Role of phosphoenolpyruvate carboxylase in malate production by the developing barley aleurone layer

The pathway of malate synthesis in the developing aleurone layer of barley (Hordeum vulgare L. cv. Himalaya) was investigated. Malate formation did not occur under anoxia. Labelling with [2-C-14]acetate showed that the glyoxylate pathway was not a significant source of malate. The partitioning of glycolytic carbon flux at the branchpoint between phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) and pyruvate kinase (PK, EC 2.7.1.40) was studied using [U-C-14]glucose. It was concluded that in aleurone from maturing, rapidly acidifying grains the nux through the PEPC branch relative to that through PK is 3-5 times greater than in young aleurone. This increase in nux can be accounted for by a 5-fold increase in PEPC protein determined by western blotting and in PEPC activity measured in vitro.