Changes in Enzyme Regulation during Growth of Maize: III. Intracellular Localization of Homoserine Dehydrogenase in Chloroplasts

Extracts of leaf tissue of Zea mays L. seedlings were fractionated on nonlinear sucrose gradients to separate subcellular organelles. Homoserine dehydrogenase (EC 1.1.1.3) was identified in those fractions containing intact chloroplasts, as judged by the presence of chlorophyll and triosephosphate isomerase activity. Neither enzyme activity was detected in fractions containing ruptured chloroplasts, mitochondria, or microbodies. Quantitative measurements of enzyme activity and chlorophyll, and electron microscopic analysis of plastid preparations support the conclusion that maize mesophyll chloroplasts contain a significant fraction of the total cellular content of homoserine dehydrogenase.