As a result of their poor aqueous solubility, Fatty acids (FAs) are quantitatively bound to plasma albumin in the circulation and to cytosolic FA-binding proteins inside the cell. The presence of FA-binding proteins on both sides of the plasma membrane complicates interpretation of FA uptake studies, and does not allow the detailed characterization accomplished with other uptake systems. However, despite these limitations, significant information can be obtained. In the past decade and a half, evidence has accumulated to document the existence of two components of FA uptake, a protein-facilitated component, referred to as “transport,” and a passive diffusion component (1 ). When assaying FA transport, it is important to keep in mind that the contribution of the transport component to cellular FA uptake will depend on several factors, including the concentration of free or unbound FA, the cell type, and so on.
