Quality protein maize: A biochemical study of enzymes involved in lysine metabolism

Quality protein maize (QPM) varieties have been produced by the introductio n of opaque-2 modifier genes. Two QPM varieties, BR451 and BR473, a wild ty pe and an opaque-2 variety, have been used to study key enzymes controlling lysine metabolism in the endosperm during development. Aspartate kinase an d homoserine dehydrogenase enzymes, which are involved in lysine and threon ine biosynthesis, respectively, exhibited identical activity patterns durin g endosperm development, with a maximum specific activity at 16 days after pollination. The QPM varieties exhibited higher levels of aspartate kinase activity in the endosperm, suggesting an increased rate of lysine biosynthe sis when compared to the opaque-2 and wild-type genotypes. Similar results were observed for the lysine ketoglutarate reductase and saccharopine dehyd rogenase enzymes, which form a single bifunctional polypetide involved in e ndosperm lysine degradation. Both enzyme activities were strongly reduced i n the opaque-2 maize variety when compared to the wild-type maize, whereas the QPM varieties exhibited even lower levels of lysine ketoglutarate reduc tase - saccharopine dehydrogenase activities when compared to the opaque-2 variety. The developmental pattern of enzyme activity showed a different pr ofile when compared to the enzymes involved in lysine biosynthesis, with ac tivity being detected only 12-16 days after pollination (DAP) and maximum a ctivities similar to 24 DAP. These results also suggest that the modifier g enes have intensified the effect of the opaque-2 mutation on lysine ketoglu tarate reductase-saccharopine dehydrogenase. These alterations lead to an i ncrease in soluble lysine in the endosperm of the QPM varieties when compar ed to the opaque-2 and wild type.