Benzo(a)pyrene (BP) in liposomes and microsomes exhibits overlapping excimer bands peaked at 495 and 520 nm, respectively, in contrast to the single band of pyrene at 480 nm. Since BP is asymmetrical and pyrene is symmetrical, the dual maxima imply that the BP molecules in the excimer must be oriented either parallel or anti-parallel to each other. Time-resolved studies of BP oxidation by cytochrome P-450 show that the 520-nm component dissipates much faster than the 495-nm component during the first 30 min of metabolism, indicating that the initial P-450-BP interaction may be BP orientation sensitive. The binding of BP to the porphyrin group of a heme protein (e.g., hemoglobin or myoglobin) appears to be stronger under oxygen saturation than under CO<sub>2</sub> saturation conditions. The transfer of BP between the membrane and the porphyrin group, as measured by means of excimer/monomer ratios, appears to be significantly different from that of pyrene. As the liposome-to-protein ratio increases, the excimer/monomer of BP steadily decreases, whereas that of pyrene increases and reaches a plateau. The significance of this phenomenon in their mechanisms of metabolism will have to be investigated further.

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