Phosphoenolpyruvate carboxykinase is involved in the decarboxylation of aspartate in the bundle sheath of maize

We recently showed that maize (Zea mays L.) leaves contain appreciable amou nts of phosphoenolpyruvate carboxykinase (PEPCK; R.P. Walker, R.M. Acheson, L.I. Tecsi, R.C. Leegood [1997] Aust I Plant Physiol 24: 459-468). In the present study, we investigated the role of PEPCK in C-4 photosynthesis in m aize. PEPCK activity and protein were enriched in extracts from bundle-shea th (BS) strands compared with whole-leaf extracts. Decarboxylation of [4-C- 14]aspartate (Asp) by BS strands was dependent on the presence of 2-oxoglut arate and Mn2+, was stimulated by ATP, was inhibited by the PEPCK-specific inhibitor 3-mercaptopicolinic acid, and was independent of illumination. Th e principal product of Asp metabolism was phosphoenolpyruvate, whereas pyru vate was a minor product. Decarboxylation of [4-C-14]malate was stimulated severalfold by Asp and 3-phosphoglycerate, was only slightly reduced in the absence of Mn2+ or in the presence of 3-mercaptopicolinic acid, and was li ght dependent. Our data show that decarboxylation of Asp and malate in BS c ells of maize occurs via two different pathways: Whereas malate is mainly d ecarboxylated by NADP-malic enzyme, decarboxylation of Asp is dependent on the activity of PEPCK.