Studies on Lp(a) distribution in ultracentrifugally separated lipoprotein fractions.

Abstract

In this study we have determined how lipoprotein( a) (Lp(a)) influences the HDL-cholesterol ' lewel when present in human plasma. Six hundred : and eighty one individuals were chosen for the : study disregarding age, sex or illness criteria. '. Their plasma was processed in order to assess the cholesterol and triglyceride concentrations in . HDL particles isolated both by MgCl2- Phospho''. tungstic acid precipitation and differential ultra' centrifugation in the density range of 1.063-1.21 I , kg/L. The presence of Lp(a) was identified noting I · the presence or not of a band with a pre-f3 elec- ' .rophoretic mobility ("sinking pre-f3" lipoprotein) i in agarose gel electrophoresis of VLDL-free pla: sma (plasma infranatant at d> 1.006 kg/L). On •r·· this basis, the samples were classified as sinking pre-f3( +) and sinking pre-f3(-), respectively. Lp(a) j was also quantified in plasma by means of an 1 enzyme-linked immunoassay and a close corre\ spondence between the existence of "sinking pre~ f3" lipoprotein, and a plasmatic concentration of ! Lp(a) 'higher than 300 mg/L was observed. The : HDL-cholesterol level obtained by ultracentrifu. g~tion was higher than the one assessed by preci, pitation in the sinking pre-f3( +) samples, but not in the sinking pre-f3(-). There was a positive lineal correlation between the difference of such HDL-cholesterol levels and plasma Lp(a) concentration. A Lp(a) concentration of 300 mg/L (value found in more than 27% of the population study) · settles an ultracentrifugation HDL-cholesterol value that was 0.097 mmol/L higher than the precipitation HDL-cholesterol level. The presence of Lp(a) in the lipoprotein fractions prepared by ultracentrifugation was determined. Lp(a) was mostly detected in the HDL fraction. This latter result together with the observation that Lp(a) precipitates along with LDL under the action of the MgCl2-Losphotungstic acid reagent, explains the discrepancy between HDL-cholesterol values commented above. The proportion of Lp(a) present in the LDL fraction (density range: 1.006-1.063 kg/L) was in av~-- rage one thire of that in HDL. Lp(a) in the VLDL fraction was detectable in plasma with a high concentration of Lp(a); in these samples, Lp(a) in VLDL was positively correlated with VLDL-triglycerides. In conclusion, Lp(a) is mostly separated along with HDL, but also LDL and even VLDL particles from ultracentrifuged plasma. This means a Lp(a) interference in the quantification of HDL-cholesterol by ultracentrifugation that is proportional to plasma Lp(a) concentration.