The Role of Receptors in Apolipoprotein B Metabolism
A new perspective on lipoprotein metabolism has been gained from the identification and characterisation of Specific high affinity lipoprotein receptors. These are located on cell membranes and serve to facilitate and control the ingress of lipoprotein cholesterol into the cell. They exhibit two important properties. First, they are subject to autoregulation so that when cellular cholesterol requirements rise, the receptors are stimulated to promote lipoprotein assimilation. Conversely, when the cell is replete with the sterol, receptor activity is suppressed. Second, they demonstrate a high degree of ligand specificity which derives from selective interaction between the receptor and apoproteins on the surface of lipoproteins. Two polypeptides, apolipoproteins B and E, have been implicated in this recognition process and two distinct membrane receptor activities have also been identified (1,2,3,4). One seems to be confined to hepatocytes and apparently operates as an efficient mechanism for the cellular assimilation of chylomicron remnants. The other which appears to be identical to the fibroblast receptor of Goldstein and Brown (1) is widely distributed throughout the tissues of the body and plays an important role in the metabolism of low density lipoproteins (LDL).
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