Characterization of the peptide acetyltransferase activity in bovine and rat intermediate pituitaries responsible for the acetylation of β-endorphin and α-melanotropin

The peptide acetyltransferase activity responsible for the NH₂-terminal acetylation of β-endorphin, adrenocorticotropic hormone (ACTH)(1-13)NH₂, and α-melanocyte-stimulating hormone (αMHS) in bovine and rat intermediate pituitary has been studied. Rat anterior and intermediate pituitary rough endoplasmic reticulum/Golgi apparatus and cytosol fractions had similar amounts of acetyltransferase activity, but only the secretory granule fraction from the intermediate pituitary had what is believed to be the acetyltransferase activity specific for the formation of αMSH and α-N-acetyl-β-endorphin in this tissue. Bovine intermediate pituitary secretory granules were used as the primary source of acetyltransferase activity. This activity, which used acetyl coenzyme A as the acetate donor, used ACTH- and β-endorphin-related peptides as the acetate acceptor. The bovine intermediate pituitary secretory granule-associated acetyltransferase activity was not membrane-bound and had a pH optimum of 7.0. ACTH(1-13)NH₂, ACTH(1-39), αMSH, β-endorphin(1-31), and β-endorphin(1-27) were among the effective peptide substrates. The products of the enzymatic reaction, using [³H]acetyl CoA as the acetate donor, were analyzed by chymotrypsin digestion followed by reverse-phase high pressure liquid chromatography; at least 75% of the radioactivity was associated with the NH₂ terminus of the peptide substrates. ACTH(18-39), lysine-rich histones, ACTH(1-2)[Ser-Tyr], ACTH(11-24), α-N,O-diacetyl-αMSH, and α-N-acetylated β-endorphin-related molecules were not acetylated. Ser-Tyr, ACTH(11-24), and α-N,O-diacetyl-αMSH inhibited the NH₂-terminal acetylation of ACTH(1-13)NH₂. Steady state kinetic experiments indicated that the K(m) value for acetyl CoA was 8 μM and the K(m) values for the peptide substrates ranged from 11 to 286 μM. The addition of α-N-acetyl-β-endorphin(1-27) inhibited the NH₂-terminal acetylation of β-endorphin(1-27), ACTH(1-18), and ACTH(1-39); the inhibition of the acetylation of ACTH(1-18) by α-N-acetyl-β-endorphin(1-27) was competitive. These results suggest that the same enzyme NH₂ terminally acetylates both ACTH- and β-endorphin-related peptides in the bovine intermediate pituitary.