Mechanisms of cellular uptake of long chain free fatty acids

Cells take up long chain free fatty acids (FFA) in vivo from the non-protei n bound ligand pools in extracellular fluid and plasma, which contain simil ar to 100 and 600 mu M albumin, respectively. The physiologic range of unbo und FFA concentrations in such fluids has traditionally been calculated at < 1 mu M. Studies of I:[H-3]-oleate uptake by hepatocytes, adipocytes, card iac myocytes and other cell types demonstrate that FFA uptake within this r ange is saturable, and exhibits many other kinetic properties indicative of facilitated transport. Within this range, the uptake kinetics of the acidi c (pKa = 0.5) FFA analog alpha(2)beta(2)omega(3)-heptafluorostearate are si milar to those of stearate. Thus, uptake of physiologic concentrations of F FA involves facilitated transport of the FFA anion (FA(-)). Over a much wid er range of unbound FFA concentrations hepatocellular [H-3] -oleate uptake exhibits both saturable and non-saturable components. Oleate binding to liv er plasma membranes (LPM) also demonstrates such components. Comparing the two components of FFA uptake to the corresponding components of binding per mits estimates of trans-membrane transport rates. T-1/2 for saturable uptak e (similar to 1 sec) is less than for non-saturable uptake (similar to 14 s ec). Others have determined the flip-flop rates of protonated FFA (FAH) acr oss small and large unilamellar vesicles (SUV, LUV) and across cellular pla sma membranes. These reported flip-flop rates, measured by the decrease in pH resulting from the accompanying proton flux, exhibit a highly significan t inverse correlation with cell and vesicle diameter (r = 0.99). Although T -1/2'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, u nder physiologic conditions, the predominant mechanism of cellular FFA upta ke is facilitated transport of FA(-); at much higher, non-physiologic FFA c oncentrations, passive flip-flop of FAH predominates. Several plasma membra ne proteins have been identified as potential mediators of facilitated FFA transport. Studies in animal models of obesity and non-insulin dependent di abetes mellitus demonstrate that tissue-specific regulation of facilitated FFA transport has important pathophysiologic consequences.