Loten E G, Regen D M, Park C R
J Cell Physiol. 1976 Dec;89(4):651-60. doi: 10.1002/jcp.1040890423.
D-allose, a glucose analogue, is not metabolized by isolated fat-cells and its distribution space at equilibrium in the cells is the same as that of triated water. Uptake of allose is inhibited by glucose and 3-O-methylglucose, stimulated by insulin and virtually eliminated by cytochalasan B. Counter transport of allose out of fat-cells against a concentration gradient can be induced by exogenous glucose but not by pyruvate. It is concluded that allose is transported into fat-cells by the same carrier mediated transport system as glucose and that it is a suitable analogue with which to study the glucose transport system. Insulin stimulated allose transport, into or out of the cell, but not basal transport, is inhibited by a brief exposure of isolated fat-cells to exogenous ATP or ADP (but not AMP or AMP-PNP). The antilipolytic effect of insulin is not affected. The ATP inhibition is slowly reversible. It is suggested that ATP phosphorylates a membrane component and thereby blocks transmission of signal from the insulin receptor to the carrier system. Indirect evidence suggests that ATP does not alter the affinity of the insulin or glucose binding sites. Insulin decreases the Km of glucose metabolism of CO2 and lipid in isolated fat-cells and increases the Vmax. However,the hormone has no effect on the Ki of glucose as an inhibitor of allose transport. The glucose analogue, 3-O-methyl-glucose, also inhibits both glucose metabolism and allose transport. The Ki for both these processes is similar and is not affected by insulin. These results support the view that the effect of insulin on glucose transport is to raise the Vmax without a change in the Km. It appears further that sugar transport is not the major rate limiting step in metabolism at high glucose concentrations in the absence of insulin, or at most glucose concentrations in the presence of the hormone.
D-阿洛糖是一种葡萄糖类似物，分离的脂肪细胞不能代谢它，其在细胞内达到平衡时的分布空间与氚化水相同。葡萄糖和3-O-甲基葡萄糖可抑制阿洛糖的摄取，胰岛素可刺激其摄取，而细胞松弛素B几乎可完全消除这种摄取。外源性葡萄糖可诱导阿洛糖逆浓度梯度从脂肪细胞中反向转运出来，但丙酮酸则不能。由此得出结论，阿洛糖与葡萄糖通过相同的载体介导转运系统转运进入脂肪细胞，并且它是研究葡萄糖转运系统的合适类似物。胰岛素刺激阿洛糖进出细胞的转运，但基础转运不受刺激，分离的脂肪细胞短暂暴露于外源性ATP或ADP（但不是AMP或AMP-PNP）可抑制基础转运。胰岛素的抗脂解作用不受影响。ATP的抑制作用是缓慢可逆的。有人提出，ATP使一种膜成分磷酸化，从而阻断信号从胰岛素受体向载体系统的传递。间接证据表明，ATP不会改变胰岛素或葡萄糖结合位点的亲和力。胰岛素降低了分离的脂肪细胞中CO2和脂质葡萄糖代谢的Km值，并增加了Vmax。然而，该激素对葡萄糖作为阿洛糖转运抑制剂的Ki值没有影响。葡萄糖类似物3-O-甲基葡萄糖也抑制葡萄糖代谢和阿洛糖转运。这两个过程的Ki值相似，且不受胰岛素影响。这些结果支持了胰岛素对葡萄糖转运的作用是提高Vmax而不改变Km的观点。进一步看来，在无胰岛素时高葡萄糖浓度下，或在有该激素时大多数葡萄糖浓度下，糖转运不是代谢中的主要限速步骤。
