Cysteine-10 on 17β-Hydroxysteroid Dehydrogenase 1 Has Stabilizing Interactions in the Cofactor Binding Region and Renders Sensitivity to Sulfhydryl Modifying Chemicals
17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) catalyzes the conversion of estrone to the potent estrogen estradiol. 17β-HSD1 is highly expressed in breast and ovary tissues and represents a prognostic marker for the tumor progression and survival of patients with breast cancer and other estrogen-dependent tumors. Therefore, the enzyme is considered a promising drug target against estrogen-dependent cancers. For the development of novel inhibitors, an improved understanding of the structure-function relationships is essential. In the present study, we examined the role of a cysteine residue, Cys10, in the Rossmann-fold NADPH binding region, for 17β-HSD1 function and tested the sensitivity towards sulfhydryl modifying chemicals. 3D structure modeling revealed important interactions of Cys10 with residues involved in the stabilization of amino acids of the NADPH binding pocket. Analysis of enzyme activity revealed that 17β-HSD1 was irreversibly inhibited by the sulfhydryl modifying agents N-ethylmaleimide (NEM) and dithiocarbamates. Preincubation with increasing concentrations of NADPH protected 17β-HSD1 from inhibition by these chemicals. Cys10Ser mutant 17β-HSD1 was partially protected from inhibition by NEM and dithiocarbamates, emphasizing the importance of Cys10 in the cofactor binding region. Substitution of Cys10 with serine resulted in a decreased protein half-life, without significantly altering kinetic properties. Despite the fact that Cys10 on 17β-HSD1 seems to have limited potential as a target for new enzyme inhibitors, the present study provides new insight into the structure-function relationships of this enzyme. 1. Introduction 17β-Hydroxysteroid dehydrogenase (17β-HSD) enzymes are involved in the interconversion of inactive and active sex-steroid hormones, thereby playing an essential role in the intracrine regulation of estrogen-, androgen-, and progesterone-dependent physiological functions [1]. 17β-HSD1 is responsible for the conversion of estrone to estradiol, the most potent natural estrogen. The highest expression of this enzyme is found in breast and ovarian tissue. Importantly, 17β-HSD1 expression showed a negative correlation with breast cancer progression and was identified as an independent prognostic marker for the disease-free and overall survival of patients with breast cancer [2, 3]. Elevated expression of 17β-HSD1 was observed in endometrial cancer [4] and nonsmall cell lung cancer [5]. Furthermore, the expression ratio of 17β-HSD1 and 17β-HSD2 was found to be a predictor of the response to tamoxifen in postmenopausal
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