Differential Regulation of Maize Homoserine Dehydrogenase under Physiological Conditions

Homoserine dehydrogenase is associated with the multibranched pathway of amino acid biosynthesis originating with aspartic acid. Like most of the related pathway enzymes, this enzyme is localized in chloroplasts. The activity and regulatory properties of the threonine-sensitive isozyme of homoserine dehydrogenase isolated from Zea mays var earliking were examined under variable conditions that could exist within chloroplasts. Catalytic activity is not significantly altered within the range of pHs that occur within these organelles, but inhibition of the enzyme by the pathway product, l-threonine, is markedly diminished at the alkaline pHs characteristic of illuminated chloroplasts. Inhibition by threonine is also subject to modulation by physiological levels of NADPH. Under conditions considered to represent the environment within unilluminated chloroplasts, the enzyme is severely inhibited by micromolar concentrations of threonine, but significant enzyme activity is retained under conditions that are likely to occur during illumination, even in the presence of millimolar levels of threonine. These results indicate that homoserine dehydrogenase may be subject to environmentally mediated regulation in vivo. Other observations support this concept and suggest that the intrinsic catalytic and regulatory properties of key enzymes could facilitate a direct link between light-dependent carbon and nitrogen assimilation and amino acid biosynthesis in chloroplasts of higher plants.