Exocrine secretion granules contain peptide amidation activity.

Exocrine secretion granules from the rat parotid gland contain a carboxyl-terminal peptide alpha-amidation enzyme resembling closely an enzyme from the pituitary (peptidyl-glycine alpha-amidating monooxygenase) that functions in post-translational processing of secretory polypeptides within neural and endocrine secretion granules. alpha-Amidation is a characteristic (often essential) chemical feature of a variety of biologically active regulatory peptides in animals. The parotid and pituitary activities exhibit very similar ascorbate and copper requirements, pH dependence, and kinetic properties. Further, like the pituitary enzyme(s), the parotid activity is found predominantly in secretion granule content and is discharged by exocytosis. These results establish the presence of a novel enzyme in exocrine secretion granules and suggest a potential role of the L-ascorbic acid contained in parotid granules. Two additional findings--the detection of similar levels of amidation activity in purified secretion granule fractions from other exocrine glands and the observation, in parotid granule fractions, of a B-type carboxypeptidase activity similar to that involved in post-translational processing in other systems--form a rational basis for considering whether exocrine secretion granules (like their neural and endocrine counterparts) serve as post-translational processing sites. The identity and functional role of the modified polypeptides remain to be determined.