Abstract
Purified piasma membranes from rat livers were able to take up cholesterol both from serum lipoproteins and cholesterol-hospholipid dispersions in vitro. The cholesterol uptake from liposomes was larger than that from lipoproteins and increased linearly by the increase of choiesterol concentration up to 2μmoles/ml. Although the cholesterol uptake from liposomes was not affected by the heat-inactivation or formalin treatment of the membranes, the uptake from lipoproteins by these denatured membranes was lower than that by the native membranes. The difference between cholesteroi uptake from lipoproteins by the native membranes and that by the denatured membranes is considered to be a process which is mediated by a carrier protein (presumably binding protein to serum lipoproteins) and is saturable with a certain amount of serum lipoproteins at 4°C.
It is suggested that the choiesterol uptakes from low density lipoprotein and high density lipoprotein which are mediated by the specific membrane proteins are larger than those from very low density lipoprotein+intermediate density lipoprotein, but the uptake from high density lipoprotein is brought about by lower affinity binding than those from low and very low+intermediate density lipoprotein fractions.
The cholesterol uptake of hepatic plasma membranes of aged rats from serum lipoproteins by such specific binding was lower than that of young rats, but non-specific uptake of cholesterol from serum lipoproteins and liposomes was almost the same in both membranes.
