Cytochrome P-450LMz was isolated from rabbit liver microsomes in a form which was shown to be homogeneous in AcA-22 Ultrogel and ultracentrifugation studies. The molecular mass determined by sedimentation equilibrium roughly corresponded to hexamer composed of 56 kDa monomers. Hexamer structure of the cytochrome was directly demonstrated by electron microscopic study. In the cytochrome P-450LM1 hexamer. monomers seem to be arranged in two layers (three monomers in the layer) in such a way that each monomer occupies a position at the vertices of a triangular antiprism with a 32 point group symmetry.Cytochrome P-450 Quaternary structure Electron microscop?

The effects of bifunctional cross‐linking reagents on the purified hexameric cytochrome P‐450LM2 in an aqueous medium and on the proteoliposomal cytochrome P‐450LM2 have been compared. In both cases, cross‐linking is shown to result in the appearance of a range of additional protein bands in SDS electrophoretograms. The number and the position of these bands seem to be similar in the solubilized and in the proteoliposomal cytochromes. No additional bands appear when the purified cytochrome P‐450 was pretreated with ?2? Emulgen 913, which decomposes cytochrome P‐450LM2 hexamers. The results indicate that the membrane‐bound cytochrome P‐450 can exist in the oligomeric (presumably hexametric) form.

Purified cytochrome P-450,,, was found to be monodisperse in 20% glycerol by analytical ultracentrifugation. Its S,,,, value was qmte similar to that of hexameric P-450,,2.At lower glycerol concentrations the P-450,,, ohgomers showed a tendency to aggregate. The P-450LM4 oligomers were immobilized on Ultrogel A4 under conditions allowing only one covalent hnk to the matrix per oligomer. In the presence of SDS, the oligomers dissociated leaving only 116th of the initial amount of bound protein on the matrix. suggesting that the purified P-450LM, is a hexamer. This was confirmed by electron microscopy. The quaternary structure of the P-450,,, was similar to that demonstrated earher for P-450LMz.

Cytochromes P450 represent a numerous family of heme-containing enzymes belonging to the group of monooxygenases. In prokaryotes, cytochromes P450 usually perform a plastic function, whereas in eukaryotes their functions are very diverse. Mammalian cytochromes P450 are components of membranes and are involved in biosynthesis and metabolism of many physiologically active substances; moreover, these cytochromes are unique in their ability to catalyze biotransformation of xenobiotics, i.e. metabolize substances of foreign origin (drugs, toxins, environmental pollutants). The latter promotes elimination of xenobiotics, but sometimes intermediates of their metabolism are even more toxic and dangerous than the original xenobiotics per se. Some catalytic features of cytochromes P450 still need unambiguous explanation, i.e. broad substrate specificity, diversity of catalytic reactions, and unusual kinetics. Under some conditions, cytochromes P450 can produce reactive oxygen species, and this is another problem attracting increasing attention. In this respect, a recent finding in mitochondria of analogs of microsomal cytochromes P450 seems especially intriguing; it was postulated that P450 can be responsible for mitochondrial dysfunction, cell apoptosis, and pathogenesis of some diseases. In this paper the present state of the art concerning these problems is considered.