Mobility and clusterlike organization of liposomal cytochrome P-450 LM2: saturation transfer EPR studies

Rotational diffusion of the electrophoretically homogenenous isozyme cytochrome P-450 LM2 from rabbit liver microsomes has been studied in buffer solution and in phospholipid vesicles by means of saturation transfer EPR spectroscopy. Sulfhydryl groups of the enzyme were selectively modified using a maleimide spin label. The effective rotational correlation time of 220 ns for the rotation of cytochrome P-450 in buffer solution is consistent with the fact that the purified free enzyme occurs as an oligomeric (6-8 monomers) aggregate. Further, the clusters rotate almost isotropically and therefore are in a first approximation spherically shaped. The effective correlation time of about 180 microseconds observed strong immobilization thus evidencing protein aggregation within the membrane. The anisotropic character of the spectra indicates a nonspherical shape and/or anisotropic rotational motion of the cluster. The results are compared with corresponding data from cytochrome P-450 in microsomal form.