Carboxymethylation of methionine residues in bovine growth hormone

Oxidation of methionine residues in bovine growth hormone by chloramine-T

The methionine residues in bovine growth hormone were chemically modified wih chloramine-T at pH 7.4 using different chloramine-T methionine molar ratios. Methionine 4 was shown to be the most reactive followed in decreasing order by methionines 148, 123 and 178. With a 50-fold molar excess of chloramine-T over methionine almost full oxidation of all the methionine residues was achieved. The oxidation of the four residues of methionine does not change the growth promoting activity of the hormone.

Studies on the role of methionine in porcine pancreatic phospholipase A2

The unique methionine-15 residue located at the N-terminal site of iso- or beta-phospholipase A2 from porcine pancrease has been specifically carboxymethylated with iodoacetic acid. The modification results in a complete inactivation of the enzymatic activity toward micellar and monomeric substrates. Spectroscopic measurements reveled that the carboxymethylated protein still binds Ca2+ and monomeric substrates with comparable affinities as the native enzyeme. The active site histidine-54 residue in the modified enzyme shows a reactivity toward the active site-directed irreversible inhibitor p-bromophenacylbromide which is identical to that of the native enzyme. The alkylated protein, however, has lost its ability to bind to lipid-water interfaces. Although circular dichroic spectra of the carboxymethylated enzyme display some changes in the tertiary structure as compared with the native enzyme, the alpha-helix content remains rather constant. It is concluded that carboxymethylation of methionine-15 destroys the interface recognition site but has only limited influence on the active site of the molecule. Therefore, it seems that methionine-15 is not involved in the catalytic events but that this residue is part of the interface recognition site which embraces the N-terminal hydrophobic part of the enzyme: Ala-Leu-Trp-Gln-Phe-Arg-Ser-Met.

Studies on pituitary prolactin. 39. Reaction of the ovine hormone with hydrogen peroxide

Three methionine-modified derivatives of ovine prolactin have been prepared: two by oxidation of the methionines by H2O2 to sulfoxide (partial and complete), and the third by complete alkylation of the metionines with iodoacetic acid to the carboxymethyl sulfonium salts. The derivatives were characterized by exclusion chromatography, amino acid composition, circular dichroism spectra, relative rates of digestion by trypsin, and biological activity. Partially oxidized prolactin, having four of its seven methionines oxidized, was very similar to the native hormone. The unmodified methionines in partially oxidized prolactin were found to be the residues at positions 36, 81 and 132. The prolactin derivatives in which all the methionines had been oxidized, or alkylated, showed major changes in all parameters examined. In addition, circular dichroism spectra indicated that complete modification of all the methionines in prolactin exposes the normally buried tryptophans.