Presence of "free" apoA-I in serum: implications for immunological quantification of HDL and its apoproteins.

Antibodies against purified apoA-I and apoA-II have been 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 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 also 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 degrees C all apoA-I could be detected as "free" apoA-I. Storage at 37 degrees C for four weeks had a similar but weaker effect. Density gradient ultracentrifugation showed that the different HDL subclasses had different hydrated densities, and apart from differences in the apoprotein composition, they also 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 between total apoA-I and apoA-II yielded a correlation coefficient of 0.43.