The metabolism of lipoprotein(a) and other apolipoprotein B-containing lipoproteins: a kinetic study in humans

Lipoprotein(a) is a risk factor for cardiovascular disease composed of an apolipoprotein B-containing lipoprotein to which a second protein, apolipoprotein(a), is attached. We investigated in seven subjects with Lp(a) levels of 39--85 mg/dl the metabolism of four apo B-containing lipoproteins (VLDL(1), VLDL(2), IDL and LDL) together with that of apo B and apo(a) isolated from Lp(a). Rates of secretion, catabolism and where appropriate, transfer were determined by intravenous administration of d(3)-leucine, mass spectrometry for measurements of leucine tracer/tracee ratios and kinetic data analysis using multicompartmental metabolic modeling. Apo B in Lp(a) was secreted at a rate of 0.28 (0.17--0.40) mg/kg per day. It was found to originate from two sources -- 53% (43--67) were derived from preformed lipoproteins, i.e. IDL and LDL, the remainder was accounted for by apo B, directly secreted by the liver. The fractional catabolic rates (FCRs) of apo B and of apo(a) prepared from Lp(a) were determined as 0.27 (0.16--0.38) and 0.24 (0.12--0.40) pools per day, respectively, which is less than half of the FCR observed for LDL. Our in vivo data from humans support the view that Lp(a) assembly is an extracellular process and that its two protein components, apo(a) and apo B, are cleared from the circulation at identical rates.