Hide/Show Apps

Characterization and modulation by drugs and other effectors of bovine liver microsomal flavin monoxygenase(fmo)

Başer, Deniz Fulya
The flavin-containing monooxygenases (FMO; E.C. are microsomal NADPH and oxygen-dependent flavoprotein enzymes that catalyze the oxidation of a wide variety of xenobiotics, including drugs and environmental toxicants. Nucleophiles containing nitrogen, sulfur, phosphorus and selenium heteroatoms are the substrates of FMO. Bovine liver microsomal FMO enzyme activity was characterized using methimazole as substrate, which is a highly specific substrate for FMO. From 12 different bovine liver samples, microsomes were prepared and the average specific activity of bovine liver microsomal FMO was found to be 2.37 ? 0.30 nmol/min/mg (Mean ? SE, n=12). The rate of reaction was linear up to 0.5 mg of bovine liver microsomal protein. The maximum FMO enzyme activity was detected at 37 ?C and at pH 8.0. Effects of detergents; Triton X-100 and Emulgen 913, on FMO activity were determined and found that enzyme activity increased by the addition of either detergent at all concentrations (0.1%-1.0%). The apparent Vmax and Km values of bovine liver microsomal FMO for methimazole substrate were found as 1.23 nmol/min/mg and 0.11 mM, respectively. Thermostability of bovine liver microsomal FMO was studied at four different temperatures; 24 ?C, 37 ?C, 50 ?C and 65 ?C. The incubation time required for the complete loss of enzyme activity was 5 minutes at 65 ?C, 10 minutes at 50 ?C and 6.5 hours at 37 ?C. 68 % of the activity was still detectable at the end of 53 hours at 24 ?C. Bovine liver microsomal activity towards two drug substrates, imipramine and chlorpromazine, was also determined and found to be 3.73 and 3.75 nmol NADPH oxidized/min/mg, respectively. Effects of two drug substrates, imipramine and chlorpromazine, on bovine liver microsomal FMO-catalyzed methimazole oxidation activity was also studied and found that they inhibit FMO activity at all concentrations studied.