Hui D Y, Innerarity T L, Mahley R W
J Biol Chem. 1981 Jun 10;256(11):5646-55.
Hepatic membranes from adult dog livers have receptors which bind to lipoproteins containing the E apoprotein (the apo-E HDLc) but lack specific receptors for the apo-B-containing low density lipoproteins (LDL). Scatchard analysis of direct binding data for 125I-apo-E-HDLc revealed nonlinearity of the binding which could be resolved into two components, suggesting the presence of two separate binding sites. The binding site for apo-E HDLc that possessed the highest affinity (Kd = 0.23 x 10(-9) M) was calcium-dependent and was sensitive to proteolytic digestion with pronase. The lower affinity (Kd = 20 x 10(-9) M) binding site for apo-E HDLc did not require calcium and was resistant to pronase digestion. Chemical modification of the arginyl or lysyl residues of the apo-E HDLc prevented the HDLc from binding to the higher affinity receptor but had no effect on their binding to the lower affinity site. Adult canine liver membranes also bound canine 125I-HDL. However, the binding of HDL was of lower affinity (Kd = 8.2 x 10(-8) M), did not require calcium, was not blocked by modification of the lysyl or arginyl residues, and may not be of physiologic significance. Although the liver membranes from normal chow-fed adult dogs did not bind canine LDL, it was possible to demonstrate specific high affinity binding of LDL under certain metabolic conditions in dogs. When adult dogs were treated with the hypocholesterolemic agent cholestyramine, the liver membranes from these animals readily bound canine LDL. The Kd for canine LDL binding to these liver membranes was 15 x 10(-9) M. Furthermore, it was possible to demonstrate high affinity binding of LDL to the liver membranes from young rapidly growing puppies (Kd = 11 x 10(-9) M). The binding of the 125I-LDL to the liver membranes from the adult cholestyramine-treated dogs or from the puppies appeared to be mediated by apo-B,E receptors which resembled the LDL receptor of human skin fibroblasts. The 125I-LDL binding of these liver membranes was competitively inhibited by the addition of unlabeled LDL or apo-E HDLc. On the other hand, 125I-apo-E HDLc, capable of binding to the apo-B,E or to the apo-E receptors, were only partially displaced by the addition of unlabeled LDL but were totally displaced by apo-E HDLc. In summary, the adult dog liver possessed only the apo-E receptor. An apo-B,E receptor capable of binding LDL and HDLc could be induced by treatment of adult dogs with cholestyramine. Similarly, the liver membranes of young growing puppies possessed the apo-E and po-B,E receptors and were capable of binding both apo-E HDLc and LDL. The mechanism responsible for the control of the expression of the hepatic apo-B,E and/or apo-E receptors remains to be determined. These data indicate that a unique receptor capable of interacting specifically with apo-E-containing lipoproteins, and not with apo-B-containing lipoproteins (LDL), exists in the adult canine liver.
成年犬肝脏的肝细胞膜具有与含E载脂蛋白的脂蛋白（载脂蛋白E高密度脂蛋白胆固醇，apo-E HDLc）结合的受体，但缺乏与含apo-B的低密度脂蛋白（LDL）的特异性受体。对125I-apo-E-HDLc直接结合数据的Scatchard分析显示结合呈非线性，可分解为两个成分，提示存在两个独立的结合位点。apo-E HDLc具有最高亲和力（Kd = 0.23×10⁻⁹ M）的结合位点依赖于钙，且对链霉蛋白酶的蛋白水解消化敏感。apo-E HDLc较低亲和力（Kd = 20×10⁻⁹ M）的结合位点不需要钙，且对链霉蛋白酶消化有抗性。apo-E HDLc精氨酰或赖氨酰残基的化学修饰阻止HDLc与高亲和力受体结合，但对其与低亲和力位点的结合无影响。成年犬肝细胞膜也能结合犬125I-HDL。然而，HDL的结合亲和力较低（Kd = 8.2×10⁻⁸ M），不需要钙，不受赖氨酰或精氨酰残基修饰的阻断，可能不具有生理意义。虽然正常喂食普通食物的成年犬的肝细胞膜不结合犬LDL，但在犬类的某些代谢条件下有可能证明LDL的特异性高亲和力结合。当成年犬用降胆固醇药物消胆胺治疗时，这些动物的肝细胞膜很容易结合犬LDL。犬LDL与这些肝细胞膜结合的Kd为15×10⁻⁹ M。此外，有可能证明LDL与快速生长的幼犬的肝细胞膜有高亲和力结合（Kd = 11×10⁻⁹ M）。125I-LDL与成年消胆胺治疗犬或幼犬的肝细胞膜的结合似乎是由类似于人皮肤成纤维细胞LDL受体的apo-B,E受体介导的。这些肝细胞膜的125I-LDL结合可被未标记的LDL或apo-E HDLc竞争性抑制。另一方面，能够结合apo-B,E或apo-E受体的125I-apo-E HDLc仅被未标记的LDL部分取代，但被apo-E HDLc完全取代。总之，成年犬肝脏仅具有apo-E受体。用消胆胺治疗成年犬可诱导出能结合LDL和HDLc的apo-B,E受体。同样，幼年生长幼犬的肝细胞膜具有apo-E和apo-B,E受体，能够结合apo-E HDLc和LDL。负责控制肝脏apo-B,E和/或apo-E受体表达的机制仍有待确定。这些数据表明成年犬肝脏中存在一种独特的受体，它能够与含apo-E的脂蛋白特异性相互作用，而不与含apo-B的脂蛋白（LDL）相互作用。
