The physiological role of malic enzyme in grape ripening

Abstract

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.

Alteration of the Interconversion of Pyruvate and Malate in the Plastid or Cytosol of Ripening Tomato Fruit Invokes Diverse Consequences on Sugar But Similar Effects on Cellular Organic Acid, Metabolism, and Transitory Starch Accumulation

“…It is suggested that in fruit ripening, NADP-ME may produce the respiratory substrates pyruvate and NADPH from malate and NADP + (Ruffner, 1982;Drincovich et al, 2001) in cytosol and maybe thus partake in malate degradation during ripening (Ruffner et al, 1984;Franke and Adams, 1995). Here, feeding experiments involving the incubation of positionally labeled Glcs showed that the strong decrease in plastidic NADP-ME levels did not lead to substantial changes in overall respiration rates and TCA cycle flux in tomato fruits.…”

Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus

“…It is possible but unlikely that carboxylation is occumng via pyruvate carboxylase, PEP carboxykinase or malic enzyme. In plants, malic enzyme seems only to function unidirectionally toward pyruvate formation, decarboxylating malate (Ruffner et al, 1984). Pyruvate carboxylase conversion of pyruvate to OAA does not seem to occur in plants (Lance and Rustin, 1984).…”

Not All Shrivels Are Created Equal—Morpho-Anatomical and Compositional Characteristics Differ among Different Shrivel Types That Develop during Ripening of Grape (<i>Vitis vinifera</i> L.) Berries

“…In such situations, tonoplast leakage is likely to increase as membranes have been proposed to be the sites of damage [123] and when it does, the leakage becomes prime cause of malate vacuolar decompartmentation resulting changes in malic acid [124]. As a result of malic acid breakdown, pH of the cell sap increases during ripening [125] and becomes abnormally high in sunlit berries [126]. This explains the high pH of sunburned berries, which was same as reported for sunexposed Merlot berries [127].…”