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Biosynthesis of estrogens from androgens is catalyzed by cytochrome P450 aromatase. Aromatase inhibition by the triazole compounds Letrozole (LTZ) and Anastrozole is a prevalent therapy for estrogen-dependent postmenopausal breast cancer.
Triazole fungicides are widely used as agricultural fungicides and antimycotic drugs that target 14α-demethylase. Some were previously shown to inhibit aromatase, thereby raising the possibility of endocrine disruptive effects. However, mechanistic analysis of their inhibition has never been undertaken.
We have evaluated the inhibitory effects of 15 common fungicides in human aromatase enzyme in comparison with the Letrozole (LTZ), the most potent inhibitor of aromatase used as anti-estrogen for breast cancer treatment using AUTODOCK software for calculation of inhibition energy on CYP19 aromatase enzyme (Egbuta, Lo, & Ghosh, 2014).
Triazole containing compounds as systemic fungicides are widely used in agriculture due to its high efficiency, broad spectrum, low toxicity and long effectiveness (Feng, Guo, Song, Hu, & Li, 2011). Currently 16 triazole fungicides: bitertanol, cyproconazole, difenoconazole, epoxiconazole, fluquinconazole, flusilazole, flutriafol, hexaconazole, metconazole, myclobutanil, penconazole, propiconazole, tebuconazole, triadimefon, triadimenol and triticonazole, are approved by Swiss Federal Office of Public Health (Zürich, Switzerland). Switzerland no longer allows the use of many chemicals that are still sprayed on American fields (Rosensteil, 2015). By 2005 was set the goal to halve the pesticide pollution of water bodies (Singer, 2002). Although, by 2014 report was released that in the five rivers in Switzerland’s found heavily polluted in spring and summer by a cocktail of different pesticides (swissinfo.ch, 2014).
Target enzymes of triazoles in steroidogenesis are the sterol 14-alfa-demethylase (encoded by the CYP51 gene) and the aromatase (encoded by the CYP19 gene).
The human aromatase enzyme is a member of the cytochrome P450 family and is the product of the CYP19A1 gene, located on chromosome 15 (Thompson & Siiteri, 1974; Chen et al., 1988). Aromatase is the only known vertebrate enzyme that can aromatize a six-membered ring; aromatase is, therefore, the sole source of estrogen in the body (Amarneh, Corbin, Peterson, Simpson, & Graham-Lorence, 1993).
Nevertheless, since aromatase was first characterized, research has been impeded by the lack of its three-dimensional structure. In 2009, Ghosh et al. successfully solved the crystallized structure of human aromatase enzyme and provides a structural basis for the specificity to androgen (Ghosh, Griswold, Erman, & Pangborn, 2009; Ghosh, Griswald, Erman, & Pangborn, 2010).
The catalytic site of aromatase is located at the juncture of the I and F helices, β-sheet 3, and as the B-C loop. Androstenedione binds into the steroid binding pocket such that its β-face orientates towards the heme group of aromatase, placing C19 within 4.0 Å of the Fe atom. This binding site is only possible if the I-helix backbone is moved 3.5 Å, creating a binding pocket that is approximately 400 Å3. This important distortion is created by residue P308, without which N309, steric hindrance would prevent catalytic activity.
This crystal structure of aromatase will not only allow better structure-based drug design than previous models, but it has also allowed a direct analysis of why some currently available aromatase inhibitors function better than others (Chumsri, Howes, Bao et al., 2011).