Flux control of the malate valve in leaf cells

The coupled processes of the chloroplast trans-envelope transport of malate and oxaloacetate and their interconversion as catalysed by the stromal NADP-linked malate dehydrogenase are quantitatively analysed by means of a steady-state model. The equation for the NADP-malate dehydrogenase reaction is developed, The empirical dependence of enzyme activity on NADPH and NADP(+) is used to determine its actual activity. The trans-envelope counter exchange of malate and oxaloacetate is described by a kinetic model of the translocator. Kinetic parameters are derived from known data, except for the K-m value and the maximum rate for oxaloacetate transport, which are estimated from oxaloacetate-dependent malate formation in isolated intact chloroplasts. Using the kinetic properties of the system and the known metabolite concentrations, the model demonstrates that photosynthetically genera-ted NADPH can be exported efficiently from the chloroplasts to the cytosol by the malate-valve system. The transfer capacity of the malate valve is estimated not to exceed 20 mu mol (mg Chl)(-1) h(-1) (or 5% of the electron transport) under normal physiological conditions, The possible role of the malate valve in leaf cells under normal conditions and duping stress is discussed. (C) 1995 Academic Press.