Phosphoenolpyruvate carboxykinase is involved in the decarboxylation of aspartate in the bundle sheath of maize
1999
Wingler, A. | Walker, R.P. | Chen, Z.H. | Leegood, R.C.
We recently showed that maize (Zea mays L.) leaves contain appreciable amounts of phosphoenolpyruvate carboxykinase (PEPCK; R.P. Walker, R.M. Acheson, L.I. Tecsi, R.C. Leegood [1997] Aust J Plant Physiol 24: 459-468). In the present study, we investigated the role of PEPCK in C(4) photosynthesis in maize. PEPCK activity and protein were enriched in extracts from bundle-sheath (BS) strands compared with whole-leaf extracts. Decarboxylation of [4-(14)C]aspartate (Asp) by BS strands was dependent on the presence 2-oxoglutarate and Mn(2+), was stimulated by ATP, was inhibited by the PEPCK-specific inhibitor 3-mercaptopicolinic acid, and was independent of illumination. The principal product of Asp metabolism was phosphoenolpyruvate, whereas pyruvate was a minor product. Decarboxylation of [4-(14)C]malate was stimulated severalfold by Asp and 3-phosphoglycerate, was only slightly reduced in the absence of Mn(2+) or in the presence of 3-mercaptopicolinic acid, and was light dependent. Our data show decarboxylation of Asp and malate in BS cells of maize occurs via two different pathways: Whereas malate is mainly decarboxylated by NADP-malic enzyme, decarboxylation of Asp is dependent on the activity of PEPCK.
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