Incorporation of leucine- 14C and lysine- 14C into protein in the developing endosperm of nomal and opaque-2 corn

Abstract

Lysine-U- 14C (2 μCi) or leucine-U- 14C (6.25 μCi) were injected into corn plants just below the ear at various stages of kernel development. The labeling of Osborne protein fractions in the endosperm of the seeds was then determined. Except for albumin, the distribution of label was more or less as might be expected from the size and amino acid composition of the fraction. Evidence was obtained that the albumin fraction undergoes turnover. When leucine- 14C was injected, virtually all the label in endosperm of both normal and opaque-2 genotypes was recovered as leucine. Within normal endosperm much of the injected lysine- 14C was converted to glutamic acid and proline; but little conversion occurred within opaque-2 endosperm. Progressively more lysine was converted in normal endosperm at later times of injection. This reflected an increase in the proportion of label in the zein fraction, since almost all label in this protein was present in glutamic acid and proline. Label in the embryo of normal kernels was mostly in lysine, suggesting that the endosperm is the site of lysine conversion. The difference in lysine metabolism in normal and opaque-2 endosperms correlated with the higher level of lysine in opaque-2.