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 catalyzed by the stromal NADP-linked malate dehydrogenase are quantitatively analyzed 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 Km 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 generated 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 mumol (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 during stress is discussed.