Monoamine oxidase. II. Copper, one of the prosthetic groups of plasma monoamine oxidase.

The nature of the prosthetic groups of monoamine oxidase has been the subject of much conjecture, and definitive experimental results have not been obtained. The results of Tabor et al. (1) suggest that pyridoxal phosphate may be the coenzyme of the enzyme, and Gorkin and Romanova (2) have suggested that the enzyme is a metalloenzyme. Others (3, 4) have proposed that monoamine oxidase is a flavoenzyme. In a previous paper (5)) we reported on a procedure for obtaining crystalline plasma monoamine oxidase. The availability of the crystalline enzyme has opened the way to a critical examination of the prosthetic groups of monoamine oxidase. In this paper, we wish to show that plasma monoamine oxidase is a copper enzyme and that the copper is involved in enzyme activity. While this work was in progress, studies similar in some respects were reported by Mann (6)) who used a partially purified preparation of plant amine oxidase.