Characterization of alkylamine-sensitive site in alpha 2-macroglobulin.

Abstract

Methylamine reacted with the [human] plasma protease inhibitor, .alpha.2-macroglobulin, to form an irreversible, covalent modification. Reaction quantitation indicated 3.9 .+-. (SD) 0.4 reactive sites/native tetrameric protein (MW = 725,000) or 1 site per subunit. The reaction was selective and specific; only 1 or 2 labeled peptides were observed on radioautography of peptide maps derived from [14C]methylamine-treated .alpha.2-macroglobulin. A single chymotryptic peptide was isolated in 56% overall yield from the labeled protein. The peptide sequence by Edman degradation was Gly-Cys-Gly-Glu-X-Asn-Met-(Val, Leu). X was the only radiolabeled phenylthiohydantoin derivative. Amino acid analysis and mass spectral derivative analysis suggested that X is .gamma.-glutamylmethylamide. Because glutamic acid and glutamine residues do not normally react with alkylamines, presumptive evidence for an alternative activated selected protein center was presented.