CYTOPLASMIC TRANSPORT OF LIPIDS - ROLE OF BINDING-PROTEINS

After entering the cell, small molecules must penetrate the cytoplasm before they are metabolized, excreted or can convey information to the cell nucleus. Without efficient cytoplasmic transport, most such mole cules would efflux back from the cell before they could reach their ta rgets. Conversely, intracellular lipids generated by hydrolysis of tri glycerides, phospholipids and other esters must be transported away fr om their site of formation to prevent toxic accumulation. Intracellula r movement of all molecules is slowed by molecular crowding, tortuosit y, and the greater viscosity of the cytosol relative to water. However , lipids and other amphipathic molecules are further slowed by their t endency to bind to cytoplasmic membranes. Cytoplasmic binding proteins reduce membrane binding by increasing the aqueous solubility of their ligands. These aqueous carriers catalyze the transport of lipid molec ules across hydrophilic water layers just as plasma membrane carriers catalyze the transport of hydrophilic molecules across the hydrophobic membrane core. They even display the principal features of carrier-me diated transport, including saturation, mutual competition, and counte rtransport. Higher concentrations of cytoplasmic binding proteins are associated with more rapid cytoplasmic transport of long-chain fatty a cids. Available data suggest that substantial intracellular concentrat ion gradients of fatty acids should exist, and that these gradients ma y help determine which metabolic pathway the fatty acid enters. Thus, cytoplasmic carrier proteins may help regulate the uptake and metaboli sm of fatty acids and other lipid molecules. Copyright (C) 1996 Elsevi er Science Inc.