Defective in vitro lipolysis of type IV hyperlipidemic human plasma by purified milk lipoprotein lipase. Studies by single vertical spin centrifugation.

In vitro interactions of purified lipoprotein lipase with plasma from type IV hyperlipidemic and normolipidemic subjects were studied using an automated cholesterol analysis adaptation to single vertical spin centrifugation. Lipolysis for 60 min of normolipidemic plasma (control) supplemented with autologous very low density lipoprotein (VLDL) resulted in a progressive conversion of VLDL to low density lipoprotein (LDL) via intermediate density lipoprotein intermediates; 20% of the original VLDL cholesterol was transferred to high density lipoprotein (HDL), resulting in a density shift from the HDL, to the HDL2 region. Lipolysis of reconstituted type IV plasma supplemented with autologous VLDL-at a triglyceride level equal to the control-resulted in accumulation of much of the displaced VLDL cholesterol as light intermediate density lipoprotein and, correspondingly, cholesterol transfer to HDL was less than for normolipidemic plasma. Differences between type IV and control plasma were minimal or even undetectable when the type IV plasma had triglyceride levels below approximately 250 mg/ 100 ml. Analysis of apolipoprotein VLDL (‘251-labeled), free and esterified cholesterol, and phospholipid in the density gradient fraction of preand postlipolyzed samples showed that 95% of ‘251-labeled protein, 86% of the cholesteryl ester, and more than 97% of the free cholesterol and phospholipid in normolipidemic VLDL were removed from the VLDL density region during lipolysis and subsequently transferred to the LDL and HDL density regions; displaced cholesteryl ester and tetramethyl urea-insoluble ‘251-label (apolipoprotein B) ended up exclusively in the LDL density region, while displaced free cholesterol, phospholipid, and tetramethyl urea-soluble ’251-label (apolipoprotein C) ended up both in the LDL and HDL density regions. In type IV hyperlipoproteinemic plasma, only 37% of the cholesteryl ester and 51-55% of the free cholesterol and phospholipid in VLDL were displaced from the VLDL density region following lipolysis; the major fraction of displaced VLDL components accumulated in the intermediate density lipoprotein density region. Following lipolysis of reconstituted plasma containing either normolipidemic VLDL and type IV VLDL infranatant (the latter containing LDL, HDL, and albumin-free protein)