Faculty

Dr. Hollenberg's laboratory is primarily concerned with investigations of the microsomal cytochrome P450-dependent mixed-function oxidases found in most mammalian tissues. These enzymes catalyze the metabolism of a wide variety of xenobiotics including drugs, anesthetics, pesticides, chemical carcinogens, and organic solvents as well as endogenous compounds of great physiological importance including steroids, fatty acids, retinoids, eicosanoids, and lipid hydroperoxides. They also serve as nitric oxide (NO) synthases. Understanding the catalytic mechanisms, regulation, and roles of the P450s in the metabolism of endogenous and exogenous compounds is of great importance to the fields of pharmacology, endocrinology, toxicology and oncology.

Epidemiological studies suggest most human cancers are due to exposure to chemicals in the environment which cause cancer. In general, these chemicals are biologically inert and require activation by the drug-metabolizing enzymes in the target tissue to express this carcinogenic potential. Compounds known to cause cancer as a result of metabolic activation by P450 include polycyclic aromatic hydrocarbons, nitrosamines, and aflatoxins. Thus, a primary objective of the laboratory is to gain a better understanding of mechanisms by which cells activate environmental chemicals to reactive forms that cause cancer and ultimately to develop approaches which could be used to protect humans against potentially carcinogenic or toxic chemicals in the environment.

More than 30 different forms of cytochromes P450 have been found in human tissues. Each form exhibits its own unique substrate specificity and even when two different forms metabolize the same substrate, they generally give very different product profiles. For some time, a major interest in our laboratory has been concerned with the relationships between the structures of the active sites of various forms of P450 and their catalytic functions. For these studies we have been using purified P450 2B1 and P450 2B4, the major forms of P450 induced by phenobarbital in rats and rabbits, respectively. We are also studying P450 2E1, a form which is thought to play a major role in the metabolic activation of many low molecular weight carcinogens and other toxic agents.

Representative Publications

1. Zhang, H., Lin, H.L., Walker, V.J., Hamdane, D., and Hollenberg, P.F.,"tert-Butylphenylacetylene is a potent mechanism-based inactivator of cytochrome P450 2B4: Inhibition of cytochrome P450 catalysis by steric hindrance", Mol. Pharmacol., 2009, In Press.

2. Lin, H.L., Zhang, H., Noon, K.R., and Hollenberg P.F., "Mechanism-based inactivation of CYP2B1 and its F-helix mutant by two tert-butyl acetylenic compounds: covalent modification of prosthetic heme versus apoprotein", J. Pharmacol. Exp. Ther., 2009, In Press.

3. Zhang, H., Kenaan, C., Hamdane, D., Hui Bon Hua, G., Hollenberg, P.F.,"Effect of Conformational Dynamics on Substrate Recognition and Specificity as Probed by the Introduction of a De Novo Disulfide Bond into Cytochrome P450 2B1", J. Biol. Chem., 2009, In Press.

4. Sheng, X., Zhang, H., Im S.C., Horner, J.H., Waskell, L., Hollenberg, P.F., Newcomb, M., "Kinetics of Oxidation of Benzphetamine by Compounds I of Cytochrome P450 2B4 and Its Mutants" J. Amer. Chem. Soc., 2009, 131, 2971.

5. Sheng, X., Zhang, H., Hollenberg, P.F., Newcomb, M., "Kinetic isotope effects in hydroxylation reactions effected by cytochrome P450 compounds I implicate multiple electrophilic oxidants for P450-catalyzed oxidations", Biochemistry, 2009, 48, 1620.

6. Snider, N.T., Nast, J.A., Tesmer, L.A., Hollenberg, P.F., "A Cytochrome P450-derived Epoxygenated Metabolite of Anandamide is a Potent Cannabinoid Receptor 2 - Selective Agonist", Mol. Pharmacol., 2009, 75, 965.

7. Lin, H.L., Zhang, H., Hollenberg, P.F. "Metabolic Activation of Mifepristone (RU486) by Mammalian P450s and the mechanism-based Inactivation of Human CYP2B6", J. Pharmacol. Exp. Ther., 2009, 329, 26.

8. Shebley, M., Kent, U.M., Ballou, D.P., Hollenberg. P.F., "Mechanistic Analysis of the Inactivation of Cytochrome P450 2B6 by Phencyclidine: Effects on Substrate Binding, Electron Transfer, and Uncoupling.", Drug Metab. Dispos., 2009, 37, 745.