Mechanisms of cellular uptake of long chain free fatty acids.

Cells take up long chain free fatty acids (FFA) in vivo from the non-protein bound ligand pools in extracellular fluid and plasma, which contain approximately 100 and 600 microM albumin, respectively. The physiologic range of unbound FFA concentrations in such fluids has traditionally been calculated at <1 microM. Studies of [3H]-oleate uptake by hepatocytes, adipocytes, cardiac myocytes and other cell types demonstrate that FFA uptake within this range is saturable, and exhibits many other kinetic properties indicative of facilitated transport. Within this range, the uptake kinetics of the acidic (pKa = 0.5) FFA analog alpha2,beta2,omega3-heptafluorostearate are similar to those of stearate. Thus, uptake of physiologic concentrations of FFA involves facilitated transport of the FFA anion (FA). Over a much wider range of unbound FFA concentrations hepatocellular [3H]-oleate uptake exhibits both saturable and non-saturable components. Oleate binding to liver plasma membranes (LPM) also demonstrates such components. Comparing the two components of FFA uptake to the corresponding components of binding permits estimates of trans-membrane transport rates. T1/2 for saturable uptake (approximately 1 sec) is less than for non-saturable uptake (approximately 14 sec). Others have determined the flip-flop rates of protonated FFA (FAH) across small and large unilamellar vesicles (SUV, LUV) and across cellular plasma membranes. These reported flip-flop rates, measured by the decrease in pH resulting from the accompanying proton flux, exhibit a highly significant inverse correlation with cell and vesicle diameter (r = 0.99). Although T1/2's in vesicles are in the msec range, those in cells are >10 sec, and thus comparable to the rates of non-saturable uptake we determined. Thus, under physiologic conditions, the predominant mechanism of cellular FFA uptake is facilitated transport of FA ; at much higher, non-physiologic FFA concentrations, passive flip-flop of FAH predominates. Several plasma membrane proteins have been identified as potential mediators of facilitated FFA transport. Studies in animal models of obesity and non-insulin dependent diabetes mellitus demonstrate that tissue-specific regulation of facilitated FFA transport has important pathophysiologic consequences.