Amino acid biosynthesis in the developing endosperm tissue of ZEA MAYS L.

A study of the enzymes involved in the synthesis of the nutritionally essential amino acids lysine, threonine and methionine in the developing maize endosperm (Zea mays L. var. DeKalb XL43) was the subject of the major portion of this thesis. The enzymes β-aspartate kinase (EC 2.7.2.4), homoserine dehydrogenase (EC 1.1.1.3) and diaminopimelate decarboxylase (EC 4.1.1.20) have been shown to exhibit changes in activity throughout maize endosperm development. Analysis of the subcellular compartmentation of these enzymes on sucrose gradients suggests that β-aspartate kinase and diaminopimelate decarboxylase activities appear, in part, to be associated with a plastid fraction, as identified by co-fractionation with triosephosphate isomerase, triosephosphate dehydrogenase and ribulose-1,5- bisphosphate carboxylase activities. Homoserine dehydrogenase activity, however, appears predominantly associated with the cytoplasmic fraction, or may be weakly associated with an organellar fraction.

The developmental-dependent changes in total extractable 3-aspartate kinase and homoserine dehydrogenase activities suggest regulation at the gene level or level of enzyme activity. Perturbation of the free pool of the end-product effector amino acid lysine, by its injection at 200 nmoles/endosperm early in development, prevents the increase in total extractable β-aspartate kinase and homoserine dehydrogenase activities without changing normal endosperm development, suggesting that it regulates at the gene level.

The presence of ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39) in the non-photosynthetic maize endosperm was demonstrated. The low specific activity suggests that the enzyme may not function in CO₂ fixation.

The non-photosynthetic maize endosperm, therefore, appears to contain some of the enzyme machinery for nitrogen and carbon metabolism.