Schenerman M A, Kilberg M S
Biochim Biophys Acta. 1986 Apr 25;856(3):428-36. doi: 10.1016/0005-2736(86)90133-1.
Plasma membrane vesicles prepared from intact rat liver or isolated hepatocytes retain transport activity by systems A, ASC, N, and Gly. Selective substrates for these systems showed a Na+-dependent overshoot indicative of energy-dependent transport, in this instance, driven by an artificially-imposed Na+ gradient. Greater than 85% of Na+-dependent 2-aminoisobutyric acid (AIB) uptake was blocked by an excess of 2-(methylamino)isobutyric acid (MeAIB) with an apparent Ki of 0.6 mM. Intact hepatocytes obtained from glucagon-treated rats exhibited a stimulation of system A activity and plasma membrane vesicles isolated from those same cells partially retained the elevated activity. Transport activity induced by substrate starvation of cultured hepatocytes was also evident in membrane vesicles prepared from those cells. The membrane-bound glucagon-stimulated system A activity decays rapidly during incubation of vesicles at 4 degrees C (t1/2 = 13 h), but not at -75 degrees C. Several different inhibitors of proteolysis were ineffective in blocking the decay of transport activity. Hepatic system N transport activity was also elevated in plasma membrane vesicles from glucagon-treated rats, whereas system ASC was essentially unchanged. The results indicate that both glucagon and adaptive regulation cause an induction of amino acid transport through a plasma membrane-associated protein.
从完整大鼠肝脏或分离的肝细胞制备的质膜囊泡通过系统A、ASC、N和甘氨酸保留转运活性。这些系统的选择性底物显示出钠依赖性过冲，表明能量依赖性转运，在这种情况下，由人工施加的钠梯度驱动。超过85%的钠依赖性2-氨基异丁酸（AIB）摄取被过量的2-（甲基氨基）异丁酸（MeAIB）阻断，表观抑制常数（Ki）为0.6 mM。从胰高血糖素处理的大鼠获得的完整肝细胞表现出系统A活性的刺激，并且从这些相同细胞分离的质膜囊泡部分保留了升高的活性。在从培养的肝细胞制备的膜囊泡中，由底物饥饿诱导的转运活性也很明显。膜结合的胰高血糖素刺激的系统A活性在囊泡于4℃孵育期间迅速衰减（半衰期=13小时），但在-75℃时不衰减。几种不同的蛋白水解抑制剂在阻断转运活性衰减方面无效。胰高血糖素处理的大鼠的质膜囊泡中肝系统N转运活性也升高，而系统ASC基本不变。结果表明，胰高血糖素和适应性调节均通过与质膜相关的蛋白质诱导氨基酸转运。
