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A number of molybdopterin enzymes, including xanthine oxidoreductase (XOR), aldehyde oxidase (AO), sulfite oxidase (SO), and mitochondrial amidoxime reducing component (mARC), have been identified as nitrate and nitrite reductases. Of these enzymes, XOR has been the most extensively studied and reported to be a substantive source of nitric oxide (NO) under inflammatory/hypoxic conditions that limit the catalytic activity of the canonical NOS pathway. It has also been postulated that XOR nitrite reductase activity extends to red blood cell (RBCs) membranes where it has been immunohistochemically identified. These findings, when combined with countervailing reports of XOR activity in RBCs, incentivized our current study to critically evaluate XOR protein abundance/enzymatic activity in/on RBCs from human, mouse, and rat sources. Using various protein concentrations of RBC homogenates for both human and rodent samples, neither XOR protein nor enzymatic activity (xanthine → uric acid) was detectable. In addition, potential loading of RBC-associated glycosaminoglycans (GAGs) by exposing RBC preparations to purified XO before washing did not solicit detectable enzymatic activity (xanthine → uric acid) or alter NO generation profiles. To ensure these observations extended to absence of XOR-mediated contributions to overall RBC-associated nitrite reduction, we examined the nitrite reductase activity of washed and lysed RBC preparations via enhanced chemiluminescence in the presence or absence of the XOR-specific inhibitor febuxostat (Uloric®). Neither addition of inhibitor nor the presence of the XOR substrate xanthine significantly altered the rates of nitrite reduction to NO by RBC preparations from either human or rodent sources confirming the absence of XO enzymatic activity. Furthermore, examination of the influence of the age (young cells vs. old cells) of human RBCs on XO activity also failed to demonstrate detectable XO protein. Combined, these data suggest: 1) that XO does not contribute to nitrite reduction in/on human and rodent erythrocytes, 2) care should be taken to validate immuno-detectable XO by demonstrating enzymatic activity, and 3) XO does not associate with human erythrocytic glycosaminoglycans or participate in nonspecific binding. [Display omitted] • Reported immunocytochemical evidence suggest the presence of XO in/on RBCs. • Assessment of rodent and human RBCs herein reveals neither XO protein nor XO/XDH activity. • Human and rodent RBCs do not demonstrate XO-dependent nitrite reductase activity. • Conclusions based on nonspecific antibodies require validation. [ABSTRACT FROM AUTHOR]

Coenzyme Q (CoQ) is ubiquitously embedded in lipid bilayers of various cellular organelles. As a redox cycler, CoQ shuttles electrons between mitochondrial complexes and extramitochondrial reductases and oxidases. In this way, CoQ is crucial for maintaining the mitochondrial function, ATP synthesis, and redox homeostasis. Cardiomyocytes have a high metabolic rate and rely heavily on mitochondria to provide energy. CoQ levels, in both plasma and the heart, correlate with heart failure in patients, indicating that CoQ is critical for cardiac function. Moreover, CoQ supplementation in clinics showed promising results for treating heart failure. This review provides a comprehensive view of CoQ metabolism and its interaction with redox enzymes and reactive species. We summarize the clinical trials and applications of CoQ in heart failure and discuss the caveats and future directions to improve CoQ therapeutics. [Display omitted] • CoQ is an indispensable component in the mitochondrial respiratory chain. • The redox status of CoQ is regulated by mitochondrial complexes and other enzymes in different subcellular compartments. • CoQ, in both reduced and oxidized forms, interacts with a myriad of reactive species. • CoQ has been implemented in treating cardiovascular disease for decades and showed potentials as a therapy for heart failure. • More rigorous clinical trials are needed to appreciate CoQ's clinical value as a therapeutic approach against heart failure. [ABSTRACT FROM AUTHOR]

Neurological injury resulting from dysfunction of brain mitochondria limits recovery after cardiac arrest (CA). Available indicators of neurological injury are inadequate for early prognostication after return of spontaneous circulation (ROSC). High diversification and essentiality of brain mitochondrial cardiolipins (CL) makes them unique candidates to quantify brain injury and predict prognosis early after ROSC. Using high-resolution liquid chromatography mass spectrometry, we determined CL content in plasma from 39 patients within 6 h of ROSC, 10 healthy controls and human heart and brain tissue. Cerebral score (C-score) was derived from brain-specific CL. Using a rat model of CA, CL was quantified in plasma and brain and results were compared with the controls. Brain and heart fell on extreme ends of CL diversity spectrum. Nine of 26 brain-specific CL were detected in plasma and correlated with brain injury. The C-score correlated with early neurologic injury and predicted neurologic outcome. In a rat CA model, a significant reduction in hippocampal CL content corresponded to CL released from the brain into systemic circulation. In conclusion brain-specific CL species accumulate in plasma after CA and can be used to predict injury severity and outcome. [ABSTRACT FROM AUTHOR]