Secretion and regulation of two biosynthetic enzyme activities, peptidyl-glycine alpha-amidating monooxygenase and a carboxypeptidase, by mouse pituitary corticotropic tumor cells

Abstract

Secretion of peptidyl-glycine alpha-amidating monooxygenase (PAM) activity and enkephalin convertase (a carboxypeptidase B-like enzyme) activity from AtT-20 mouse corticotrope tumor cells was compared with secretion of pro-ACTH/endorphin-derived peptides. Upon stimulation of secretion by corticotropin-releasing factor, a beta-adrenergic agonist, or barium ions, secretion of PAM activity, enkephalin convertase activity, and immunoactive hormone rose in parallel. During inhibition of stimulated secretion with glucocorticoids or cobalt ions, secretion of PAM activity, enkephalin convertase activity, and immunoactive hormone fell in parallel. No measurable secretion of cytosolic, mitochondrial, or lysosomal marker enzymes was detected, indicating that the secretion of PAM activity and enkephalin convertase activity was specific and not a result of cell damage or lysis. The kinetic characteristics, apparent mol wt, and enzymatic properties determined for the secreted enzymes were similar to those for the enzymes from secretory granules. In AtT-20 cells treated with glucocorticoids for up to 8 days, cellular levels of immunoactive ACTH/endorphin-related peptides decreased to one third of control levels. Levels of PAM activity in cell extracts declined in parallel with levels of hormone. In contrast, levels of enkephalin convertase activity, cell protein, lactate dehydrogenase, fumarase, and lysosomal carboxypeptidase did not decline in response to chronic glucocorticoid exposure.