A peptide alpha-amidating enzyme was purified to apparent homogeneity from porcine pituitary. This enzyme is a glycoprotein with a mol wt of 64,000, a metal prosthetic group, and a dependence upon ascorbate and molecular oxygen. The purified enzyme has a strong preference for peptides ending in glycine. It also catalyzes the oxidation of valylglycine bonds more rapidly than prolylglycine bonds, and demonstrates a primary isotope effect greater than 5 when the alpha-hydrogens of glycine are replaced by deuterium. Kinetic analysis is consistent with a ping-pong or double displacement catalytic mechanism in which both the peptide substrate and ascorbate are competitive inhibitors with respect to each other. With respect to its kinetic properties, catalytic mechanism, and cofactor requirements, the purified amidating enzyme is very similar to dopamine beta-hydroxylase, a finding which supports the previous suggestion that the peptide alpha-amidating enzyme be classified as a peptidyl glycine monooxygenase.