Estrogen receptor alpha (ER alpha) is present in the nucleus, the cytosol and in mitochondria. The rat ER alpha ligand binding domain was employed as bait in a bacterial two-hybrid screening of a human heart cDNA library to detect novel protein-protein interaction partners of ER alpha in the heart. 17 beta-Hydroxysteroid dehydrogenase type 10 (17 beta-HSD10), which converts potent (17 beta-estradiol) to less potent estrogens (estrone), co-localized with 17 beta-HSD10 in the mitochondria of rat cardiac myocytes. GST pull-down experiments confirmed the interaction of ER alpha and 17 beta-HSD10. These findings suggest that the ER alpha estrogen receptor might be involved in regulating intracellular estrogen levels by modulating 17 beta-HSD10 activity. (C) 2009 Elsevier Inc. All rights reserved.