Determination of kinetic parameters of apolipoprotein B metabolism using amino acids labeled with stable isotopes

Metabolism of apolipoprotein B-100 and of triglyceride-rich lipoprotein particles in the absence of functional lipoprotein lipase

Cholesterol and triglyceride in the various lipoprotein fractions were determined in five patients without functional lipoprotein lipase (LPL) while on their habitual therapeutic diet of 'low fat' content (20-25 g/day). They were also studied following 3 days on either a 'minimal fat' diet (< 15 g/day) or a 'moderate fat' diet (45-50 g/day). Values obtained were compared with the respective levels measured in five control subjects on a 'normal fat' (70-90 g/day) diet. The patients had hypertriglyceridaemia (type V hyperlipoproteinaemia) under all dietary conditions. Cholesterol and triglyceride levels in plasma and in the chylomicron fraction increased in the patients with increasing dietary fat. In the very low density lipoprotein (VLDL) fraction from the patients, triglyceride levels also increased with the dietary fat intake, but cholesterol levels were similar under all dietary conditions. In the patients, cholesterol concentrations in the low (LDL) and high density (HDL) lipoprotein fractions were significantly lower than the respective levels in controls, but the ratio of cholesterol to triglyceride levels in both of these lipoprotein fractions decreased with the dietary fat intake. VLDL apolipoprotein B-100 (apo B-100) pool size was similar in the patients on the two test diets (P = 0.95) and 3.5-fold higher than in five healthy volunteers on a normal fat diet. Using a stable isotope enrichment method, the kinetics of apo B-100 were investigated in the patients under the last two dietary conditions. The fractional and absolute secretion rates of the apolipoprotein in the patients did not vary with fat intake, but fractional secretion rates were significantly lower and the absolute secretion rates were significantly higher in the patients than the respective values in the controls. These results are consistent with the hypothesis that in the absence of LPL activity the metabolism of chylomicron and VLDL particles in the circulation results in triglyceride-rich LDL and HDL particles that are taken up by the liver at increased rates, thus reducing the plasma LDL and HDL cholesterol concentrations, whereas the products of hydrolysis of these particles induce an increased rate of synthesis of triglyceride and an increased rate of secretion of VLDL apo B-100.

No evidence for feedback inhibition of hepatic apolipoprotein B (apo B) production after extracorporeal low density lipoprotein precipitation as determined by [1-13C]leucine infusion in normal volunteers

To determine the impact of an acute reduction of the circulating mass of apolipoprotein B (apo B) on apo B metabolism we studied six healthy male volunteers before (day 0), 1 day after (day 2), and 7 days after (day 8) an LDL apheresis treatment which reduced apo B mass by 59%. Appearance of newly synthesized apo B in plasma VLDL and LDL was studied using a primed-constant infusion of [1-13C]-leucine. VLDL apo B pool size and fractional VLDL apo B production rate calculated using a one-compartment model were similar on all 3 study days. Absolute VLDL apo B production was not statistically different throughout the study (19.7 +/- 12.3, 19.5 +/- 7.5, 29.1 +/- 17.7 mg kg-1 day-1). LDL apo B fractional production rate was increased on day 2 (0.38 +/- 0.17, 0.68 +/- 0.08, 0.37 +/- 0.06 pools day-1 on days 0, 2, and 8; P < 0.01). Absolute LDL apo B production, however, remained constant throughout the study (10.8 +/- 3.3, 11.0 +/- 1.9, 10.8 +/- 3.1 mg kg-1 day-1). We conclude that in healthy male volunteers acute reduction of the circulating apo B mass by LDL apheresis does not affect apo B metabolism significantly.

Stable isotopes show a direct relation between VLDL apoB overproduction and serum triglyceride levels and indicate a metabolically and biochemically coherent basis for familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) may be genetically and metabolically more heterogeneous than previously thought. A consistent feature is an increase in circulating very-low-density lipoprotein (VLDL) apolipoprotein (apo) B, which could be due to either an increase in apoB production or a decrease in its catabolism. Therefore, we directly measured VLDL apoB production in the postabsorptive state in seven FCHL subjects (four male, three female) and seven normal control subjects (three male, four female) by using L-[1-13C]leucine as an endogenous label. Mean age and body mass index did not differ significantly between the two groups. The mean total cholesterol levels were 4.7 +/- 0.8 and 8.8 +/- 1.6 mmol/L (+/- SD, P < .01) and the mean triglyceride levels were 0.84 +/- 0.14 and 3.30 +/- 1.10 mmol/L (+/- SD, P < .01) in the control and FCHL groups, respectively. Although the fractional production rate of VLDL apoB was 38% lower in the FCHL group than in the control subjects (0.11 +/- 0.03 versus 0.18 +/- 0.02 pool/h; mean +/- SD, P < .01), its absolute production rate was 2.7 times greater (534 +/- 193 micrograms/kg per hour in FCHL versus 196 +/- 71 micrograms/kg per hour in control subjects; mean +/- SD, P < .01). There was a linear relation (r = 0.8, P = .03) between triglyceride levels and the VLDL apoB production rate in FCHL, the slope of which indicated a similar VLDL triglyceride-to-apoB ratio in the FCHL and control groups. We conclude that FCHL is a metabolically coherent disorder and that the increase in circulating apoB and triglyceride levels in FCHL is due to secretion of an increased number of VLDL particles, each containing, on average, a normal amount of triglyceride and one molecule of apoB.(ABSTRACT TRUNCATED AT 250 WORDS)