Insulin-stimulating protein from human plasma. Chemical characteristics and biological activity.
A protein that potentiates the action of insulin in vitro was purified from human plasma. When reduced with 2-mercaptoethanol and then carboxymethylated, it yielded a single subunit, indicating that it was composed of two identical subunits connected by a single disulfide bond. This modified subunit tended to inhibit rather than stimulate insulin activity. A distinctive feature of the amino acid composition of this protein (H-ISP) was the absence of histidine, arginine, and tryptophan. The molecular mass, subunit composition, the characteristic amino acid composition and the N-terminal amino acid residue of H-ISP are very similar to those of human plasma apolipoprotein A-II (apo A-II). The isoelectric point of H-ISP was estimated to be 4.91, which is identical with that of the major apo A-II isoform. H-ISP did not itself have insulin-like activity in increasing CO2 liberation from labeled glucose and 2-deoxyglucose uptake by isolated rat adipocytes, but it potentiated the action of insulin in these parameters. It had no appreciable affect on the binding or degradation of 125I-labeled insulin by adipocytes. Like H-ISP, apo A-II isolated from human plasma also had no insulin-like activity by itself, but stimulated the effect of insulin on CO2 production from labeled glucose in isolated rat adipocytes. From these results, it is concluded that H-ISP is identical with the major apo A-II isoform. Incubation of isolated adipocytes with H-ISP resulted in marked increase in the activity of pyruvate dehydrogenase in a dose-dependent manner in the absence of added insulin. H-ISP also stimulated pyruvate dehydrogenase activity in a subcellular system consisting of plasma membranes and mitochondria from rat adipocytes. The effect of H-ISP on pyruvate dehydrogenase activity could be produced by treatment of the isolated mitochondrial fraction alone.
D000273:Adipose Tissue / Q000378:metabolism; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001692:Biological Transport; D002851:Chromatography, High Pressure Liquid; D004591:Electrophoresis, Polyacrylamide Gel; D005947:Glucose / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007328:Insulin / Q000502:physiology*; D036341:Intercellular Signaling Peptides and Proteins; D007525:Isoelectric Focusing; D008297:Male; D013058:Mass Spectrometry; D008970:Molecular Weight; D010084:Oxidation-Reduction; D010455:Peptides / Q000097:blood* / Q000502:physiology; D011768:Pyruvate Dehydrogenase Complex / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains
D000596:Amino Acids; D007328:Insulin; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D011768:Pyruvate Dehydrogenase Complex; C047279:insulin-stimulating peptide, human; D005947:Glucose