Absorption spectra of highly purified liver microsomal cytochrome P-450 in non-equilibrium conformational states at low temperatures.

Absorption spectra of highly purified liver microsomal cytochrome P-450 in non-equilibrium states were obtained at 77 K by reduction with trapped electrons, formed by gamma-irradiation of the water-glycerol matrix. In contrast to the equilibrium form of ferrous cytochrome P-450 with the heme iron in the high-spin state the non-equilibrium ferrous state has a low-spin heme iron. The absorption spectrum of the non-equilibrium ferrous cytochrome P-450 is characterized by two bands at 564 (alpha-band) and 530 nm (beta-band). When the temperature is increased to about 278 K this non-equilibrium form of the reduced enzyme is relaxed to the corresponding equilibrium form with a single absorption band at 548 nm in the visible region characteristic for a high-spin heme iron.