Differences in the binding of substrates to cytochromes P-450b and P-450e from rat liver microsomes.

Two phenobarbital-induced isoenzymes of the rat cytochrome P-450 have been isolated using a modification of a method described earlier. Titration of cytochromes P-450 with different compounds (followed by changes in the second derivative spectrum in the near ultraviolet region) reveals that tyrosyl residues are affected. These residues behave differently in the isoenzymes P-450b and P-450e. In the former, after addition of a substrate the tyrosyl residues become hidden in the nonpolar parts of the protein; however, then become accessible with increasing substrate concentration. In the latter, the exposure to a polar medium decreases with increasing substrate concentration.