Gluconeogenesis from Amino Acids in Germinating Castor Bean Endosperm and its Role in Transport to the Embryo

Abstract

During germination of the castor bean all of the contents of the endosperm are ultimately transported to the embryo through the cotyledon or respired. A net loss of nitrogen from the endosperm begins about the 4th day, i.e. at the time when embryo growth and fat breakdown are also beginning. Amino acid analysis of the exudate from the cotyledons, still enclosed in the endosperm, showed that the amounts of aspartate, glutamate, glycine, and alanine were very low and that glutamine made up 40% of the amino acids in the exudate. Amino acids labeled with 14C were applied to intact excised endosperms to follow utilization. Aspartate, glutamate, alanine, glycine, serine, and leucine were converted to sugar to varying extents. Proline, arginine, valine, and phenylalanine were not appreciably converted to sugars. Proline and glutamate were converted to glutamine. When 14C-glutamate, aspartate, and alanine were added to the outer endosperm of intact seedlings, only sugars and glutamine contained appreciable label in the exudate. When 14C-valine was added, it was virtually the only labeled compound in the exudate. The results show that amino acids which on deamination can give rise to intermediates in the pathway of conversion of fat to sucrose are largely converted to sucrose and the nitrogen transported as glutamine. Other amino acids released from the endosperm protein are transported intact into the seedling axis. Some carbon from the gluconegenic amino acids is also transported as glutamine.