Fridlyand L E, Backhausen J E, Scheibe R
Institute of Experimental Botany, Academy of Sciences of the Belarus, Minsk, Belarus.
Arch Biochem Biophys. 1998 Jan 15;349(2):290-8. doi: 10.1006/abbi.1997.0482.
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.
利用稳态模型对叶绿体跨包膜转运苹果酸和草酰乙酸以及它们在基质中与NADP相连的苹果酸脱氢酶催化下的相互转化这一耦合过程进行了定量分析。推导了NADP - 苹果酸脱氢酶反应的方程。利用酶活性对NADPH和NADP⁺的经验依赖性来确定其实际活性。通过转运体的动力学模型描述了苹果酸和草酰乙酸的跨包膜反向交换。动力学参数由已知数据得出，但草酰乙酸转运的Km值和最大速率是根据分离的完整叶绿体中草酰乙酸依赖性苹果酸形成来估算的。利用该系统的动力学特性和已知的代谢物浓度，该模型表明光合产生的NADPH可以通过苹果酸阀系统有效地从叶绿体输出到细胞质中。在正常生理条件下，苹果酸阀的转运能力估计不超过20 μmol（mg Chl）⁻¹ h⁻¹（或电子传递的5%）。讨论了正常条件下和胁迫期间苹果酸阀在叶细胞中的可能作用。
