A reexamination of the cytochrome P-450-catalyzed free radical production from a dihydropyridine. Evidence of trace transition metal catalysis.

Radical production from 3,5-bis(ethoxycarbonyl)-4-ethyl-2,6-dimethyl-1,4- dihydropyridine (DDEP) in rat liver microsomes has been attributed to one-electron oxidation via cytochrome P-450 followed by extrusion of an ethyl radical. In the presence of the spin trap alpha-(4-pyridyl 1-oxide)-N-tert-butylnitrone (4-POBN), this radical was detected as the 4-POBN/ethyl radical adduct by electron paramagnetic resonance spectroscopy. The addition of catalase resulted in over a 50% decrease in radical production. The concentration of the 4-POBN/ethyl radical adduct increased about 9-fold in the presence of EDTA and approximately 2-fold in the presence of either diethylenetriaminepentaacetic acid (DTPA) or deferoxamine mesylate. When DDEP was incubated with deferoxamine mesylate-washed microsomes and NADPH in Chelex-treated incubation buffer, much less 4-POBN/ethyl radical formation occurred. Addition of either DTPA, EDTA, or the ferric complexes of these chelators greatly stimulated production of the 4-POBN/ethyl radical adduct in this system. These results suggest that the ethyl radicals produced from DDEP in a microsomal system arise via trace transition metal-catalyzed reactions.