Monoamine oxidases deaminate many amines, including neurotransmitters, by oxidation followed by spontaneous breakdown of the imine product. The reduced enzyme is reoxidized slowly by oxygen, but in the presence of amines, the rate of reoxidation is markedly enhanced. The extent of enhancement depends on the amine substrate, kynuramine enhancing the rate 125-fold, but 5-hydroxytryptamine only 6-fold. Here we describe the properties of human liver monoamine oxidase A which has been cloned into and overexpressed in yeast. The purified enzyme has a higher Km for oxygen than does the placental enzyme, but the steady-state parameters for the endogenous amines are the same. Tertiary amines are oxidized at slightly different rates by the two enzymes. The consequences of the branched pathway mechanism with substrate-dependent enhancement of reoxidation for the steady-state levels of the various enzyme species is discussed.