PRESENCE OF FREE APOA-I IN SERUM - IMPLICATIONS FOR IMMUNOLOGICAL QUANTIFICATION OF HDL AND ITS APOPROTEINS

  • 1 January 1980
    • journal article
    • research article
    • Vol. 7  (2) , 139-160
Abstract
Antibodies against purified apo[apolipoprotein]A-I and apoA-II were produced. Immunological studies revealed that apoA-I exists in serum as apoA-I, bound in high density lipoprotein (HDL) particles containing apoA-II, apoC and apoE, in HDL [high density lipoprotein] particles deficient in apoA-II, and as apoA-I particles associated with some lipid, referred to as free apoA-I. ApoA-II was found only in HDL particles which contained ApoA-I. All apoA-I and apoA-II in the different HDL particles were immunologically detectable without delipidation. The concentration of free apoA-I in fresh serum was 10-30% of total apoA-I. Free apoA-I had electrophoretic mobility different from the other HDL particles. Following delipidation with 1, 1, 3, 3-tetramethylurea (TMU), or heating at 56.degree. C all apoA-I could be detected as free apoA-I. Storage at 37.degree. C for 4 wk had a similar but weaker effect. Density gradient ultracentrifugation showed that the different HDL subclasses had different hydrated densities; apart from differences in the apoprotein composition, they differed in cholesterol concentration. Quantitative determination of apoA-I bound in HDL, free apoA-I, total apoA-I, apoA-II and of apoA-I following delipidation with TMU were performed in 100 sera from normal individuals. The correlation coefficient between total apoA-I (free plus HDL-bound) and apoA-I following delipidation was 0.81. Regression analysis [An inverse relationship exists between the level of circulating plasma HDL and the propensity to develop coronary heart disease.].