Formation of isoaspartate 99 in bovine and porcine somatotropins.

Asparagine 99 in bovine (BST) and porcine somatotropins (PST) was converted to an isoaspartate residue during incubation at neutral or alkaline pH. Isoaspartate 99 BST or isoaspartate 99 PST was resolved from the normal somatotropin by reversed-phase high-performance liquid chromatography (HPLC). The altered peptide of residues 96-108 which contains isoaspartate 99 was detected by tryptic peptide mapping of the modified BST or PST. Amino acid sequencing, amino acid analysis, mass spectrometry, and co-elution with a chemically synthesized peptide containing isoaspartate 99 were used to demonstrate the existence of isoaspartate in the modified peptides. Peptide bond cleavage between Asn 99 and Ser 100 also occurred during incubation of BST and PST at neutral or alkaline pH. This chemically cleaved product was resolved on reversed-phase HPLC from both the isoaspartate 99 and normal somatotropin molecules.