Description of Research Expertise

Research Summary
Steroid Hormone Transforming Aldo-Keto Reductases
The aldo-keto reductase (AKR) superfamily contains mammalian hydroxysteroid dehydrogenases (HSDs). For each sex steroid there are a pair of HSDs, which by acting as reductases or oxidases can convert potent steroid hormones into their cognate inactive metabolites or vice versa. When found in steroid target tissues they can regulate the occupancy and trans-activation of steroid hormone receptors, providing a pre-receptor regulation of steroid hormone action. Many HSDs are considered therapeutic targets. For example, prostatic oxidative 3alpha-HSD converts 3alpha-androstanediol (a weak androgen) to 5alpha-dihydrotestosterone (a potent androgen) and regulates occupancy of the androgen receptor. cDNA cloning performed in Dr. Penning's laboratory indicates that rat liver and human prostate 3alpha-HSDs share 75% sequence identity. Crystal structures for the apoenzyme, the EoNADP+ and the EoNADP+ testosterone complexes have been determined for the rat liver enzyme. The group is relating structure of HSDs to function, using the techniques of site-directed mutagenesis, affinity-labeling, and phage-display. Active site information is being exploited to design selective inhibitors for human HSDs in the AKR superfamily.

Dihydrodiol Dehydrogenases and Polycyclic Aromatic Hydrocarbon (PAH) Activation
Dihydrodiol dehydrogenases are members of the AKR superfamily. They convert PAH-trans-dihydrodiols (proximate carcinogens) to reactive and redox active o-quinones. By entering into futile redox-cycles the o-quinones can amplify the production of reactive oxygen species (e.g., superoxide anion, hydrogen peroxide and hydroxyl radical). The pro-oxidant state may provide a mechanism by which PAH can act as complete carcinogens. Similar metabolic activation has been observed for the structurally related catechol estrogens and diethylstilbestrol. The cytotoxicity and genotoxicity of PAH o-quinones are being studied. Methods include cell culture, high-resolution NMR, EPR, mass-spectrometry, PAH-DNA adduct chemistry, and mutagenesis paradigms.

Mechanistic Studies on Prostaglandin H2 Synthase
Prostaglandin H2 synthase converts arachidonic acid to prostaglandins which mediate symptoms of inflammation and is the target for nonsteroidal anti-inflammatory drugs. Existing mechanisms of catalysis involve the generation of intermediate-I (FeV+) and intermediate II (FeIV+) of peroxidases with concomitant production of a tyrosyl radical to activate arachidonic acid. Stopped-flow and EPR studies are being used to assign unique spectral intermediates in synthases that lack the peroxidase activity but can still make prostaglandins.

Also, visit www.med.upenn.edu/akr