Changes in Enzyme Regulation during Growth of Maize

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.A survey of representative kinetic, regulatory, and physical properties did not reveal any significant differences between enzyme released from isolated, undamaged chloroplasts and that obtained from soluble cellular fractions.Examination of enzyme prepared from chloroplasts of different age seedlings indicated that the sensitivity of homoserine dehydrogenase to inhibition by the feedback modifier l-threonine was progressively diminished during growth of the plants. This systematic change in regulatory properties of the enzyme occurred to the same extent for the enzymes obtained from chloroplasts and soluble fractions.