Methionine sulfoxide in the hinge-ligament protein of molluscan bivalve species

Abstract

Methionine sulfoxide, an oxidized form of methionine, was detected at extremely high levels (15–30 mol% of total amino acids) in the protein from the elastic hinge-ligaments of molluscan bivalve species. The presence of methionine sulfoxide in the intact protein was confirmed by amino acid analysis of the NaOH-hydrolysate of the protein, non-destructive analyses of the ligament with solid-state ¹³C-NMR and IR-spectrometries and by the observation that the protein was resistant to the BrCN treatment. The conversion of methionine into its sulfoxide is almost complete. The oxidation process may be non enzymatic because two diastereomers, (5R)-L- and (5S)-L-methionine sulfoxide, were detected in the protein. The methionine sulfoxide residues might contribute to keep the protein highly hydrophilic and to promote the swelling of the ligaments.