Distribution of apolipoprotein A-IV between the lipoprotein and the lipoprotein-free fractions of rat plasma: possible role of lecithin:cholesterol acyltransferase

Abstract

Plasma samples were incubated under various conditions to study the effect of in vitro incubation on apolipoprotein A-IV distribution between the lipoprotein and lipoprotein-free fractions. When plasma was fractionated immediately after bleeding, apolipoprotein A-IV was present in equal concentrations in the lipoprotein and lipoprotein-free fractions. After a 4-hr, 37 degrees C incubation, greater than 90% of total plasma apolipoprotein A-IV was present in the lipoprotein fraction and the percentage of plasma cholesterol present as cholesteryl ester increased from 58% to 74%. When plasma was incubated for 4 hr at 37 degrees C in the presence of 1.5 mM 5,5-dithiobis(2-nitrobenzoic acid) (DTNB), greater than 90% of total plasma apoA-IV was present in the lipoprotein-free fraction, whereas plasma cholesteryl ester concentration did not change. Incubating heat-inactivated plasma for 4 hr also resulted in the redistribution of apolipoprotein A-IV from the lipoprotein fraction to the lipoprotein-free fraction, concurrent with no change in cholesterol esterification. When heat-inactivated plasma was incubated in the presence of a purified lecithin:cholesterol acyltransferase preparation, cholesterol esterification was restored and apolipoprotein A-IV was redistributed from the lipoprotein-free fraction to the lipoprotein fraction in such a manner that greater than 90% was present in the lipoprotein fraction. No changes in apolipoprotein A-I and apolipoprotein E distributions were found under any of the above conditions. Thus, the in vitro plasma incubations show that apolipoprotein A-IV can move bidirectionally between lipoprotein and lipoprotein-free fractions; the direction of this movement depends on the condition of the incubation.(ABSTRACT TRUNCATED AT 250 WORDS)