Peptidyl-glycine alpha-amidation activity in tissues and serum of the adult rat

Abstract

Bioactive peptides frequently terminate in a carboxyl-terminal alpha-amide. The tissue distribution of enzymatic activity capable of converting [125I]D-Tyr-Val-Gly into [125I]D-Tyr-Val-NH2 has been determined. Assay conditions have been established so that enzyme activity can be measured in crude homogenates. In adult male rats, the highest concentrations of activity are found in the anterior and neurointermediate lobes of the pituitary. Lower concentrations of activity are found in the hypothalamus, submandibular glands, and the rest of the brain. Enzyme activity is also easily assayed in serum. Taking into account the mass of each tissue, the submandibular glands and the brain are the major tissue sources of enzymatic activity; serum contains more enzymatic activity than is found in the pituitary gland. In all tissues and in serum, enzyme activity is stimulated by the addition of copper sulfate and ascorbate and is dependent on molecular oxygen. This activity is, therefore, referred to as peptidyl glycine alpha-amidating monooxygenase (PAM) activity. The pH optima for PAM activity in the pituitary and submandibular glands are near neutrality, whereas the pH optima for PAM activity in serum and hypothalamus are broad, with substantial activity at alkaline pH.