Capacity of the malate/oxaloacetate shuttle for transfer of reducing equivalents across the envelope of leaf chloroplasts

Abstract

A steady-state model is used to estimate the capacity of the malate/oxaloacetate shuttle for indirect transfer of reducing equivalents between cytoplasm and chloroplast stroma of leaf cells. The dependence of transenvelope exchange on the malate and oxaloacetate gradients is described by a kinetic model of the dicarboxylate translocator. This model accounts for competition of transferable compounds, for counterexchange, and for saturation kinetics. Kinetic parameters are derived from known data, except the K m value for oxaloacetate, which is estimated from oxaloacetate-dependent oxygen evolution of isolated intact chloroplasts. Transfer capacity of the translocator is estimated not to exceed 10 μmol/mg chlorophyll per hour under physiological conditions. Using known data on stromal pyridine nucleotide concentrations it is shown that the stromal NADP-linked malate dehydrogenase cannot contribute to oxidation of malate in the stroma. Stromal malate can be oxidized by NAD-linked malate dehydrogenase, as the NAD system is more oxidized than the NADP system. However, according to this calculation, rates of steady-state transport do not exceed 3–4 μmol/mg chlorophyll per hour. Likewise, rates of export of reducing equivalents to the cytosol are shown to be physiologically insignificant. It is concluded that the malate/oxaloacetate shuttle cannot contribute significantly to the communication of cytosolic and stromal reducing equivalents.