Here we describe for the first time the formation of a complex of reduced CYP102 (cytochrome P450 BM-3) heme domain with molecular oxygen. To stabilize the oxycomplex, the experiments had to be done under argon atmosphere at cryogenic temperatures (-25 degrees C) in the presence of 50% glycerol. The spectral properties of this species were different from those of another P450-type autosuffisant enzyme, i.e., the neuronal nitric oxide synthase, On the contrary, the oxyferrous complex of CYP102 possesses spectral properties similar to those of complexes of microsomal cytochromes P450, e.g., CYP2B4. (C) 1999 Academic Press.