Free fatty acid uptake by polyethylene: what can one learn from this?

Previous experiments have shown that fatty-acid uptake by isolated hepatocytes is inhibited by albumin, but this inhibition was less than expected from the decrease in the equilibrium concentration of fatty acid. The possible explanation of this observation by the effects of codiffusion of protein-bound and unbound fatty acid across the unstirred layer surrounding these isolated cells has recently been challenged on the basis of experiments in which uptake by monolayers of hepatocytes was compared with that by a polyethylene sheet [F.J. Burczynski et al., Am. J. Physiol. 257 (Gastrointest. Liver Physiol. 20): G584-G593, 1989]. In the present report, we reevaluate the theoretical basis for interpretation of these experiments by solving the differential equations describing diffusion into a sheet behind a linear barrier. The diffusion coefficient for palmitate in polyethylene is estimated to be approximately 10(-9) cm2/s. We conclude that when proteins are absent from the aqueous phase, diffusion across the unstirred layer is rate limiting for removal of fatty acids by cellular monolayers, and also rate limiting for net flux across the water-polyethylene interface. In contrast, if the aqueous phase contains either 5 microM albumin or 125 microM beta-lactoglobulin, diffusion within the polyethylene sheet will become rate limiting. The net flux of fatty acids into a polyethylene sheet becomes insensitive to an increase in protein concentration if the latter rises above a certain threshold. The polyethylene data provide no additional insight into the manner in which hepatocytes take up free fatty acids.