Direct determination of lipoprotein(a) cholesterol by ultracentrifugation and agarose gel electrophoresis with enzymatic staining for cholesterol

Abstract

Lipoprotein(a) [(Lp(a)], a low-density lipoprotein (LDL)-like particle, contains in addition to LDL a specific protein component, apolipoprotein(a) [apo(a)]. Conventionally. Lp(a) has been measured by immunological methods that distinguish between Lp(a) and LDL by dealing with apo(a) as an antigen. We describe a new method to determine Lp(a) on the basis of its cholesterol content. Very-low-density lipoproteins were removed from serum by preparative ultracentrifugation at a density of 1.006 kg/L. The infranate was subjected to agarose gel electrophoresis to separate Lp(a) and LDL. Lp(a) cholesterol was then determined by direct enzymatic staining for cholesterol. On electrophoresis of the >1.006 kg/L (bottom) fraction, Lp(a) migrates to the pre-β position, regardless of the genetic apo(a) isoform. The interassay CVs of Lp(a) cholesterol determinations ranged from 6,9% to 11.5%, and the results correlated well with the Lp(a) concentrations measured by immunonephelometry (r = 0.937). There was an inverse relation between the molecular mass of the genetically determined apo(a) isoforms and Lp(a) cholesterol concentrations. Patients with angiographically proven coronary artery disease (CAD) had significantly more Lp(a) cholesterol than healthy controls did. The ratio of Lp(a) cholesterol to immunologically determined Lp(a) tended to be lower in CAD patients, suggesting that Lp(a) particles contained less cholesterol than apo(a). In addition, the new method allows determination of LDL cholesterol without contamination by Lp(a).