Oleic acid uptake and binding by rat adipocytes define dual pathways for cellular fatty acid uptake

Oleic acid (OA) uptake by rat adipocytes and the proportions of intracellular unesterified ( 3 H)OA and its 3 H-labeled esters were determined over 300 s. Uptake was linear for 20-30 s, with rapid esterification indicating entry into normal metabolic pathways. Initial rates of OA uptake and its binding to plasma membranes were studied over a spectrum of oleic acid:bovine serum albumin (BSA) ratios, and expressed as functions of unbound OA concentrations calculated with both the 1971 OA:BSA association constants of Spector, Fletcher, and Ashbrook and more recent con- stants (e.g., the 1993 constants of Richieri, Anel, and Klein- feld), which generate concentrations 10- to 100-fold lower. In either case, uptake was the sum of saturable and linear processes, with > 90% occurring via the saturable pathway when the OA:BSA molar ratio was within the physiologic range (0.5-3.0). Within this range, rate constants for satura- ble transmembrane influx ( k s ), calculated from both sets of constants, were similar (2.9 s 2 1 ) and were 10- to 30-fold faster than those for nonsaturable uptake ( k ns 5 0.26-0.10 s 2 1 , t 1/2 5 2.7-6.6 s, based on the constants of Spector et al. and Richieri et al., respectively). The rate of oleic acid flip- flop into rat adipocytes ( k ff 5 0.16 6 0.02 s 2 1 , t 1/2 5 4.3 6 0.5 s), computed from published data, was similar to k ns . Thus, OA uptake occurs by both a saturable mechanism and passive flip-flop. This conclusion is independent of the OA:BSA association constants used to analyze the experi- mental measurements. —Stump, D. D., X. Fan, and P. D. Berk. Oleic acid uptake and binding by rat adipocytes de- fine dual pathways for cellular fatty acid uptake. J. Lipid Res. 2001. 42: 509-520.