Bosner M S, Gulick T, Riley D J, Spilburg C A, Lange L G
Department of Medicine, Jewish Hospital of Saint Louis, Washington University Medical Center, MO 63110.
Proc Natl Acad Sci U S A. 1988 Oct;85(20):7438-42. doi: 10.1073/pnas.85.20.7438.
Molecular mechanisms regulating the binding, amphipathic stabilization, and metabolism of the major neutral lipids (e.g., cholesteryl esters, triglycerides, and fatty acids) are well studied, but the details of their movement from a binding compartment to a metabolic compartment deserve further attention. Since all neutral lipids must cross hydrophilic segments of plasma membranes during such movement, we postulate that a critical receptor-like site exists on the plasma membrane to mediate a step between binding and metabolism and that membrane-associated heparin is a key part of this mediator. For example, intestinal brush border membranes containing heparin bind homogeneous human pancreatic 125I-labeled cholesterol esterase (100 kDa) and 125I-labeled triglyceride lipase (52 kDa). This interaction is enzyme concentration-dependent, specific, and saturable and is reversed upon addition of soluble heparin. Scatchard analysis demonstrates a single class of receptors with a Kd of 100 nM and a Bmax of approximately 50-60 pmol per mg of vesicle protein. In contrast, enzymes associated with the hydrolysis of hydrophilic compounds such as amylase, phospholipase A2, and deoxyribonuclease do not bind to intestinal membranes in this manner. Human pancreatic cholesterol esterase also binds specifically and saturably to cultured intestinal epithelial cells (CaCo-2), and soluble heparin significantly diminishes the cellular uptake of the resultant hydrophobic reaction products (cholesterol and free fatty acids). We conclude that a physiological role for intestinal heparin is that of a mediator to bind neutral lipolytic enzymes at the brush border and thus promote absorption of the subsequent hydrolyzed nutrients in the intestine. This mechanism may be a generalizable pathway for transport of neutral lipids into endothelial and other cells.
调节主要中性脂质（如胆固醇酯、甘油三酯和脂肪酸）的结合、两亲性稳定及代谢的分子机制已得到充分研究，但其从结合区室向代谢区室移动的细节值得进一步关注。由于所有中性脂质在这种移动过程中都必须穿过质膜的亲水性片段，我们推测质膜上存在一个关键的受体样位点，以介导结合与代谢之间的一个步骤，并且膜相关肝素是该介质的关键部分。例如，含有肝素的肠刷状缘膜能结合纯化的人胰腺125I标记的胆固醇酯酶（100 kDa）和125I标记的甘油三酯脂肪酶（52 kDa）。这种相互作用是酶浓度依赖性的、特异性的且可饱和的，加入可溶性肝素后可逆转。Scatchard分析表明存在一类单一的受体，其解离常数（Kd）为100 nM，每毫克囊泡蛋白的最大结合量（Bmax）约为50 - 60 pmol。相比之下，与亲水性化合物水解相关的酶，如淀粉酶、磷脂酶A2和脱氧核糖核酸酶，不会以这种方式与肠膜结合。人胰腺胆固醇酯酶也能特异性且可饱和地结合培养的肠上皮细胞（CaCo - 2），可溶性肝素显著减少了所得疏水反应产物（胆固醇和游离脂肪酸）的细胞摄取。我们得出结论，肠肝素的生理作用是作为一种介质，在刷状缘结合中性脂解酶，从而促进肠道中后续水解营养物质的吸收。这种机制可能是中性脂质转运进入内皮细胞和其他细胞的一种普遍途径。
