Catabolism of human very low density lipoproteins in vitro: a fluorescent phospholipid method for monitoring lipolysis

Very-low-density lipoprotein of uremic patients is a poor substrate for bovine lipoprotein lipase in vitro

Very-low-density lipoprotein (VLDL) from 10 hemodialysis patients and 10 healthy controls was studied with respect to the substrate characteristics for bovine milk lipoprotein lipase (LPL). Compared with the control subjects, the hemodialysis patients had significantly higher serum triglyceride and apolipoprotein B-associated apolipoprotein CIII concentrations (1.03 +/- 0.31 v 1.98 +/- 0.86 mmol/L and 0.004 +/- 0.002 v 0.011 +/- 0.005 g/L, respectively), lower serum high-density lipoprotein (HDL) cholesterol and apolipoprotein AI concentrations (1.33 +/- 0.37 v 0.95 +/- 0.31 mmol/L and 1.29 +/- 0.25 v 1.09 +/- 0.23 g/L, respectively), and lower postheparin plasma LPL activity (82 +/- 24 v 35 +/- 14 milliU/milliL). There were also significant increases in the relative fat content and diameter of VLDL particles from patients versus controls. VLDL was labeled with a fluorescent phospholipid analog, DHPE, and the rate of the lipolytic reaction with purified bovine milk LPL was estimated from the increase in fluorescence intensity at 490 nm. There was no significant difference between initial reaction velocities in the study groups, but VLDL particles from hemodialysis patients were lipolyzed to a significantly lesser extent than those from healthy controls (mean increase in fluorescence intensity after completion of the reaction, 95 +/- 36 v 140 +/- 43 arbitrary units). These results are in accordance with the accumulation of remnant particles reported to occur in uremia despite only a moderately increased serum triglyceride concentration.

Apolipoprotein AI and AII metabolism in patients with primary high-density lipoprotein deficiency associated with familial hypertriglyceridemia

Plasma high-density lipoproteins (HDL) and their major proteins--apolipoprotein (apo) AI and apo AII--are subnormal in most patients with familial hypertriglyceridemia. However, the pathophysiology of low-plasma apo AI and apo AII is unclear. The kinetic parameters (turnover) of HDL apo AI and apo AII were studied in six lean patients with primary HDL deficiency associated with familial hypertriglyceridemia and five normolipidemic controls. Autologous 125I labeled HDL were injected intravenously (IV; 25 microCi) and blood samples drawn ten minutes after the injection and periodically thereafter for 12 days. Urine samples were collected daily and their radioactivity measured. Kinetic parameters were calculated from the area under the decay curve using three exponentials. Mean plasma apo AI and apo AII were significantly lower (P less than 0.001) in patients than normals (70.4 +/- 2.7 v 106.9 +/- 7.0; 24.2 +/- 1.6 v 39.2 +/- 0.9 mg/dL, respectively). The mean fractional catabolic rates (FCR) obtained from plasma 125I-HDL, apo AI, apo AII radioactivity decay curves and by Berson and Yalow's method (urine/plasma radioactivity ratios) were significantly greater (P less than 0.05) in patients than in controls (0.387 v 0.299; 0.391 v 0.309; 0.361 v 0.275; 0.272 v 0.207/d; respectively). The mean synthetic rates (SR) of apo AI and apo AII were significantly lower in patients than in controls (11.12 v 14.17 mg/kg body weight/d, P less than 0.05; 3.53 v 4.68 mg/kg body weight/d, P less than 0.05, respectively). In vitro lipolysis of triglyceride (TG) rich lipoproteins by bovine lipoprotein lipase, and measurement of hepatic TG lipase and lipoprotein lipase in postheparin plasma were similar in patients and controls, indicating no abnormality in these factors that are linked to HDL and TG catabolism. However, a significant positive correlation between hepatic TG lipase and the FCR of apo AI and apo AII was found. The data suggest that in this series of patients with HDL deficiency the low plasma HDL-cholesterol, apo AI, and apo AII levels resulted from decreased synthesis and an increased fractional catabolic rate of apo AI and apo AII, the major proteins of HDL.

Effect of desialylation of very low-density lipoproteins on their catabolism by lipoprotein lipase

Very low-density lipoproteins (VLDL) contain sialylated apolipoproteins (apo) (eg, apo CIII1-3) that inhibit apo CII activation of lipoprotein lipase (LPL) and also uptake of triglyceride (TG)-rich lipoproteins by the liver. Hypertriglyceridemic patients can have an excess of sialylated apo CIII (apo CIII1 or apo CIII2) in VLDL. These observations have prompted the notion that sialic acid in VLDL may impede LPL or receptor-mediated clearance of VLDL and thus result in hypertriglyceridemia. The aim of this study was to determine whether desialylation of VLDL altered their property as a substrate for LPL. VLDL isolated from five hypertriglyceridemic patients was desialylated with neuraminidase, labeled with a fluorescent probe, dansyl phosphatidylethanolamine and 600 micrograms of labeled VLDL TG were incubated with a constant amount of purified bovine LPL. The change in fluorescence against time was monitored on a recorder to yield curves representing continuous lipolysis of VLDL by LPL. Mean initial velocity of reaction (Vi) and extent of lipolysis measured as total increase in fluorescence over baseline at 30 minutes (F30/FO) were similar (Vi = 10.2 +/- 0.37 control v 10.2 +/- 0.42 u/min desialylated VLDL; F30/FO = 4.1 +/- 0.15, control v 4.1 +/- 0.07 desialylated VLDL; n = 5). Thus, sialic acid does not influence VLDL catabolism by LPL. Our study does not exclude a possible role of the sialic acid in receptor mediated uptake of remnants produced by initial catabolism of VLDL by LPL.