Fast photochemical reactions of cytochrome P450 at subzero temperatures.

Several reactions of the cytochrome P450 multi-step cycle have been studied by fast light activation combined with subzero temperatures. A flash device was adapted to an Aminco-Chance DW 2 spectrophotometer equipped for subzero temperature thermostatisation. The first electron can be introduced into the cycle by non specific reducing agents such as reduced flavin mononucleotide (FMNH2) or methylviologen radical (MV.). This first reduction remains a fast process even at subzero temperatures. The oxy-compound Fe2+-O2 can thus be formed either directly from Fe2+ or via the photodissociation of the carboxy-ferro adduct. Fe2+-O2 is stable at subzero temperatures towards spontaneous autoxidation as well as further reduction by FMNH2 or MW.. In addition, the recombination of CO after flash photodissociation of Fe2+-CO was used to study in more details the specific behaviors of the purified microsomal cytochrome.