Quantitative analysis of uptake of free fatty acid by mammalian cells: lauric acid and human erythrocytes

Quantitative aspects of the binding of free fatty acid to human erythrocytes were studied by measuring the distribution of various amounts of [1-(14)C]lauric acid between washed human erythrocytes and defatted human plasma albumin. Incubations were done at 37 degrees C in an isotonic phosphate-buffered salt solution. Laurate uptake approached a steady state value within 1 hr of incubation over the range of laurate-albumin molar ratios that were tested. Uptake was due primarily to a transfer of laurate from albumin to the cell, not to incorporation of the intact laurate-albumin complex. The fatty acid binding sites of the erythrocyte are located predominantly on or within the cell membrane. The binding model which best fitted the laurate uptake data consisted of two classes of erythrocyte binding sites. This model contains a small number of sites, 2.0 x 10(-13) moles/10(6) cells, that have an average apparent association constant of 1.8 x 10(6) m(-1) for laurate. Thus, the average strength of these sites is of the same order of magnitude as the stronger laurate binding sites of albumin. The binding model also contains a relatively large number of weaker fatty acid binding sites, 1.3 x 10(-11) moles/10(6) cells, that have an average apparent association constant of 1.3 x 10(4) m(-1) for laurate. These sites are too weak to bind appreciable amounts of laurate unless the fatty acid-albumin molar ratio is elevated.