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. von Weymarn, L.B., Zhang, Q.-Y., Ding, X. and Hollenberg, P.F., "Effects of 8-methoxypsoralen on cytochrome P450 2A13: Potential implications for tobacco-induced lung cancer", Carcinogenesis, 2005, 26, 621.

2. Lin, H.-l., Kent, U.M. and Hollenberg, P.F., "The grapefruit juice effect is not limited to P450 3A4: Evidence for bergamottin-dependent inactivation, heme destruction and covalent binding to protein in P450s 2B6 and 3A5",  J. Pharmacol. Exp. Ther., 2005, 313, 154.

3. Bhakta, M.N., Hollenberg, P.F. and Wimalasena, K., "P450/NADPH/O2 and P450/PhlO-catalyzed N-dealkylations are mechanistically distinct", J. Amer. Chem. Soc., 2005, 127, 1376.

4. Blobaum, A.L., Harris, D.L. and Hollenberg, P.F., "P450 active site architecture and reversibility: The inactivation of cytochromes P450 2B4 and 2B4 T302A by tert -butyl acetylenes", Biochemistry, 2005, 44, 3831.

5. Bumpus, N.N., Sridar, C., Kent, U.M. and Hollenberg, P.F., "The naturally occurring P450 2B6 K262R mutant of P450 2B6 exhibits alterations in substrate metabolism and inactivation", Drug Metab. Dispos., 2005, 33, 795.

6. Lin, H.-l., Zhang, H., Waskell, L. and Hollenberg, P.F., "The highly conserved Glu-149 and Tyr-190 residues contribute to peroxynitrite-mediated nitrotyrosine formation and the catalytic activity of cytochrome P450 2B1", Chem. Res. in Toxicology, 2005, 18, 1203.