Molecular Pharmacology, Vol 8, 711-721, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

The Incomplete Conversion of Hepatic Cytochrome P-450 to P-420 by Mercurials

¹ Department of Biochemistry, The University of Texas Southwestern Medical School at Dallas, 5323 Harry Hines Blvd, Dallas, Texas 75235

The conversion of hepatic microsomal cytochrome P-450 to P-420 by the mercurial compound mersalyl (sodium o[(3-hydroxymercuri-2-methoxypropyl)carbamoyl]phenoxyacetate) was incomplete. In contrast, high concentrations of urea caused complete conversion of cytochrome P-450 to P-420. During the conversion the total heme concentration of microsomes did not change, but the sum of the cytochrome concentration measurable as P-450 and P-420 showed a net decrease. The lack of complete conversion of cytochrome P-450 to P-420 was independent of the nature of the mercurial sulfhydryl reagent employed and was largely unaffected by changes in the type of buffer, ionic strength, and the presence of type I or type II substrates. The loss of the ability to demonstrate a type I but not a type II binding spectrum paralleled the loss of cytochrome P-450. The conversion of cytochrome P-450 to P-420 could not be completely reversed by the addition of reduced glutathione. The transition of the iron of the cytochrome P-450 from a low-spin to a high-spin state, observed in the electron paramagnetic resonance spectrum, which occurs upon the mercurial conversion of cytochrome P-450 to P-420, was investigated and compared with both the spin state changes obtained by lowering the pH and the optically determined conversion of cytochrome P-450 to P-420 by mercurials.

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ACKNOWLEDGMENTS The author thanks Mrs. Nancy J. Mahanay for her technical assistance, and Dr. Ronald W. Estabrook for his interest and advice.