Mechanism of monoclonal antibody inhibition/stimulation of reactions catalyzed by cytochrome P450IIB1.

We describe two monoclonal antibodies (MAbs) against rat cytochrome P450IIB1 and investigate the mechanisms by which they influence P450IIB1-mediated catalysis. MAb ce9 partially inhibits the activities toward p-nitroanisole, 7-ethoxycoumarin, and benzphetamine as well as NADPH oxidation. These findings can be explained by the observation that ce9 cross-links P450 to form large aggregates resulting in the inhibition of the functional interaction with NADPH cytochrome P450 reductase. Binding of ce9 to P450IIB1 does not affect the spin state of the P450 heme, as revealed by comparing the magnetic circular dichroism (MCD) spectra of free and antibody-bound P450IIB1. On the other hand, the second antibody tested, MAb 14E10, induces a remarkable low to high spin transition upon binding to P450IIB1, as shown by MCD difference spectroscopy. This MAb stimulates activities toward p-nitroanisole and 7-ethoxycoumarin without affecting the rate of NADPH oxidation. This observation indicates that MAb 14E10 may increase the efficiency of electron utilization by P450IIB1. Benzphetamine metabolism remains unchanged in the presence of MAb 14E10.