In vitro metabolism of l-aspartate by maize kernels

Abstract

The metabolic fate of l-[ 14C]aspartate in isolated developing maize ( Zea mays) kernels was studied using pulse-chase methods. At the beginning of the chase period, all radioactivity was recovered in the basic (amino acid-containing) fraction of pedicel extracts. Radioactivity recovered in the acid (organic acid-containing) fraction increased steadily from 0 to 24% over the following two hours. Only minor amounts of radioactivity were recovered in the neutral (sugar-containing) fraction over this same period. Alanine was transiently labelled early in the chase period, while glutamine accounted for 15% of the radioactivity recovered in the basic fraction after two hours. The evolution of 14CO 2 from [ 14C]aspartate and the accumulation of aspartate-derived 14C in glutamine, glutamate and alanine were inhibited by the aminotransferase inhibitor aminooxyacetic acid. Seventy-one to 76% of the 14C supplied to the pedicel as [ 14C]aspartate appeared in the endosperm basic fraction, mainly as glutamine plus glutamate. Direct uptake of glutamine by the endosperm of kernels with pedicels removed was inhibited by 2,4-dinitrophenol and by p-chloromercuribenzenesulphonic acid, suggesting active transport of glutamine by the basal endosperm transfer cells. The role of pedicel amino acid metabolism in maize kernel nitrogen assimilation is discussed.