Evidence for an opiomelanotropin acetyltransferase in the rat pituitary neurointermediate lobe.

Results of this study demonstrate two distinct forms of acetyltransferase activity which will acetylate alpha-MSH. These acetyltransferases are distinguished by pH optima, subcellular distribution and sensitivity to magnesium and several solubilizing detergents. A general acetyltransferase, as characterized in the rat pituitary anterior lobe and lens, has a pH optima of 7.4 and is inhibited by magnesium. Subcellular fractionation of anterior and neurointermediate lobes revealed that this acetyltransferase is primarily localized in the cytosol fraction of these tissues. An alpha-MSH acetyltransferase (MAT) and a beta-endorphin acetyltransferase (EAT) have a pH optima of 6.0-6.6, are inhibited by detergents, and are specifically localized in the secretory granules of the neurointermediate lobe. Comparative studies of MAT and EAT suggest a single enzyme responsible for the acetylation of opioid and melanotropin peptides, and we term this enzyme opiomelanotropin acetyltransferase (OMAT).