Another look at the monoamine oxidases and the monoamine oxidase inhibitor drugs

Abstract

Publisher Summary This chapter discusses the monoamine oxidases (MAO) and the MAO inhibitor drugs. Phenylethylamines and indolethylamines appear to be the endogenous substrates for tissue MAO. The m-O-methylated catecholamine metabolites have lower values than the parent amines. Normetanephrine is a preferred substrate for MAO A as is norepinephrine. Oxygen is also a required substrate and flavin-adenine dinucleotide is found in most MAOs. The relative efficiency with which the enzyme oxidizes low concentrations of amine increases as the oxygen concentrations fall rendering the enzyme relatively insensitive to large fluctuations of local oxygen concentrations. When crude preparations of MAO or MAO prepared by polyacrylamide gel electrophoresis are subjected to high temperatures and then tested for activity, a multiphasic loss of activity is observed suggesting the existence of multiple forms of the enzyme. By using specific inhibitor drugs and substrates, it appears that MAO B is more sensitive to heat than MAO A. The thermostability of the enzymes might be a useful indicator of the molecular nature of the enzymes. It is demonstrated that the thermostability of MAO preparations was dependent on the presence of phospholipid.