The control of the production of lactate and ethanol by higher plants

Factors controlling the production of ethanol and lactate have been examined using cell free extracts prepared from pea seeds (Pisum sativum var Alaska) and parsnip roots (Pastinaca sativa). The result suggest that under aerobic conditions pyruvate decarboxylase is inactive. With the onset of anaerobiosis glycolysis leads to an accumulation of lactate with a corresponding fall in pH. The fall in pH activates pyruvate decarboxylase and initiates competition between lactate dehydrogenase and pyruvate decarboxylase for pyruvate. The effect of pyruvate concentration on the partitioning has been analysed in terms of a modified Wegscheider rule and shows that the ratio lactate dehydrogenase activity/pyruvate decarboxylase activity bears an inverse relationship to the pyruvate concentration. The decrease in ratio which occurs when the pyruvate concentration rises is enhanced by the co-operativity which is exhibited by pyruvate decarboxylase. The pH optimum of lactate dehydrogenase is alkaline whilst the pH optimum of pyruvic decarboxylase is acid, thus the two enzymes function as a pH-stat. The possibility of excessive production of lactic acid is further controlled by the response of lactate dehydrogenase to ATP; the enzyme is inhibited by ATP and the inhibition increases as the pH decreases. It is suggested that this mechanism functions to protect the plant from excess production of acid.