Hsu S C, Molday R S
Department of Biochemistry and Molecular Biology, Faculty of Medicine, University of British Columbia, Vancouver, Canada.
J Biol Chem. 1994 Jul 8;269(27):17954-9.
Glucose metabolism in the photoreceptor rod outer segment produces both ATP (GTP) and NADPH to support phototransduction and NADPH-requiring processes in this organelle. Glycolysis in isolated bovine rod outer segments produces 44.0 +/- 6.4 nmol of ATP/min/mg of protein or 5.7 mM ATP/min. This rate of ATP production is more than sufficient to maintain the basal rate of cGMP synthesis (0.86 mM cGMP/min) in the dark requiring 1.7 mM ATP/min. Following photoexcitation, the 4.5-fold increase in the turnover of cGMP requires an ATP synthesis rate of up to 7.7 mM ATP/min (Ames, A., Walseth, T. F., Heyman, R. A., Barad, M., Graeff, R. M., and Goldberg, N. D. (1986) J. Biol. Chem. 261, 13034-13042). Under these conditions the rate of ATP production by glycolysis as measured in isolated rod outer segments is not sufficient for the regeneration of cGMP. Additional energy is most likely provided by the phosphocreatine shuttle which transports high energy phosphate groups in the form of creatine phosphate from the rod inner segment to the rod outer segment for conversion to ATP. The hexose monophosphate pathway in bovine rod outer segments can produce up to 39.8 +/- 2.2 nmol of NADPH/min/mg of protein. This rate of NADPH production is sufficient to support both the reduction of retinal to retinol (1.2 +/- 0.2 nmol of NADPH/min/mg of protein) following the photobleaching of rhodopsin and glutathione reduction (1.1 +/- 0.1 nmol of NADPH/min/mg of protein) for the protection of rod outer segments from oxidative damage. These studies provide insight into the contribution of anaerobic glycolysis and the hexose monophosphate pathway in providing energy and nucleotides for phototransduction and other outer segment processes.
光感受器视杆细胞外段中的葡萄糖代谢可产生ATP（GTP）和NADPH，以支持该细胞器中的光转导及需要NADPH的过程。分离出的牛视杆细胞外段中的糖酵解过程每分钟每毫克蛋白质可产生44.0±6.4 nmol的ATP，即每分钟产生5.7 mM的ATP。这种ATP生成速率足以维持黑暗中cGMP合成的基础速率（每分钟0.86 mM cGMP），而这一过程每分钟需要1.7 mM的ATP。光激发后，cGMP周转增加4.5倍，这需要高达每分钟7.7 mM ATP的合成速率（艾姆斯，A.，瓦尔塞思，T. F.，海曼，R. A.，巴拉德，M.，格雷夫，R. M.，和戈德堡，N. D.（1986年）《生物化学杂志》261卷，13034 - 13042页）。在这些条件下，分离出的视杆细胞外段中通过糖酵解产生ATP的速率不足以满足cGMP的再生需求。额外的能量很可能由磷酸肌酸穿梭提供，该穿梭机制将高能磷酸基团以磷酸肌酸的形式从视杆细胞内段转运至视杆细胞外段，用于转化为ATP。牛视杆细胞外段中的磷酸戊糖途径每分钟每毫克蛋白质最多可产生39.8±2.2 nmol的NADPH。这种NADPH生成速率足以支持视紫红质光漂白后视网膜还原为视黄醇（每分钟每毫克蛋白质1.2±0.2 nmol的NADPH）以及谷胱甘肽还原（每分钟每毫克蛋白质1.1±0.1 nmol的NADPH），从而保护视杆细胞外段免受氧化损伤。这些研究深入了解了无氧糖酵解和磷酸戊糖途径在为光转导及其他外段过程提供能量和核苷酸方面的作用。
