Product inhibition studies and the reaction sequence of rabbit adrenal norepinephrine N-methyl transferase isozymes

Abstract

The effects of reaction products on the steady-state kinetic properties of the five charge isozymes of rabbit adrenal norepinephrine N-methyl transferase have been investigated. Qualitative and quantitative differences were observed for the isozymes. The only characteristic that was common to all isozymes was the competition between S-adenosylmethionine and S-adenosylhomocysteine for the binding site. In most instances, the product inhibition constants were sufficiently low to suggest that product inhibition may be an important factor in regulating the activities of the isozymes. A reaction model is proposed for rabbit adrenal norepinephrine N-methyl transferase which is consistent with results observed in investigations of the steady-state kinetic properties of the five charge isozymes. The proposed model is that of an ordered sequential reaction sequence in which the active center contains a binding site for S-adenosylmethionine and S-adenosylhomocysteine, and a binding site for norepinephrine and epinephrine. The proposed model includes the formation of a number of abortive complexes between enzyme and substrate and product, but not all of the abortive complexes are significant kinetically in the case of some of the isozymes. The differences in the steady-state kinetic characteristics of the isozymes are attributed to differences in the magnitudes of the rate constants of some of the individual steps.