Membrane-associated forms of peptidylglycine alpha-amidating monooxygenase activity in rat pituitary. Tissue specificity.

Abstract

Membrane-associated peptidylglycine alpha-amidating monooxygenase (PAM) activity was investigated in rat anterior and neurointermediate pituitary tissues and in pituitary AtT-20/D-16v and GH3 cell lines. A substantial fraction of total pituitary PAM activity was found to be membrane-associated. Triton X-100, N-octyl-beta-D-glucopyranoside, and Zwittergent were effective in solubilizing PAM activity from crude pituitary membranes. The distribution of enzyme activity between soluble and membrane-associated forms was tissue-specific. In the anterior pituitary lobe and pituitary cell lines, 40-60% of total PAM activity was membrane-associated while only 10% of the alpha-amidating activity in the neurointermediate lobe was membrane-associated. Soluble and membrane-associated forms of PAM shared nearly identical characteristics with respect to copper and ascorbate requirements, pH optima, and Km values. Upon subcellular fractionation of anterior and neurointermediate pituitary lobe homogenates on Percoll gradients, 12-18% of total PAM activity was found in the rough endoplasmic reticulum/Golgi fractions and 42-60% was localized to secretory granule fractions. For both tissues, membrane-associated PAM activity was enriched in the rough endoplasmic reticulum/Golgi pool, whereas most of the secretory granule-associated enzyme activity was soluble.