Extracellular ATP-induced production of hydrogen peroxide in porcine thyroid cells

Regulation of dual oxidase hydrogen peroxide synthesis results in an epithelial respiratory burst

Redox status is a central determinant of cellular activities and redox imbalance is correlated with numerous diseases. NADPH oxidase activity results in formation of H₂O₂, that, in turn, sets cellular redox status, a key regulator of cellular homeostasis and responses to external stimuli. Hydrogen peroxide metabolism regulates cell redox status by driving changes in protein cysteine oxidation often via cycling of thioredoxin/peroxiredoxin and glutathione; however, regulation of enzymes controlling synthesis and utilization of H₂O₂ is not understood beyond broad outlines. The data presented here show that calcium-stimulated epithelial Duox H₂O₂ synthesis is transient, independent of intracellular calcium renormalization, H₂O₂ scavenging by antioxidant enzymes, or substrate depletion. The data support existence of a separate mechanism that restricts epithelial H₂O₂ synthesis to a burst and prevents harmful changes in redox tone following continuous stimulation. Elucidation of this H₂O₂ synthesis tempering mechanism is key to understanding cellular redox regulation and control of downstream effectors, and this observation provides a starting point for investigation of the mechanism that controls H₂O₂-mediated increases in redox tone.

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DUOX1 and DUOX2 H₂O₂ synthesis decreases after calcium stimulation even in the continued presence of elevated calcium.

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Decreasing DUOX1 and DUOX2 activity in elevated calcium is not due to substrate depletion or assay limitations.

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Calcium activation of DUOX1 and DUOX2 H₂O₂ synthesis results in a respiratory burst independent of calcium renormalization.

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The data support a mechanism that limits Duox H₂O₂ synthesis in the presence of continually elevated calcium stimulation.

Quinine inhibits ovulation and produces oxidative stress in the ovary of cyclic Sprague-Dawley rats

BACKGROUND

Quinine has been reported to possess anti-spermatogenic activities.

OBJECTIVES

This study was carried out to determine the effect of quinine on ovarian function in Sprague-Dawley rats.

METHODS

Twenty rats with regular 4-days oestrous cycle divided into 4 groups (N=5) were used. Group I received quinine at 30 mg/kg body weight by gavage for 28 days after which they were sacrificed. The ovaries were excised for biochemical oxidation of glutathione peroxidase (GSH), superoxide dismutase (SOD), catalase and malondialdehyde (MDA). Group II received single dose quinine at 30 mg/kg body weight at 0900 hrs on day of proestrus. Blood was obtained at 1800 hrs for hormonal assay of FSH and LH. The animals were sacrificed the next morning on estrus: oviducts were examined for ova count. Groups III and IV served as controls.

RESULTS

Quinine treated rats recorded zero number of ova compared to control. Serum concentration of LH reduced significantly in the quinine treated group compared to the control. Furthermore, quinine significantly decreased the oxidant status of GSH, SOD and catalase and significantly increased MDA levels in the ovary compared to the control group.

CONCLUSION

Quinine completely blocks ovulation, suppresses LH surge, and produces oxidative stress in the ovary.

Prostaglandin pathways in duodenal chemosensing

Acid-sensing pathways, which trigger mucosal defense mechanisms in response to luminal acid, involve the rapid afferent-mediated "capsaicin pathway" and the sustained "prostaglandin (PG) pathway." Luminal acid quickly increases protective PG synthesis and release from epithelia, although the mechanism by which luminal acid induces PG synthesis is still mostly unknown. Acid exposure augments purinergic ATP-P2Y signaling by inhibition of intestinal alkaline phosphatase activity. Since P2Y activation increases intracellular Ca2+, we further hypothesized that ATP-P2Y signals increase the generation of H2O2 derived from dual oxidase, a member of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family activated by Ca2+. Our recent studies suggest that acid exposure increases H2O2 output, followed by phospholipase A2 and cyclooxygenase activation, increasing PG synthesis. Released prostaglandin E2 augments protective HCO3- and mucus secretion via EP4 receptor activation. Thus, the PG pathway as a component of duodenal acid sensing consists of acid-related intestinal alkaline phosphatase inhibition, ATP-P2Y signals, dual oxidase 2-derived H2O2 production, phospholipase A2 activation, prostaglandin E2 synthesis, and EP4 receptor activation. The PG pathway is also involved in luminal bacterial sensing in the duodenum via activation of pattern recognition receptors, including Toll-like receptors and nucleotide-binding oligomerization domain 2. The presence of acute mucosal responses to luminal bacteria suggests that the duodenum is important for host defenses and may reduce bacterial loading to the hindgut using H2O2, complementing gastric acidity and anti-bacterial bile acids.

Unique pattern of component gene disruption in the NRF2 inhibitor KEAP1/CUL3/RBX1 E3-ubiquitin ligase complex in serous ovarian cancer

The NFE2-related factor 2 (NRF2) pathway is critical to initiate responses to oxidative stress; however, constitutive activation occurs in different cancer types, including serous ovarian carcinomas (OVCA). The KEAP1/CUL3/RBX1 E3-ubiquitin ligase complex is a regulator of NRF2 levels. Hence, we investigated the DNA-level mechanisms affecting these genes in OVCA. DNA copy-number loss (CNL), promoter hypermethylation, mRNA expression, and sequence mutation for KEAP1, CUL3, and RBX1 were assessed in a cohort of 568 OVCA from The Cancer Genome Atlas. Almost 90% of cases exhibited loss-of-function alterations in any components of the NRF2 inhibitory complex. CNL is the most prominent mechanism of component disruption, with RBX1 being the most frequently disrupted component. These alterations were associated with reduced mRNA expression of complex components, and NRF2 target gene expression was positively enriched in 90% of samples harboring altered complex components. Disruption occurs through a unique DNA-level alteration pattern in OVCA. We conclude that a remarkably high frequency of DNA and mRNA alterations affects components of the KEAP1/CUL3/RBX1 complex, through a unique pattern of genetic mechanisms. Together, these results suggest a key role for the KEAP1/CUL3/RBX1 complex and NRF2 pathway deregulation in OVCA.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Reactive oxygen species are indispensable in ovulation

Ovulation is stimulated by the preovulatory surge of the pituitary luteinizing hormone (LH). Because the ovulatory response is commonly identified with inflammation, we explored the involvement of reactive oxygen species (ROS) in this process. Our experiments show that administration of broad-range scavengers of oxidative species into the ovarian bursa of mice, hormonally induced to ovulate, significantly reduced the rate of ovulation. LH-induced cumulus mucification/expansion, a necessary requirement for ovulation, was prevented by antioxidants both in vivo and in an ex vivo system of isolated intact ovarian follicles. Along this line, H(2)O(2) fully mimicked the effect of LH, bringing about an extensive mucification/expansion of the follicle-enclosed cumulus-oocyte complexes. Impaired progesterone production was observed in isolated follicles incubated with LH in the presence of the antioxidant agents. Furthermore, LH-stimulated up-regulation of genes, the expression of which is crucial for ovulation, was substantially attenuated upon ROS ablation. This system was also used for demonstrating the role of ROS in phosphorylation and activation of the EGF receptor as well as its downstream effector, p42/44 MAPK. Together, our results provide evidence that ovarian production of ROS is an essential preovulatory signaling event, most probably transiently triggered by LH.

Introduction to oxidative stress and mitochondrial dysfunction

Oxidative stress and mitochondrial dysfunction are discussed in this article. Mitochondria are the major producers of free radicals and the major target of oxidative damage. Oxidative stress is simply the elevation of free radicals (reactive oxygen species/reactive nitrogen species) found in cells that accumulate to higher than normal levels. Excessive or inappropriate oxidative stress damages cells and tissues, specifically mitochondria, cell membranes, DNA, proteins, and lipids.

Nucleotide receptors stimulation by extracellular ATP controls Hsp90 expression through APE1/Ref-1 in thyroid cancer cells: a novel tumorigenic pathway

Nucleotide receptors signaling affects cell proliferation, with possible implications on tumorigenic processes. However, molecular targets and action mechanisms of the extracellular nucleotides are still poorly elucidated. We have previously shown in ARO cells that APE1/Ref-1, a transcriptional coactivator responsible for the maintenance of the cellular proliferative rate, is functionally controlled by P2-mediated signaling. Here, we demonstrate that extracellular ATP has a mitogenic effect on ARO cells, increasing ERK phosphorylation, AP1 activation, and cyclin D1 expression. Using the ATP/ADPase apyrase and the P2 receptor antagonist suramin, we show that the extracellular ATP, physiologically released by ARO cells, exerts mitogenic effects. A differential proteomic approach was used to identify molecular events associated with the ATP-induced cell proliferation. Among other proteins, Hsp90 was found upregulated upon ATP stimulation. Pretreatment with suramin completely blocked the ATP-induced Hsp90 activation, confirming the involvement of cell-surface P2 nucleotide receptors in the ATP-mediated activation of ARO cells. Treatment of proliferating ARO cells with suramin and apyrase significantly reduced the intracellular levels of Hsp90, suggesting an autocrine/paracrine mechanism of control on Hsp90 expression by extracellular ATP. The influence of Hsp90 on ATP-induced cell proliferation was also demonstrated by its specific inhibition with 17-AAG. The molecular pathway by which ATP stimulates cell proliferation was further investigated by siRNA strategies showing that Hsp90 is a target of APE1/Ref-1 functional activation. Stimulation of ARO cells with specific nucleotide receptors agonists evidenced a major involvement of P2Y1 and P2Y2 receptors in controlling the Hsp90 activation. Accordingly, these two receptors resulted significantly upregulated in sample biopsies from different thyroid tumors.

Activation of APE1/Ref-1 is dependent on reactive oxygen species generated after purinergic receptor stimulation by ATP

Apurinic apyrimidinic endonuclease redox effector factor-1 (APE1/Ref-1) is involved both in the base excision repair (BER) of DNA lesions and in the eukaryotic transcriptional regulation. APE1/Ref-1 is regulated at both the transcriptional and post-translational levels, through control of subcellular localization and post-translational modification. In response to stress conditions, several cell types release ATP, which exerts stimulatory effects on eukaryotic cells via the purinergic receptors (P2) family. By using western blot and immunofluorescence analysis on a human tumour thyroid cell line (ARO), we demonstrate that purinergic stimulation by extracellular ATP induces quick cytoplasm to nucleus translocation of the protein at early times and its neosynthesis at later times. Continuous purinergic triggering by extracellular ATP released by ARO cells is responsible for the control of APE1/Ref-1 intracellular level. Interference with intracellular pathways activated by P2 triggering demonstrates that Ca²⁺ mobilization and intracellular reactive oxygen species (ROS) production are responsible for APE1/Ref-1 translocation. The APE1/Ref-1 activities on activator protein-1 (AP-1) DNA binding and DNA repair perfectly match its nuclear enrichment upon ATP stimulation. The biological relevance of our data is reinforced by the observation that APE1/Ref-1 stimulation by ATP protects ARO cells by H₂O₂-induced cell death. Our data provide new insights into the complex mechanisms regulating APE1/Ref-1 functions.

Targeting of the dual oxidase 2 N-terminal region to the plasma membrane

Dual oxidase 2 (Duox2) is a cell surface glycoprotein that probably provides thyroperoxidase with the H2O2 required to catalyze thyroid hormone synthesis. No functional H2O2-generating system has yet been obtained after transfecting Duox2 into non-thyroid cell lines, because it is retained in the endoplasmic reticulum (ER). We investigated the level of maturation of various Duox2 truncated proteins in an attempt to identify the region of Duox2 responsible for its remaining in the ER. Duox2-Q686X mutant, corresponding to the N-terminal ectodomain including the first putative transmembrane domain, was expressed in different cell lines. Carbohydrate content analysis revealed that complex type-specific Golgi apparatus (GA) oligosaccharides were present on pig Duox2-Q686X, whereas human truncated Duox2 carried only high mannose-type sugar chains characteristic of the ER. Further characterization using surface biotinylation and flow cytometry assays indicated that pig Duox2-Q686X was present at the plasma membrane, whereas human Duox2-Q686X remained inside the cell. The replacement of the last 90 residues of the human Duox2-Q686X with the pig equivalent region allowed the chimerical peptide to reach the Golgi apparatus. Pig mutants containing the complete first intracellular loop with or without the second transmembrane domain accumulated in the ER. These findings show that 1) the human Duox2-Q686X region encompassing residues 596-685 prevents mutant exportation from the ER and 2) there is a pig Duox2 retention domain in the first intracellular loop. In addition, missense mutations of four cysteines (Cys-351, -370, -568, or -582) completely inhibited the emergence of pig Duox2-Q686X from the ER compartment, indicating their importance in Duox2 maturation.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Increase of [Ca(2+)](i) via activation of ATP receptors in PC-Cl3 rat thyroid cell line

In PC-Cl3 rat thyroid cell line, ATP and UTP provoked a transient increase in [Ca(2+)](i), followed by a lower sustained phase. Removal of extracellular Ca(2+) reduced the initial transient response and completely abolished the plateau phase. Thapsigargin (TG) caused a rapid rise in [Ca(2+)](i) and subsequent addition of ATP was without effect. The transitory activation of [Ca(2+)](i) was dose-dependently attenuated in cells pretreated with the specific inhibitor of phospholipase C (PLC), U73122. These data suggest that the ATP-stimulated increment of [Ca(2+)](i) required InsP(3) formation and binding to its specific receptors in Ca(2+) stores. Desensitisation was demonstrated with respect to the calcium response to ATP and UTP in Fura 2-loaded cells. Further studies were performed to investigate whether the effect of ATP on Ca(2+) entry into PC-Cl3 cells was via L-type voltage-dependent Ca(2+) channels (L-VDCC) and/or by the capacitative pathway. Nifedipine decreased ATP-induced increase on [Ca(2+)](i). Addition of 2 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment of the cells with TG or with 100 microM ATP in Ca(2+)-free medium. These data indicate that Ca(2+) entry into PC-Cl3 stimulated with ATP occurs through both an L-VDCC and through a capacitative pathway. Using buffers with differing Na(+) concentrations, we found that the effects of ATP were dependent of extracellular Na(+), suggesting that a Na(+)/Ca(2+) exchange mechanism is also operative. These data suggest the existence, in PC-Cl3 cell line, of a P2Y purinergic receptor able to increase the [Ca(2+)](i) via PLC activation, Ca(2+) store depletion, capacitative Ca(2+) entry and L-VDCC activation.

Biochemical characterization of a Ca2+/NAD(P)H-dependent H2O2 generator in human thyroid tissue

An NAD(P)H-dependent H2O2 forming activity has been evidenced in thyroid tissue from patients with Grave's disease. Its biochemical properties were compared to those of the NADPH oxidase previously described in pig thyroid gland. Both were Ca2+-dependent and activated by inorganic phosphate anions in the same range of concentrations. Both are flavoproteins using FAD as cofactor, but the human enzyme was also able to utilize FMN. The apparent Km for NADPH of the human enzyme (100 microM) was 5-10 times higher than that of porcine enzyme. Vm was 3 to 10 times higher in pig (150 nmol x h(-1) x mg(-1)) than in man (14 to 45). Total content in human tissue was 7 to 9% of that in porcine tissue. An unidentified inhibitor has been detected in the 3000 g particulate fraction from most patients, which could account for this apparently low enzyme content. An NADH-dependent H2O2 production has also been observed in porcine and human thyroid tissues. This activity was only partly Ca2+-dependent (man, 50-70%; pig, 80-90%) and presented similar apparent Km values for NADH (man, 100 microM; pig, 200 microM). In pig thyrocytes, the expression of the Ca2+-dependent part of the NADH-oxidase activity was induced by TSH and down-regulated by TGFbeta, as was the NADPH oxidase activity. Furthermore, NADPH and NADH-dependent activities were not additive. We conclude that a single, inducible, NAD(P)H-oxidase can use NADPH or NADH as substrate to catalyse H2O2 formation, and that human and porcine NAD(P)H-oxidases are highly similar. Differences observed could be attributed to minor differences in enzyme structure and/or in membrane microenvironment. The NADH-dependent Ca2+-independent activity observed in human and porcine thyroid fractions could be attributed to a distinct and constitutive enzyme.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Impairment of ATP-induced Ca2+ -signalling in human thyroid cancer cells

Extracellular nucleotides like ATP that activate the Ca2+ -phosphatidylinositol (PI) signalling pathway have been suggested to participate in the regulation of normal human thyroid function. We examined, whether P2y-purinergic receptors are expressed on human thyroid cancer cells and whether post-receptor Ca2+ signalling is altered by malignant transformation. Extracellular ATP caused a biphasic increase in cytosolic free Ca2+ ([Ca2+]i) in normal human thyrocytes and in human follicular (FTC) and papillary (PTC) thyroid carcinoma cells. In FTC and PTC cell lines the dose-response curves for ATP-induced changes in [Ca2+]i were shifted to the right when compared with normal thyrocytes, whereas in undifferentiated thyroid carcinoma (UTC) cells even high concentrations of ATP (500 microM) failed to stimulate a rise in [Ca2+]i. By contrast, ATP stimulated inositol 1,4,5-trisphosphate (IP3) formation and capacitative Ca2+ entry was operational as judged by thapsigargin in normal thyrocytes and all thyroid cancer cells. Thus, P2y-purinergic receptors are expressed on thyroid tumor cells independent of degree of differentiation. In UTC cells, however, impairment in the Ca2+ -phosphatidylinositol (PI) signalling cascade occurs distal to the formation of IP3 and proximal to the activation of capacitative Ca2+ entry. Disturbed ATP-induced Ca2+ -signalling and alterations in the Ca2+ -PI signalling cascade may contribute to decreased expression or loss of specific thyroid functions in thyroid cancer cells.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

Species differences of the thyroid protein kinase C heterogeneity

Protein kinase C (PKC), the mediator of the phosphoinositide transduction pathway, is a family of at least 11 isozymes and its heterogeneity has been described in many tissues and cells. We studied here the heterogeneity of PKC in thyroid glands from three different species, rat, pig, and dog. By combining immunological and biochemical approaches, we identified in rat thyroids, the PKC alpha, beta II, delta, epsilon, and zeta subspecies, in pig thyroids, the alpha, epsilon, and zeta isozymes, and in dog thyroids, only the alpha and zeta isozymes. The observed species differences of the thyroid gland PKC heterogeneity could be related to the reported species differences in the activation of the phosphoinositide regulatory cascade by TSH and other thyroid cell regulators.

A comparative study of free radicals in vertebrates--II. Non-enzymatic antioxidants and oxidative stress

1. The three main non-enzymatic endogenous soluble antioxidants and three estimators of oxidative stress were measured in the liver, lung and brain of seven animal species of different vertebrate classes. 2. The more concentrated antioxidant was GSH, followed by ascorbate and finally by uric acid. Liver showed higher levels of GSH and uric acid than the other two organs in the majority of the species. 3. GSSG/GSH ratio was highest in lung, probably due to the high pO2 prevalent in the tissue. Nevertheless, this did not result in higher tissue peroxidation, suggesting that the lung antioxidants are capable of coping with a high tissue pO2. 4. Tissue peroxidation was maximal in the brain when assayed by the TBA test, but this was not confirmed by HPLC of malondialdehyde (MDA). HPLC resulted in much lower MDA values than TBA.

A comparative study of free radicals in vertebrates--I. Antioxidant enzymes

1. Five antioxidant enzymes and cytochrome oxidase were measured in three vital organs of seven animal species of different vertebrate classes. 2. Minimal superoxide dismutase activities were found in the brain of homeotherms and in the lung of amphibia. Catalase (CAT) was maximal in liver and minimal in brain. 3. Possession of both Se dependent and independent glutathione peroxidase (GPx) is widespread in vertebrate organs. Similarities in tissue distribution were found among enzymes which use hydroperoxides (Se and non-Se GPx and CAT) or glutathione (both GPx and glutathione reductase) as substrates. 4. The results also suggest that the high aerobic capacity of the liver strongly influences the activities of the antioxidant enzymes in this tissue across vertebrate species, whereas other factors such as tissue pO2 can be more important in the lung.

Oxygen radicals, a failure or a success of evolution?

Oxygen radicals are no doubt involved in the development of many pathological states. Nevertheless, the possibility that oxygen radical production was selected for during biological evolution in order to perform useful roles in relation to cellular metabolism is contemplated; previous data on this subject are briefly reviewed. The concept of an "oxygen radical cycle" is proposed as a useful theoretical model.

Brown fat thermogenesis and exercise: two examples of physiological oxidative stress?

Both brown fat tissue (BAT) and skeletal muscle experience large increases of oxygen consumption and oxygen radical generation during activation. This, together with the relatively low activities of antioxidant enzymes in these two tissues and the high lipid content and free fatty acid liberation of BAT, can produce a physiological oxidative stress. Increases of in vivo or in vitro (BAT) lipid peroxidation have been described in these tissues after activation. They react to this oxidative stress in an adaptive way after chronic stimulation. Cold acclimation increases antioxidant enzymes, ascorbate, and especially reduced glutathione (GSH) in BAT. There is controversy about the variations of antioxidants in skeletal muscle after acute exercise. Nevertheless, exercise training seems to increase muscle antioxidant enzymes and GSH. Many reports show that vitamin E levels decrease in the muscle and increase in plasma during exercise. Studies of vitamin E deficiency and supplementation strongly suggest that this vitamin is of protective value during exercise.