The activity of rat liver monoamine oxidase was increased in an uncompetitive manner with increasing concentrations of oxygen. However, the value of the Michaelis constant for oxygen, estimated from determinations of enzyme activity with 6 oxygen concentrations and 5–6 amine substrate concentrations, was dependent upon the amine substrate used to assay for activity. In an attempt to determine the nature of these differences, the value of the Michaelis constants for oxygen have been related to the k cat (rate constant of the limiting step in the overall enzyme-catalysed reaction) values of the enzyme towards the different amine substrates. The results are consistent with the hypothesis that the two forms of monoamine oxidase in rat liver are not independent enzyme forms, but interact one with the other in some way.
