COMPARATIVE-STUDY OF 2 HIGHLY PURIFIED FORMS OF LIVER MICROSOMAL CYTOCHROME-P-450 - CIRCULAR-DICHROISM AND OTHER PROPERTIES

The 2 major forms of rabbit liver microsomal cytochrome P-450, P-450LM2 and P-450LM4, differing in their absorption spectra, electrophoretic and immunochemical properties and substrate specificities, were further characterized by several methods. The 2 cytochromes had different CD [circular dichroism] spectra in the ferric state but similar spectra when reduced. Upon conversion of P-450LM2 to P-420 by treatment with sodium dodecyl sulfate, the CD spectrum was greatly diminished except in the far UV region; the conversion of P-450LM4 to P-420 with this detergent resulted in a spectrum with a new positive band in the visible region. P-450LM4 had a much higher tryptophan content than P-450LM2; the fluorescence spectra of the proteins were similar in magnitude. Upon denaturation, the fluorescence of P-450lM4 increased, indicating a large quenching effect in the native protein. Studies on the interaction of dilauroylglyceryl-3-phosphorylcholine with the cytochromes showed that P-450LM2 gave a much stronger Type I difference spectrum than did P-450LM4. This phospholipid had no significant effect on the state of aggregation of the cytochromes as judged by calibrated gel filtration. The CD spectra of P-450LM2 and P-450LM4 were unchanged in the visible region but were enhanced in the far UV region upon the addition of phosphatidylcholine. The results indicated an increase in .alpha.-helical content, particularly with P-450LM4, in the presence of the phospholipid.