Purification of a peptidylglycine alpha-amidating enzyme from transplantable rat medullary thyroid carcinomas

A peptidyl glycine alpha-amidating activity has been isolated from total tissue extracts of rat medullary thyroid carcinoma (MTC). Purification of the activity by ammonium sulfate fractionation, Sephacryl S-300 chromatography, and strong anion-exchange chromatography at pH 6.0 has resolved at least four peaks of activity. The activity associated with peak III has been further purified to apparent homogeneity by strong anion-exchange chromatography at pH 8.0. The purified peak III enzyme has an apparent molecular mass of 75,000 Da as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The identity of the 75,000-Da band as the alpha-amidating enzyme has been confirmed by recovery of activity from a nondenaturing polyacrylamide gel. The enzyme is catalytically active as a monomer, exhibits a pH optimum between 5.0 and 5.5, and has a turnover number of 300 min-1 for N-dansyl-Tyr-Val-Gly amidation at pH 5.5. The larger size, more acidic pH optimum, and higher specific activity distinguish the purified peak III rat MTC enzyme from the enzymes isolated from bovine and porcine pituitary or from frog skin.