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Environmental toxicants on Leydig cell function
Published in C. Yan Cheng, Spermatogenesis, 2018
Leping Ye, Xiaoheng Li, Xiaomin Chen, Qingquan Lian, Ren-Shan Ge
CYP11A1 is a complex enzyme, requiring NADPH as the cofactor to provide the electrons (Figure 20.2). The electrons are transferred from NADPH to CYP11A1 via two electron transfer proteins: adrenodoxin reductase and adrenodoxin.22,23 All three proteins constitute the cholesterol side-chain cleavage complex.23 The complex catalyzes three sequential reactions using three molecules of cofactor NADPH for electron transfers via the mitochondrial electron transfer system.14 The reaction catalyzed by this enzyme occurs in the inner membrane of the mitochondria. Two hydroxyl groups are added to cholesterol (at C20 and C22) first. Then, the cleavage between the added hydroxyl groups is followed, leading to the formation of pregnenolone.14 CYP11A1 is the first enzyme for the formation of all steroids and it expresses in adrenals, gonads (including Leydig cells), and placentas. Thus, the disruption of CYP11A1 not only impairs Leydig cell functions but also the functions of other endocrine systems.
Synthesis, Enzyme Localization, and Regulation of Neurosteroids
Published in Sheryl S. Smith, Neurosteroid Effects in the Central Nervous System, 2003
Adrenodoxin mRNA was found in virtually all rodent tissues examined,149 including the brain. This is not surprising, as this protein functions with all mitochondrial P450s, not just the steroidogenic P450s. Similarly, as expected, adrenodoxin protein has also been found in brain homogenates.150 The exact site of expression of these proteins has not been systematically examined either in adult brain or during development. The expression of adrenodoxin reductase has not been documented yet in
The Stimulation of Steroid Biosynthesis by Luteinizing Hormone
Published in Mario Ascoli, Luteinizing Hormone Action and Receptors, 2019
Anita H. Payne, Patrick G. Quinn, John R. D. Stalvey
The purified enzyme, which is an integral membrane protein, has been successfully reconstituted in liposomes.104,116,119,120 The P-450scc is typically isolated as a mixture of the high- (substrate-bound) and low-spin (substrate-free) forms, which can be monitored spectrally.121 The P-450scc can be converted to the substrate-free form by incubation with adrenodoxin, adrenodoxin reductase, and NADPH, followed by reisolation of the cytochrome P-450 by gel filtration.122 This substrate free form of P-450 is then used to monitor binding of cholesterol. All of the P-450 in the reconstituted vesicle is capable of being reduced by adrenodoxin and adrenodoxin reductase, which suggests that the adrenodoxin binding site of P-450 is exposed, since adrenodoxin and its reductase are not integral membrane proteins.116 On the other hand, cholesterol can be bound and metabolized only when it is reconstituted in the same vesicles as the P-450, suggesting that the cholesterol binding site is within the membrane. Maximal activity is observed at a 1.1:1 ratio of adrenodoxin to P-450 and a 0.1:1 ratio of adrenodoxin to adrenodoxin reductase.116 Essentially all of the adrenodoxin is in the reduced state and electron flow is, therefore, not rate-limiting for the reaction, but can be enhanced by 1 mM Ca2+. The binding of cholesterol to P-450scc enhances the binding of adrenodoxin to P-450 by a factor of ~20, and the binding of adrenodoxin enhances the binding of cholesterol to the P-450scc. The substrate-free P-450 is not readily reduced, whereas the P-450-cholesterol complex is readily reduced by adrenodoxin.119 Free, oxidized adrenodoxin is rapidly reduced by adrenodoxin reductase. These cooperative-binding interactions help to prevent futile cycling and the generation of damaging reactive oxygen species.
Kinetics of dextromethorphan-O-demethylase activity and distribution of CYP2D in four commonly-used subcellular fractions of rat brain
Published in Xenobiotica, 2019
Barent N. DuBois, Farideh Amirrad, Reza Mehvar
In addition to the possibility of the presence of a different CYP2D isoform in the mitochondria, other factors may have also contributed to the apparent lower affinity of mitochondrial CYP2D observed in our study. For instance, post-translational modification of CYP2D for targeting to the mitochondria (Anandatheerthavarada et al., 1999; Boopathi et al., 2000; Dasari et al., 2006) may affect its affinity. Another contributing factor to the lower affinity in the mitochondria might be related to the major differences between the mitochondria and microsomes in their CYP450 electron transferring mechanisms. In the microsomes, cytochrome P450 reductase (CPR) is responsible for the electron transfer to CYP450, and both CYP450 and CPR are integrally membrane-bound proteins. However, the electron transfer system in mitochondria consists of two soluble proteins, adrenodoxin (Adx) and adrenodoxin reductase (Adr), which are present in the inner membrane matrix. During the preparation of mitochondrial fractions, it is possible that the inner and/or outer membranes of some mitochondria are disrupted, resulting in a reduction in the concentrations of Adx and Adr proteins in the mitochondrial matrix (Dasari et al., 2006; Sangar et al., 2009). Consequently, the apparently low affinity (Figure 2(B)) and intrinsic clearance (Figure 2(C)) of the CYP2D-mediated DOD in mitochondria, compared with the respective values in the microsomes, could be partially related to some loss of Adx and Adr during the preparation of mitochondria.
Mechanistic studies on the drug metabolism and toxicity originating from cytochromes P450
Published in Drug Metabolism Reviews, 2020
Chaitanya K. Jaladanki, Anuj Gahlawat, Gajanan Rathod, Hardeep Sandhu, Kousar Jahan, Prasad V. Bharatam
During CYP450 mediated drug metabolism (Meunier et al. 2004; Shaik et al. 2010) (Figure 3), initially, the drug (R-H) binds to heme iron (Loew and Harris 2000) center and displaces a water molecule, and 1st reduction takes place (Shaik et al. 2005). The covalent bond formation takes place with molecular oxygen followed by 2nd reduction. From the catalytic cycle above, it is evident that a CYP450 cycle requires two electrons. In the biological system, CYP450 family does not accept direct electrons derived from NAD(P)H but their transfer occurs through the various redox proteins. These redox proteins can have one or two component shuttle system. (Paine et al. 2005; Waskell and Kim 2015) One component shuttle system involves a single membrane-bound enzyme i.e. NADPH cytochrome P450 reductase (CPR). While two component shuttle system involves a cascade of two different redox proteins and carried out in two ways: First type involves two proteins i.e. Adrenodoxin reductase and Adrenodoxin. Adrenodoxin reductase present in mitochondria of eukaryotic cell, which takes electrons/protons from NADPH via FAD cofactor and transfer an electron to Adrenodoxin (iron-sulfur protein), which further transfer to CYP450 catalytic cycle (Miller and Chunk 2016).Second type involves other two proteins i.e cytochrome b5 reductase and cytochrome b5. The cytochrome b5 can also take electron from CYP450 reductase. There are reports which suggest that in the cellular system, cytochrome b5 protein can transfer second electron faster than NADPH CYP450 reductase (Im and Waskell 2011; Manikandan and Nagini 2018).
The hunt for radiation biomarkers: current situation
Published in International Journal of Radiation Biology, 2020
Gabriela Kultova, Ales Tichy, Helena Rehulkova, Alena Myslivcova-Fucikova
FDXR primarily functions as a soluble electron carrier between the NADPH-dependent adrenodoxin reductase and several cytochromes P450, which makes it a crucial part of the steroid hormones biosynthesis in the adrenal mitochondria of vertebrates (Grinberg et al. 2000). FDXR was found to be induced by DNA damage in a p53-dependent manner (Liu and Chen 2002). Zhang et al. published a study, which showed that knockdown of FDXR reduce mammary tumor cell growth (Zhang et al. 2015).