The metabolism of low density lipoprotein in endogenous hypertriglyceridaemia.

The metabolism of low density lipoprotein (LDL) was studied in eighteen hypertriglyceridaemic patients by injecting autologous radioiodinated LDL. Over 95% of the label was bound to the protein moiety of LDL and therefore the metabolic data reflect the fate and distribution of LDL apoprotein (apo B). The hypertriglyceridaemic subjects included ten with Type V, five with Type IV, two with Type III and one with Type IIb hyperlipoproteinaemia. For comparison identical studies were carried out in seven normal subjects and five patients with heterozygous familial hyperbetalipoproteinaemia (Type IIa). The groups differed considerably in mean LDL-cholesterol concentration. The patients with Type V lipoprotein pattern had significantly lower LDL-cholesterol concentration (mean 0.754 g/1) than the normal group (mean 1.237 g/1). Raised LDL-cholesterol levels were observed in all patients with heterozygous familial hyperbetalipoproteinaemia. The synthetic rate of LDL-apoprotein was found to be similar in all three groups (hypertriglyceridaemic, normal and hypercholesterolaemic). The highest synthetic rate was observed in the patient with Type IIb pattern. However, the fractional catabolic rate (FCR) of LDL-apoprotein differed significantly. The highest mean FCR was found in the Type V group (0.65 +/- 0.17 day-1) compared with 0.41 +/- 0.09 day-1 in the normal group and 0.185 +/- 0.05 day-1 in the Type IIa group. A strong inverse correlation was found between FCR and LDL apoprotein concentration in the whole series (r = -0.90, p less than 0.001) as well as within the Type V group (r = -0.87, p less than 0.01). These data indicate that the low plasma levels of LDL frequently observed in patients with very high plasma triglyceride levels are due to a high removal rate of LDL in these patients rather than to abnormal LDL synthesis.