ORGANIC ACID METABOLISM IN PLANTS

Physiological studies on acid metabolism in green plants IX. The distribution of 14 C in malate of darkened Kalanchoё leaf fragments after infiltration with labelled pyruvate

When carboxyl-, carbonyl- or methyl-labelled pyruvates are supplied to darkened fragments of Kalanchoё crenata Haw. leaves, malate usually becomes the most heavily labelled metabolite. To help in elucidating the processes involved in the labelling, the distribution of 14 C within the malate molecule has been determined. Investigations were made for leaves in which dark acidification led to a net synthesis of malate and for leaves in which dark deacidification led to a net consumption of malate. Some experiments were carried out in the presence of aerobic atmospheres containing 20 % CO 2 . The results are consistent with the oxidation of the pyruvate by pyruvic oxidase and enzymes associated with the conventional TCA cycle. In addition to such slight net synthesis of malate as may have occurred by this sequence there appeared to be two other distinct synthetic processes involving β -carboxylations. In one of these, 14 CO 2 that arose from pyruvate or a labelled product thereof was fixed into malate by the enzymic system responsible for the massive CO 2 fixation and resulting acidification in darkened Kalanchoё leaves. In this reaction, apparently, the carbon skeleton of pyruvate was not directly incorporated into the malate. In the other carboxylative synthesis the carbon skeleton of labelled pyruvate appeared to be incorporated as a whole into malic acid. Only a very small proportion of the total fixation of 14 CO 2 by the leaves was attributable to this synthesis. It is suggested that the first of these carboxylations involves phosphoenolpyruvic carboxylase acting in conjunction with other enzymes Support is provided for the concept that most of the malate synthesized when CO 2 is fixed during dark acidification is transferred to a storage pool. The present analyses suggest that malate synthesized from the labelled pyruvate by action of malic enzymeor in oxidative reactions of the TCA cycle is relatively rapidly metabolized. The implications of the possible occurrence of the acids in Kalanchoё leaves partly in storage pools, where they are not subject to metabolic transformations, and partly in active pools, where they are rapidly transformed in metabolic reactions, are discussed briefly with respect to the interpretation of results observed when labelled metabolites are supplied to plant tissues.

The physiological role of the isoenzymes of lactate dehydrogenase in potatoes

Four of the five isoenzymes of lactate dehydrogenase present in potato tubers have been isolated and their kinetic properties examined. The pyruvate-reductase activity of isoenzyme-4 is greatly reduced at low pH, the affinity for both pyruvate and NADH is reduced and ATP has a stronger inhibitory effect. If the design properties of an enzyme dictate a high affinity for substrates, then the Km values for lactate, glyoxylate and NAD are consistent with an oxidative role for isoenzyme-4. The same considerations do not permit a conclusion about the physiological role of isoenzymes-1 to-3. However, an overview of the kinetic properties of these isoenzymes indicates that isoenzyme-1 is best adapted for the role of pyruvate reductase. Consideration of the relationships between kinetic constants and electrophoretic mobilities of the isoenzymes, leads us to predict that isoenzyme-5 is well adapted for a role in the oxidation of lactate or glyoxylate. The lactate dehydrogenase of potato leaves appears to consist prodominantly of an isoenzyme with the same mobility as isoenzyme-2 of the tubers and the two isoenzymes are probably identical. The kinetic properties of this isoenzyme are consistent with roles in either oxidation or reduction.