Thermodynamics of fatty acid transfer.

There is continuing controversy about the mechanism for transfer of fatty acids (FA) between plasma and the interior of cells and vice versa. One view is that this is a spontaneous process. The generally accepted view is that each step of the process is facilitated by a specialized protein. Whether uptake is spontaneous or facilitated, the components of the uptake system, e.g., albumin, water, FA, plasma membrane, and putative transport proteins of the plasma membrane, must behave according to the rules of the physical chemistry of the system. We review these features to illustrate the constraints they impose on the design of experiments to adduce the mechanism of uptake. Analysis of the literature in the context of the physical chemistry of the uptake system indicates that arguments for a facilitated mechanism of uptake for FA are not supported by any data extant. By contrast, comparison of the rates for individual steps of the pathway traversed by FA moving from albumin to the inside of a cell (or vesicles of a model system) with rates of uptake of FA of tissues in the steady state shows that the rates of the former are sufficient to account for the rate of the latter.