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.EXPERIMENTAL PROCEDURE ~~~uterials-Partially purified and crystallinc plasma monoamine oxidase preparations were prepared as previously described (5).Hydroxylapatite was prepared by the method of Tiselius, Hjerten, and Levin (7).Sodium diethyldithiocarbamate and 2,2'-bipyridyl were purchased from Eastman Organic Chemicals.Cuprizone,l neocuproine hydrochloride, and o-phenanthroline were purchased from the G. Frederick Smith Chemical Company.Sodium azide, 8-hydroxyquinoline sulfate, and dithiozone were obtained from the Matheson Company, Inc., and sodium thioglycolate was a product of Wako Pharmaceutical Company, Osaka, Japan.p-Chloromercuriphenylsulfonic acid was purchased from the Sigma Chemical Company.Distilled and deionized water was used during the enzyme purification as well as in all experiments described in this paper. MethodsEnzyme Assay and Protein Determination-The enzyme assays and the protein determinations were performed spectrophotometrically as previously described (5).One unit was defined