Different arachidonate and palmitate binding capacities of the human red cell membrane

Human red cell membrane bindings of arachidonate and palmitate at pH 7.3 are investigated at temperatures between 0 and 38 degrees C by equilibrating ghosts with the long-chain fatty acids bound to bovine serum albumin in molar ratios (v) within the physiological range (< 1.7). Linearized relations of ghost uptakes and fatty acid monomer concentrations in buffer provide estimates of the binding capacities and corresponding equilibrium dissociation constants (Kdm). The temperature-independent arachidonate binding capacity, 5.5 +/- 0.5 nmol g-1 packed ghosts, is approximately fivefold smaller than that of palmitate, 26.6 +/- 2.0 nmol g-1. While Kdm of arachidonate binding 5.1 +/- 0.5 nM is temperature independent, Kdm of palmitate increases with temperature from 3.7 nM at 0 degrees C to 12.7 nM at 38 degrees C. The large difference in binding capacities suggests the presence of at least two different fatty acid binding domains in human red cell membranes.