Protein binding of palmitate measured by transmembrane diffusion through polyethylene

Thin polyethylene membranes permit ready diffusion of protonated long-chain fatty acids but are impermeable to protein and ions. This circumstance recommends polyethylene for measuring the free fraction of fatty acids in the presence of a binding protein and for estimating the ionization constant with which to compute the equilibrium constant for the binding of fatty acid anions. As an example of this approach we report the binding of tracer palmitate to bovine albumin and bovine beta-lactoglobulin. We find a binding constant for the high-affinity site on albumin that is close to that calculated by others from heptane:H2O partition ratios. Our procedure is simpler, however, and free of the theoretical objection that heptane may alter the binding characteristics of the protein. Our estimate of the pKa for palmitic acid is 4.9, a finding that conforms to the widely predicted but heretofore unconfirmed expectation that long-chain fatty acids should have a pKa of about 4.8. Unidirectional flux measurements exclude direct exchange of palmitate between albumin and polyethylene.