Characterization of brain beta-carboline-2-N-methyltransferase, an enzyme that may play a role in idiopathic Parkinson's disease.

The activity of beta-carboline-2-N-methyltransferase results in the formation of neurotoxic N-methylated beta-carbolinium compounds. We have hypothesized that these N-methylated beta-carbolinium cations may contribute to the development of idiopathic Parkinson's disease. This report describes experiments undertaken to optimize assay conditions for bovine brain beta-carboline-2-N-methyltransferase activity. The activity of beta-carboline-2-N-methyltransferase is primarily localized in the cytosol, has a pH optimum of 8.5-9, and obeys Michaelis-Menten kinetics with respect to its substrates, 9-methylnorharman (9-MeNH) and S-adenosyl-L-methionine (SAM). Kinetic constants, KM and Vmax, with respect to 9-MeNH, are 75 microM and 48 pmol/h/mg protein, respectively. The KM for SAM is 81 microM and the Vmax is 53 pmol/h/mg protein. In addition, enzyme activity is inhibited by S-adenosyl-L-homocysteine (SAH) or zinc, and is increased 2-fold in the presence of iron or manganese. Enzyme characterization is a prerequisite to the purification of this N-methyltransferase from bovine brain as well as comparison of its activity in human brain from control and Parkinson's disease individuals.