An endogenous peptide that stimulates lanthanum-resistant calcium uptake in vascular tissue

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Abstract
A recent report has described the preparation of an extract from hemolyzed erythrocytes that has a stimulatory effect on lanthanum-resistant calcium uptake by vascular tissue in vitro and a hypertensive effect when injected into normotensive rats. The compound having a stimulatory effect on calcium uptake was further fractionated by molecular sieve and ion exchange chromatography, precipitation with CaCl2, high voltage paper electrophoresis, and high performance liquid chromatography (HPLC). HPLC yielded only a single fraction containing biological activity. This fraction was ninhydrin positive and acid labile. The amino acid composition was as follows: Asp/Asn (1.41), Ser (1.02), Glu/Gln (1.00), and Gly (2.00). Based on the assumption that the compound contains a single glutamic acid or glutamine residue, concentration–response data indicated that only nanomolar amounts of material were necessary to achieve significant stimulation. There was a marked increase in stimulatory activity of the resolubilized compound following calcium precipitation. The compound became inactive or showed a reduction in activity after being applied to a cation exchange column to remove calcium. Subsequent reprecipitation with CaCl2 and resolubilization restored the lost activity. Thus, we conclude that the compound is a small, acidic, calcium-dependent peptide that is extremely potent in stimulating lanthanum-resistant calcium uptake in vascular tissue.