Metabolism of Chromatographically Separated Rat Serum Lipoproteins Specifically Labeled with 125I-apolipoprotein E

Radioiodinated apolipoprotein E, added in small amounts to rat serum, rapidly associates with the serum very low density lipoproteins and high density lipoproteins in proportion to their content of endogenous and apolipoprotein E. The labeled lipoproteins can be separated, without ultracentrifugation, by molecular sieve chromatography. When these labeled lipoproteins are injected intravenously into intact rats, the labeled apolipoprotein E rapidly exchanges with apolipoprotein E in the alternate lipoprotein fraction and is removed from the blood at a slow rate, comparable to that observed for apolipoprotein A-I in similarly labeled high density lipoproteins. No evidence was found for a rapidly turning over component of high density lipoproteins containing apolipoprotein E. When very low density lipoproteins, labeled endogenously with [3H]cholesterol and with radioiodinated apolipoprotein E, are added to perfusates of isolated rat livers, the labeled cholesteryl esters and apolipoprotein E are removed from the perfusate rapidly and at the same rate. Therefore, the slow removal of labeled apolipoprotein E in very low density lipoproteins in vivo is the result of rapid exchange with apolipoprotein E in high density lipoproteins.