Effect of alpha-fetoprotein and albumin on the uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal rat hepatocytes.

The uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal hepatocytes has been studied using albumin and alpha-fetoprotein (AFP) as carrier proteins. Both types of cells took up linoleic (18:2(n-6)) and linolenic (18:3(n-3)) fatty acids at the same rate when they were added complexed either to albumin or AFP at a 1:1 molar ratio. At 37 degrees C a greater incorporation of arachidonic acid (20:4(n-6)) and mainly docosahexaenoic acid (22:6(n-3)) was observed when these acids were bound to albumin (6.5 nmol 20:4(n-6); 6.4 nmol 22:6(n-3) /million cells) as compared to AFP (5.5 nmol 20:4(n-6); 4.3 nmol 22:6(n-3)/million cells). The 20:4(n-6) and 22:6(n-3) uptake seems to be inversely related to the apparent association constants (k'a) between these fatty acids and both proteins (1.3 and 1.5 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 6.4 and 54.0 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP). Experiments carried out at 4 degrees C indicated that binding of 20:4(n-6) (0.83 nmol/million cells in presence of albumin; 2.16 nmol/million cells in presence of AFP) and 22:6(n-3) (0.83 nmol/million cells in presence of albumin; 1.32 nmol/million cells in presence of AFP) by cell membranes was also inversely related to the k'a of these proteins. At 4 degrees C, the k'a of AFP and albumin for 20:4(n-6) and 22:6(n-3) changed considerably (12.7 and 9.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 3.9 and 14.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP) with respect to the k'a calculated at 37 degrees C. Hence, k'a values were higher for albumin and lower for AFP than the corresponding values at 37 degrees C. It was concluded that uptake by cells and interaction of fatty acids with cell membranes depend mainly on the k'a of fatty acids and carrier proteins at equilibrium.