Our hypothesis is further supported by previous data from our laboratory showing that (PhSe)2-induced

LDH inhibition was attenuated or abolished by NADH (Lugokenski et al., GSK J4 in vivo 2011). These data indicate that NADH can modulate enzyme conformation preventing the critical thiols from the attack by organochalcogens. Based on the presented results, we suggest that organochalcogen-induced mitochondrial complex I inhibition is linked to their interaction with critical thiol groups present in the active site of the NADH:ubiquinone oxidoreductase (Lin et al., 2002). As mentioned above, the complex I inhibition by organochalcogens was more pronounced than complex II. We suggest that, despite of succinate dehydrogenase (SDH) being described to possess sulfhydryl group essential for catalytic activity, located in the substrate site (Le-Quoc et al., 1981), the organochalcogens-induced mitochondrial complex II inhibition could be due to their interaction with other thiols critical to enzyme activity, than that located in the active site of the SDH (Lin et al., 2002). Our data are further supported by previous data showing that complex II is less prone to inactivation than complex I (Cadenas and Davies, 2000, Orrenius et al., 2007 and Zhang

et al., 1990). Thus, based on the presented results (Fig. 5 and Fig. 7) we suggest that both complexes I and II were directly www.selleckchem.com/products/Trichostatin-A.html affected by the organochalogens, being the thiols groups the molecular site of action for the organochalcogens. Our hypothesis is further supported by the data showing that organochalcogens induced complex I inhibition was not mediated by ROS formation (Figs. 4A–C). However, as seen in Fig. 6 and Fig. 8, (PhSe)2

has differential Dipeptidyl peptidase effect on complex II in liver and kidney. At the present moment, these results are not completely understood, but they can be related to differences in the molecular composition of mitochondria obtained from different tissues (Benard et al., 2006). Thus, we speculate that the liver and kidney could present different contents and isoforms of complex II enzymes, which resulted in different inhibition by (PhSe)2. Our assumption is based on previous data showing that, at least, two different isoforms of complex II have been reported in the literature (Tomitsuka et al., 2003a, Tomitsuka et al., 2003b and Tomitsuka et al., 2009). In addition to complexes I and II, the activities of the mitochondrial complexes III and IV (both from rat liver and kidney) were practically not targeted by organocompounds. In fact, mitochondrial complex III was minimally inhibited by the treatment with studied compounds, whereas complex IV was nearly unchanged. Thus, organochalcogens possibly did not inhibit mitochondrial complexes III and IV due to steric hindrance of their sulfhydryl groups to the organochalcogens (Lin et al., 2002). Our findings are supported by previous report showing that thiol groups from complex IV are less prone to oxidation than that from complex I (Orrenius et al., 2007).