An Apolipoprotein Homolog of Rat Apolipoprotein A-IV in Human Plasma.

A protein with properties similar to apoprotein A-IV of rat high-density lipoproteins has been isolated from the lipoprotein fraction (e < 1.006 g/ml) of individual non-fasting hypertriglyceridaemic subjects. The Mr 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 pI of the major human A-IV band in 6 M urea is &ap; 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 e < 1.006 g/ml isolated by a single ultracentrifugal spin from non-fasting subjects, but is lost from lipoproteins of e < 1.006 g/ml upon recentrifugation. Irnmunoelectrophoretic studies show that most of human apolipoprotein A-IV is present in the fraction of e < > 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. [ABSTRACT FROM AUTHOR]