Microsomal oxidation of 2-dimethylamino-3-chloro-1,4-naphthoquinone. The possibility of substrate activation by cytochrome P-450

Abstract

1. 2-Dimethylamino-3-chloro-1,4-naphthoquinone (DCNQ) is bound to microsomal cytochrome P-450 as a type I substrate (λ_max = 391 nm, λ_min = 420 nm). The K_s is 40.2 μm.

2. In a rat-liver microsomal system, the N-demethylation of DCNQ produces formaldehyde (rate 225 pmol/min per mg of protein). Induction by phenobarbital increases the rate of formation, while addition of metyrapone and SKF-525A into the system decreases the rate by 52% and 35%, respectively.

3. The microsomal N-demethylation of DCNQ is not inhibited by CO. Under full anaerobiosis, the microsomal oxidation of DCNQ again gives formaldehyde (rate 416 pmol/min per mg of protein). The anaerobic oxidation of DCNQ is inhibited by metyrapone and SKF-525A.

4. The microsomal, chemical and electrochemical reduction of DCNQ to the corresponding semiquinones and hydroquinones have been studied. Non-enzymic DCNQ reduction is insufficient for the formation of formaldehyde. Under anaerobic conditions the microsomal DCNQ oxidation is assumed to occur via the intramolecular oxazole bond which is then hydrolysed, yielding formaldehyde. This may be a new example of substrate activation by cytochrome P-450.