Enzymatic methyl esterification of specific glutamyl residue in corticotropin.

Ovine corticotropin (alpha s-ACTH) was enzymatically methylated with purified calf brain protein methylase II (protein O-methyltransferase; S-adenosyl-L-methionine: protein-carboxyl O-methyltransferase, EC 2.1.1.24) and S-adenosyl-L-[methyl-14C]methionine. After incubation for 60 min at 37 degrees C, 30 mol % of the hormone was methylated on the basis of the [14C]methyl incorporation. In order to assess the location of methylation, the modified peptide was digested with pepsin. Analytical results derived from studies on the peptic digest led to the suggestion that the alpha s-ACTH-(6--28) peptide fragment was esterified. Because there is only one available methylation site at Glu-28, these results indicate that Glu-28 of alpha s-ACTH was specifically methyl esterified to yield [Glu(OMe)28]-alpha s-ACTH.