Schnell K F, Besl E, von der Mosel R
J Membr Biol. 1981;61(3):173-92. doi: 10.1007/BF01870522.
The concentration dependence and the pH dependence of the phosphate transport across the red cell membrane were investigated. The unidirectional phosphate fluxes were determined by measuring the 32P-phosphate self-exchange in amphotericin B (5 mumol/liter) treated erythrocytes at 25 degrees C. The flux/concentration curves display an S-shaped increase at low phosphate concentrations, a concentration optimum in the range of 150 to 200 mM phosphate and a self-inhibition at high phosphate concentrations. The apparent half-saturation concentrations, P(0.5), range from 50 to 70 mM and are little affected by pH. The self-inhibition constants, as far as they can be estimated, range from 400 to 600 mM. The observed maximal phosphate fluxes exhibit a strong pH dependence. At pH 7.2, the actual maximal flux is 2.1 X 10(-6) moles . min-1 . g cells-1. The ascending branches of the flux/concentration curves were fitted to the Hill equation. The apparent Hill coefficients were always in the range of 1.5-2.0. The descending branches of the flux/concentration curves appear to follow the same pattern of concentration response. The flux/pH curves were bell-shaped and symmetric with regard to their pH dependence. The pH optimum is at approximately pH 6.5-6.7. The apparent pK of the activator site is in the range of 7.0 to 7.2, while the apparent pK for the inactivating site is in the range of 6.2 to 6.5. The pK-values were not appreciably affected by the phosphate concentration. According to our studies, the transport system possesses two transport sites and probably two modifier sites as indicated by the apparent Hill coefficients. In addition, the transport system has two proton binding sites, one with a higher pK that activates and one with a lower pK that inactivates the transport system. Since our experiments were executed under self-exchange conditions, they do not provide any information concerning the location of these sites at the membrane surfaces.
研究了红细胞膜上磷酸盐转运的浓度依赖性和pH依赖性。通过在25℃下测量两性霉素B（5微摩尔/升）处理的红细胞中32P - 磷酸盐的自交换来测定单向磷酸盐通量。通量/浓度曲线在低磷酸盐浓度下呈S形增加，在150至200毫摩尔磷酸盐范围内有一个浓度最佳值，在高磷酸盐浓度下出现自抑制。表观半饱和浓度P(0.5)范围为50至70毫摩尔，且受pH影响较小。自抑制常数（就其可估计的范围而言）为400至600毫摩尔。观察到的最大磷酸盐通量表现出强烈的pH依赖性。在pH 7.2时，实际最大通量为2.1×10(-6)摩尔·分钟-1·克细胞-1。通量/浓度曲线的上升分支拟合到希尔方程。表观希尔系数始终在1.5 - 2.0范围内。通量/浓度曲线的下降分支似乎遵循相同的浓度响应模式。通量/pH曲线呈钟形，在pH依赖性方面是对称的。pH最佳值约为pH 6.5 - 6.7。激活位点的表观pK在7.0至7.2范围内，而失活位点的表观pK在6.2至6.5范围内。pK值不受磷酸盐浓度的明显影响。根据我们的研究，如表观希尔系数所示，转运系统具有两个转运位点，可能还有两个调节位点。此外，转运系统有两个质子结合位点，一个具有较高的pK可激活转运系统，另一个具有较低的pK可使转运系统失活。由于我们的实验是在自交换条件下进行的，所以它们没有提供关于这些位点在膜表面位置的任何信息。
