CHARACTERIZATION OF BRAIN BETA-CARBOLINE-2-N-METHYLTRANSFERASE, AN ENZYME THAT MAY PLAY A ROLE IN IDIOPATHIC PARKINSONS-DISEASE

The activity of beta-carboline-2-N-methyltransferase results in the fo rmation of neurotoxic N-methylated beta-carbolinium compounds. We have hypothesized that these N-methylated beta-carbolinium cations may con tribute to the development of idiopathic Parkinson's disease. This rep ort describes experiments undertaken to optimize assay conditions for bovine brain beta-carboline-2-N-methyltransferase activity. The activi ty of beta-carboline-2-N-methyltransferase is primarily localized in t he cytosol, has a pH optimum of 8.5-9, and obeys Michaelis-Menten kine tics with respect to its substrates, 9-methylnorharman (9-MeNH) and S- adenosyl-L-methionine (SAM). Kinetic constants, K-M and V-max, with re spect to 9-MeNH, are 75 mu M and 48 pmol/h/mg protein, respectively. T he K-M for SAM is 81 mu M and the V-max is 53 pmol/h/mg protein. In ad dition, enzyme activity is inhibited by S-adenosyl-L-homocysteine (SAH ) or zinc, and is increased 2-fold in the presence of iron or manganes e. Enzyme characterization is a prerequisite to the purification of th is N-methyltransferase from bovine brain as well as comparison of its activity in human brain from control and Parkinson's disease individua ls.