Membrane transport of long-chain fatty acids: evidence for a facilitated process

In mammalian cells, membrane uptake of long-chain fatty acids is mediated b y two separate components; a passive component that is a linear function of the concentration of free fatty acid in the extracellular medium and a sat urable component that exhibits the characteristics of a protein-facilitated process, This review summarizes the body of work that has accumulated rela ted to the mechanism of fatty acid transport. Evidence in support of a faci litated uptake process is presented with relation to the different cell typ es or membrane systems where it was collected. The evidence includes satura tion kinetics, competition between different substrates, and sensitivity to a variety of inhibitors. Recent knowledge related to membrane proteins tho ught to be implicated in the uptake process is reviewed. Factors that may m odulate uptake or alter the relative contribution of passive versus facilit ated components are briefly discussed. These include the molar ratio of fat ty acid to its physiological carrier, plasma albumin and the metabolic or h ormonal milieu.