An Apolipoprotein Homolog of Rat Apolipoprotein A‐IV in Human Plasma

Abstract

A protein with properties similar to apoprotein A‐IV of Rat high‐density lipoproteins has been isolated from the lipoprotein fraction (δ < 1.006 g/ml) of individual non‐fasting hypertriglyceridaemic subjects. The M r of human apolipoprotein A‐IV determined by polyacrylamide‐gel electrophoresis in sodium dodecylsulfate and by column chromatography in 6 M guanidine hydrochloride is 46000. The amino acid composition is significantly different from any previously characterised human apolipoprotein but resembles that of rat apolipoprotein A‐IV. The N‐terminal amino acid of human and rat apolipoprotein A‐IV is glutamate or glutamine. The p I of the major human A‐IV band in 6 M urea is ∼ 5.15. Immunochemically apolipoprotein A‐IV is not identical with any of the well known human apolipoproteins. Human apolipoprotein A‐IV is present in all fractions of δ < 1.006 g/ml isolated by a single ultracentrifugal spin from non‐fasting subjects, but is lost from lipoproteins of δ < 1.006 g/ml upon recentrifugation. Immunoelectrophoretic studies show that most of human apolipoprotein A‐IV is present in the fraction of δ > 1.21 g/ml and is unassociated with the major lipoprotein fractions in serum. Charge‐shift electrophoresis however clearly demonstrates the hydrophobic character of human apolipoprotein A‐IV. There is evidence that apolipoprotein A‐IV is a constituent of chylomicrons that is removed from the particle surface when chylomicrons enter the plasma compartment.