Very Low Density Lipoprotein

Abstract

The fate of rat plasma very low density lipoprotein (VLDL) constituents was determined in the isolated perfused rat heart. VLDL was labeled with [¹⁴C]palmitate, ³²P-phospholipids, [³H] cholesterol, and ¹²⁵I-apolipoprotein C (apoC). Perfusions were performed with an albumin-containing buffer and without plasma. Radioactivity was followed in fractions of d < 1.019, d 1.019-1.04, d 1.04-1.21, and d > 1.21 g/ml, prepared by ultracentrifugation. VLDL triglycerides were progressively hydrolyzed to fatty acids (10-120-min perfusions). Concomitantly, phospholipids, cholesterol (predominantly unesterified), and apoC were removed from the VLDL to all other fractions. About 30-35% of the phosphatidylcholine was hydrolized to lysophosphatidylcholine and was recovered at d > 1.21 g/ml. The phosphatidylcholine-and triglyceride-hydrolyzing activities were confined to membrane supported enzyme(s). The other 60-65% of the phosphatidylcholine was removed unhydrolyzed and was found in fractions of d 1.019-1.04 (10-15%), d 1.04-1.21 (25-30%), and d > 1.21 g/ml (15-20%). [³²P]Sphingomyelin accumulated at the fraction of d 1.04-1.21 g/ml. Unesterified cholesterol was found in the fraction of d 1.04-1.21 g/ml. ApoC was recovered predominantly in fractions of d 1.04-1.21 (50-60%) and d > 1.21 g/ml (30-40%). Cholesteryl esters were associated with VLDL during the hydrolysis of 50-70% of the triglycerides, but with advanced lipolysis, appeared in higher densities, mainly d 1.019-1.04 g/ml. The fraction of d 1.04-1.21 g/ml, (containing phosphatidylcholine, sphingomyelin, unesterified cholesterol, and apoC) contained by negative staining, many disk-like structures. The study demonstrated that removal of surface constituents (phospholipids, unesterified cholesterol, and apoC) during lipolysis of VLDL is an intrinsic feature of the lipolytic process, and is independent of the presence of plasma. It also indicated that surface constituents may be removed in a particulated form.