as the decrease in the amount of threonine inhibition. Since K + stimulation of HSD decreased during soybean organ development and K + enhanced threonine inhibition, this might account for the observed decrease in threonine inhibition.