Aspartate stimulation of malate decarboxylation in Zea mays bundle sheath cells: possible role in regulation of C4 photosynthesis.

Aspartate stimulated by as much as three fold the rate of malate decarboxylation by Zea mays bundle sheath cells. Both the basal and aspartate stimulated rates of malate decarboxylation were light-dependent. Stimulation appeared to be due to aspartate as such, rather than depending on aspartate metabolism, and was due partly to a reduction in the malate concentration required for maximum decarboxylation and partly to an increased maximum velocity of decarboxylation. The extractable activities of NADP malic enzyme, glyceraldehyde phosphate dehydrogenase, and 3-phosphoglycerate kinase recoverable from cells were not increased by preincubating cells with aspartate, and aspartate did not affect the activity of these enzymes in cell-free extracts. It is suggested that aspartate may influence the transport of either malate into or pyruvate out of bundle sheath chloroplasts.