Enzymatic methyl esterification of specific glutamyl residue in corticotropin.

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Abstract
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.degree. C, 30 mol % of the hormone was methylated on the basis of the [14C]methyl incorporation. 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 1 available methylation site at Glu-28, Glu-28 of .alpha.s-ACTH was specifically methyl esterified to yield [Glu(OMe)28]-.alpha.s-ACTH.