Permeability characteristics of the adipocyte cell membrane and partitioning characteristics of the adipocyte triglyceride core

The unidirectional rates of passive permeation of a homologous series of saturated fatty acids and bile acids into rat epididymal adipocytes were measured to determine the permeability characteristics of this mammalian cell membrane. For fatty acids containing 5 to 12 carbon atoms the logarithm of the permeability coefficient was a linear function of the number of carbons in the fatty acid chain: fatty acids with less than five carbon atoms, showed anomalously high permeabilities. Using the data for the fatty acids with 5 to 12 carbon atoms, the incremental free energy of transfer (δΔF_w→l) of the -CH₂ moiety from the aqueous environment into the fat cell was calculated to equal -547 cal mole^-1. The δΔF_w→l of the -OH moiety calculated from data using bile acids as the probe molecules was +1,225 cal mole^-1. After rupturing the fat cells by freeze-thawing, partition ratios also were measured between buffer and the lipid phase of the adipocyte core using both ratios, δΔF_w→l for the -CH₂ and -OH substituent groups was calculated to equal -830 and +2,070 cal mole^-1, respectively. On the basis of these studies, two, conclusions were drawn. First, like many epithelial surfaces and the erythrocyte membrane, the fat cell membrane exhibits anomalously high permeabilities to small molecular weight, polar compounds. Since this behavior in the adipocyte, as in the erythrocyte, cannot be attributed to structures such as tight junctions, it must be explained on the basis of some physico-chemical feature of the cell membrane itself. Secondly, the values of the δΔF_w→l indicate that the adipocyte membrane is less polar than the intestinal and gallbladder membranes but more polar than the membranes of Nitella and the erythrocyte.