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J. Biol. Chem., Vol. 267, Issue 21, 14669-14676, Jul, 1992
AI Voznesensky and JB Schenkman
Attempts to covalently link NADPH-cytochrome P450 reductase to cytochrome
P450 2B4 using a water-soluble carbodiimide, 1-ethyl-3-(3-
dimethylisopropyl)carbodiimide, were unsuccessful, despite the fact that
under the same conditions about 30% of P450 2B4 could be covalently linked
with cytochrome b5 in a functionally active complex (Tamburini, P. P., and
Schenkman, J. B. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 11-15). This
suggested that the functional electron transfer complex between P450 2B4
and reductase is not stabilized by electrostatic forces. Raising the ionic
strength of the medium is disruptive to salt bridges and was used to
further test whether P450 2B4 and the reductase form charge-pairing
complexes. Instead of inhibiting electron transfer, high ionic strength
increased the apparent fast phase rate constant and the fraction of P450
2B4 reduced in the fast phase. The possibility that electron transfer
between NADPH- cytochrome P450 reductase and P450 2B4 is diminished by
charge repulsion was examined. Consistent with this hypothesis, the Km of
P450 2B4 for reductase was decreased 26-fold by increasing the ionic
strength from 10 to 100 mM sodium phosphate without affecting the Vmax. The
rate of benzphetamine N-demethylation also was increased by elevation of
the ionic strength. Electron transfer from the reductase to other charged
redox acceptors, e.g. cytochrome c and ferricyanide, was also stimulated by
increased ionic strength. However, no similar stimulation was observed with
the uncharged acceptor 1,4-benzoquinone. Polylysine, a polypeptide that
binds to anionic sites, enhanced electron transfer from NADPH to
ferricyanide and the apparent fast phase of reduction of cytochrome P450.
The results are consistent with the hypothesis that charges on
NADPH-cytochrome P450 reductase and cytochrome P450 decrease the stability
of the electron transfer complex.
The cytochrome P450 2B4-NADPH cytochrome P450 reductase electron transfer complex is not formed by charge-pairing
Department of Pharmacology, University of Connecticut Health Center, Farmington 06030.
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