Inhibition by lead of phenylethanolamine-N-methyltransferase

Phenylethanolamine-N-methyltransferase (PNMT) catalyzes the conversion of norepinephrine to epinephrine. The enzyme, obtained from bovine adrenal gland, was incubated with PbCl2 at 23 degrees for various times prior to assay at 37 degrees. Inhibition developed slowly and reached a maximum after 45 min. In the presence of 4.5 nmoles PbCl2 (15 microM), 5.8 microgram protein was inhibited 50%, and inhibition was complete at 18 nmoles PbCl2 (60 microM). At maximum inhibition the PbCl2: protein ratio was 3.1 nmoles PbCl2/microgram protein. In the presence of PbCl2, the graph of enzyme activity versus protein concentration intercepted the abscissa to the right of the origin, indicating that lead is an irreversible or very slowly reversible inhibitor. The activity of PNMT which had been exposed to PbCl2 (2.6 or 5.2 nmoles PbCl2/microgram protein) was not restored by the addition of EDTA, DL-penicillamine or 1,3-dithiothreitol even when the concentration of the chelator was in 10 to 200-fold mole excess over PbCl2. DL-Penicillamine and 1,3-dithiothreitol were unable to prevent PbCl2 inhibition of the enzyme when combined with PbCl2 1 hr before addition of enzyme. EDTA could prevent 40% of PbCl2 inhibition but a decrease in total enzyme activity was noted in the presence of this chelator. Dialysis of the PbCl2-inhibited enzyme against buffer alone or buffer plus DL-penicillamine did not result in restoration of PNMT activity.