The protein subunit of human serum lipoproteins of density 1. 125–1. 200 gram/ml

Abstract

Formation of lipoprotein A+ and elevation of lipoprotein fraction O in locust (Locusta migratoria migratorioides) haemolymph as induced by adipokinetic hormone (AKH) includes the participation of non-lipid carrying proteins (fraction C), which was examined in more detail. By using gel filtration chromatography, the rather heterogenous C-proteins were resolved into three protein fractions, only one of which (C2) appeared to be actually involved in the lipoprotein reassociation. The changes in amino acid composition of the elevated lipoprotein fractions as compared with those from the lipoproteins in the resting situation are accounted for by the contribution of the rather specific amino acid composition of this C2-fraction. Polyacrylamide gel electrophoresis (PAGE) indicates that the C2-protein is migrating as only one band; SDS-PAGE revealed that the C2-protein consists of one single polypeptide chain with an approximate molecular weight of 20,000. This chain is also recovered in the subunit structure of the lipoprotein fractions induced by AKH-injection (A+, OAKH) in contrast with that of the lipoprotein fractions in resting haemolymph. Unlike the other C-proteins, protein C2 displayed immunoreactivity with antiserum raised against lipoprotein A+. From carbohydrate analyses, C2 appeared to be a glycoprotein containing approx. 12.5% carbohydrate. In vivo pilot studies on the dynamics of C2-proteins using 3H-labelled glycoprotein C2 gave evidence for the incorporation of radiolabel into both A+ and OAKH. Possible functions of the involvement of the glycoprotein to A+ formation are discussed.