The physiological role of malic enzyme in grape ripening

The high specificity of malic enzyme (ME; EC 1.1.1.40) from grape berries (Vitis vinifera L.) for the naturally occurring L-enantiomer of malic acid, its very selective C4-decarboxylation, and certain allosteric properties, reported previously, favour the conjecture of a regulatory function of ME in fruit malic acid degradation. On the other hand, high ME activity was detected even during the acid-accumulating phase of berry development. Also, the in vitro reversibility of the reaction supports the possibility of malate formation under conditions facilitating carboxylation of pyruvate, notably high CO2/HCO 3 (-) and NADPH/NADP ratios. However, a very limited incorporation of (14)C into malate and the uniform labeling pattern of the dicarboxylic acid after administration of [U-(14)C] alanine to grape berries before and after the onset of ripening, indicate that the 'reverse" reaction does not contribute essentially to grape malate synthesis. A regulatory mechanism mediating malic acid remetabolization on the basis of cosubstrate availability, comparable to the control of the hexose monophosphate shunt, is discussed.