KCNQ subunits encode for the M current (I KM)

Anticancer therapy with doxorubicin (DOX) is limited by severe cardiotoxicity, presumably reflecting the intramyocardial formation of drug metabolites that alter cell constituents and functions. In a previous study, we showed that NADPH-supplemented cytosolic fractions from human myocardial samples can enzymatically reduce a carbonyl group in the side chain of DOX, yielding a secondary alcohol metabolite called doxorubicinol (DOXol). Here we demonstrate that DOXol delocalizes low molecular weight Fe(II) from the [4Fe-4S] cluster of cytoplasmic aconitase. Iron delocalization proceeds through the reoxidation of DOXol to DOX and liberates DOX-Fe(II) complexes as ultimate by-products. Under physiologic conditions, cluster disassembly abolishes aconitase activity and forms an apoprotein that binds to mRNAs, coordinately increasing the synthesis of transferrin receptor but decreasing that of ferritin. Aconitase is thus converted into an iron regulatory protein-1 (IRP-1) that causes iron uptake to prevail over sequestration, forming a pool of free iron that is used for metabolic functions. Conversely, cluster reassembly converts IRP-1 back to aconitase, providing a regulatory mechanism to decrease free iron when it exceeds metabolic requirements. In contrast to these physiologic mechanisms, DOXol-dependent iron release and cluster disassembly not only abolish aconitase activity, but also affect irreversibly the ability of the apoprotein to function as IRP-1 or to reincorporate iron within new Fe-S motifs. This damage is mediated by DOX-Fe(II) complexes and reflects oxidative modifications of -SH residues having the dual role to coordinate cluster assembly and facilitate interactions of IRP-1 with mRNAs. Collectively, these findings describe a novel mechanism of cardiotoxicity, suggesting that intramyocardial formation of DOXol may perturb the homeostatic processes associated with cluster assembly or disassembly and the reversible switch between aconitase and IRP-1. These results may also provide a guideline to design new drugs that mitigate the cardiotoxicity of DOX.

Biliverdin reductase-A is a pleiotropic enzyme involved not only in the reduction of biliverdin-IX -alpha into bilirubin-IX-alpha, but also in the regulation of glucose metabolism and cell growth secondary to its serine/threonine/tyrosine kinase activity. Together with heme oxygenase, whose metabolic role is to degrade heme into biliverdin-IX-alpha, it forms a powerful system involved in the cell stress response during neurodegenerative disorders. In this paper, an up-regulation of the biliverdin reductase-A protein levels was found in the hippocampus of the subjects with Alzheimer disease and arguably its earliest form, mild cognitive impairment. Moreover a significant reduction in the phosphorylation of serine, threonine and tyrosine residues of biliverdin reductase-A was found, and this was paralleled by a marked reduction in its reductase activity. Interestingly, the levels of both total and phosphorylated biliverdin reductase-A was unchanged as well as its enzymatic activity in the cerebella. These results demonstrated a dichotomy between biliverdin reductase-A protein levels and activity in the hippocampus of subjects affected by Alzheimer disease and mild cognitive impairment, and this effect likely is attributable to a reduction in the phosphorylation of serine, threonine and tyrosine residues of biliverdin reductase-A. Consequently, not just the increased levels of biliverdin reductase-A, but also its changed activity and phosphorylation state, should be taken into account when considering potential biomarkers for Alzheimer disease and mild cognitive impairment.

For over half a century, pyridine-4-carboxy hydrazide (isonicotinyl hydrazide; isoniazid - INH) has been a front-line weapon in the battle against tuberculosis. Its metabolism has been the subject of important research, much of which has focused on the pharmacodynamic and toxicological aspects of certain INH metabolites. Since 1952, when the drug was first introduced, multiple INH metabolites have been identified, including hydrazine (HZ), isonicotinic acid (INA), ammonia, the acetylated derivative N(1)-acetyl-N(2)-isonicotinylhydrazide (AcINH), hydrazones with pyruvic and ketoglutaric acids (INH-PA and INH-KA, respectively), monoacetylhydrazine (AcHZ), diacetylhydrazine (DiAcHZ), and oxidizing free radicals. Their formation is the result of hydrolysis (INA, HZ), cytochrome P450 (CYP)-dependent oxidation (HZ, NH(3), oxidizing free radicals), and N-acetyltransferase (NAT) activity (AcINH, AcHZ, DiAcHZ). Doubts remain about isonicotinamide (INAAM) as an INH metabolite in mammals. Quantitatively speaking, one of the major metabolites is AcINH, which is produced by the enzyme NAT. It has virtually no antitubercular activity and is far less toxic than INH. Its formation and elimination are genetically controlled, and its elimination profile is trimodal (rapid, intermediate, and slow acetylation). Slow acetylation, which is transmitted as an autosomal recessive trait, increases the risk for peripheral neurotoxicity and hepatotoxicity in INH users. Thus far, there is no conclusive pharmacogenetic evidence that the formation of HZ and oxidizing radicals are linked to CYP polymorphisms. This article examines INH, HZ and its mono- and diacetylated metabolites, and ammonia (which in vitro and in vivo studies indicate as another derivative of HZ) in terms of their potential to cause neurotoxic and hepatotoxic effects (the two major forms of INH toxicity observed in animals and humans). INH hepatotoxicity seems to be related mainly to HZ, AcHZ, and other HZ metabolites that are capable of generating free radicals. The pathological aspects of slow INH acetylation will be discussed in relation to the drug's hepato- and neurotoxic effects. The mechanism underlying INH neurotoxicity has yet to be fully defined. The metabolite(s) involved in this phenomenon remain obscure although a major role is clearly played by HZ (and possibly also by the ammonia it releases). There is some evidence of the involvement of gamma-glutamyl HZ and of a chemical analogue of a Schiff base formed by INH and pyridoxal-phosphate. Recent findings have also revealed important interactions between INH and the various isoforms of CYP, and these may play a role in clinically relevant interactions between INH and several other drugs. All of these aspects of INH will be covered in the review.

Biliverdin reductase-A (BVR-A) is a pleiotropic enzyme and plays pivotal role in the antioxidant defense against free radicals as well as in cell homeostasis. Together with heme oxygenase, BVR-A forms a powerful system involved in the cell stress response during neurodegenerative disorders including Alzheimer's disease (AD), whereas due to the serine/threonine/tyrosine kinase activity the enzyme regulates glucose metabolism and cell proliferation. In this paper, we report results that demonstrate BVR-A undergoes post-translational oxidative and nitrosative modifications in the hippocampus, but not cerebellum, of subjects with AD and amnestic mild cognitive impairment (MCI). A significant increase of nitrated BVR-A was demonstrated only in AD and MCI hippocampi, whereas no significant modifications were found in cerebellar tissue. In addition, a significant reduction in protein carbonyl-derivatives of BVR-A was found in both AD and MCI hippocampi (15% and 18%, respectively). Biliverdin reductase-bound 4-hydroxynonenals were not modified in hippocampi and cerebella from AD and MCI subjects. These results supported the hypothesis of a prevalence of nitrosative stress-induced modifications on BVR-A structure, and this evidence was confirmed by a significant upregulation of inducible nitric oxide synthase in hippocampal tissue of subjects with AD and MCI that was not present in cerebellum. In conclusion, nitrosative stress-induced modifications on hippocampal BVR-A are an early event in the pathogenesis of AD since they appear also in MCI subjects and could contribute to the antioxidant and metabolic derangement characteristic of these neurodegenerative disorders.