Nadph Dependent Cytoghbdme P-450 Reactions = Mode Oe Inhibition By The N—Butanol Fraction Of Desmodium Adscendens

ABSTRACT The n-butanol fraction (nBF) of Desmodium adscendens , a plant used for the management of asthma, is' an inhibitor of NADFH-dependent cytochrcme P-450 (CYP) reactions. Its mechanism of action as an inhibitor is hcwever not kncwn. In this stud/, flavoprotein reductase activity, spectral changes associated with binding and spectral prcperties of reduced cytochrcme c and CYP were used to investigate the mode of inhibition. nBF reduced cytochrome c but not CYP directly. In the presence of NADPH, the rate of formation of CYP2* (the reduced form of CYP) was enhanced fcy the addition of nBF. These effects were observed in reactions without substrate, indicating that in the absence of a substrate, nBF does not prevent the NADPH reduction of CYP, but rather enhances it. In the presence of a substrate, as exemplified fcy a spectrophotcmetric assay of the ERCD reaction, nBF was found to change the spectrum of oxidized CYP. The results indicate that in the presence of nBF, the binding site of CYP is altered to prevent substrate binding. xiv Therefore, the node of inhibition of nBF relates to substrate binding, nff could be inhibiting the NADPH-dependent CYP reactions ky binding to CYP and preventing substrate binding. nBF could also inhibit the flow of electrons frcm NADFH to substrate-bound CYP fcy interfering with the flavoprotein reductase activity when it is bound to CYP. Cn the other hand, nBF could have the same effect cn the rate of reduction of CYP to CYP2' in the presence of a substrate as it did in the absence of a substrate, leading to the formation of the CYP-Q-substrate ternary carplex, in which case the effect of nBF would be to prevent the breaking up of the ternary complex to form products, the build-up of which causes the substrate to rsnain unmetabolized.