The evidence to date: implications of l-ascorbic acid in the pathophysiology of aging

L-Ascorbic acid, commonly known as vitamin C, has been used not only for disease prevention and in complementary and alternative medicine, but also for anti-aging purposes. However, the scientific evidence is not yet sufficient. Here, we review the physiological functions of vitamin C and its relationship with various pathological conditions, including our previous findings, and discuss the prospects of its application in healthy longevity. In summary, vitamin C levels are associated with lifespan in several animal models. Furthermore, clinical studies have shown that the blood vitamin C levels are lower in middle-aged and older adults than in younger adults. Lower blood vitamin C levels have also been observed in various pathological conditions such as chronic kidney disease and chronic obstructive pulmonary disease in the elderly. These observations suggest the implications of vitamin C in age-related pathological mechanisms owing to its physiological functions.

Biomedical role of L-carnitine in several organ systems, cellular tissues, and COVID-19

Carnitine is a conditionally necessary vitamin that aids in energy creation and fatty acid metabolism. Its bioavailability is higher in vegetarians than in meat-eaters. Deficits in carnitine transporters occur because of genetic mutations or in conjunction with other illnesses. Carnitine shortage can arise in health issues and diseases-including hypoglycaemia, heart disease, starvation, cirrhosis, and ageing-because of abnormalities in carnitine control. The physiologically active form of L-carnitine supports immunological function in diabetic patients. Carnitine has been demonstrated to be effective in the treatment of Alzheimer's disease, several painful neuropathies, and other conditions. It has been used as a dietary supplement for the treatment of heart disease, and it also aids in the treatment of obesity and reduces blood glucose levels. Therefore, L-carnitine shows the potential to eliminate the influences of fatigue in COVID-19, and its consumption is recommended in future clinical trials to estimate its efficacy and safety. This review focused on carnitine and its effect on tissues, covering the biosynthesis, metabolism, bioavailability, biological actions, and its effects on various body systems and COVID-19.

Determination of tissue-specific interaction between vitamin C and vitamin E in vivo using senescence marker protein-30 knockout mice as a vitamin C synthesis deficiency model

Vitamin E (α-tocopherol; VE) is known to be regenerated from VE radicals by vitamin C (L-ascorbic acid; VC) in vitro. However, their in vivo interaction in various tissues is still unclear. Therefore, we alternatively examined the in vivo interaction of VC and VE by measurement of their concentrations in various tissues of senescence marker protein-30 (SMP30) knockout (KO) mice as a VC synthesis deficiency model. Male SMP30-KO mice were divided into four groups (VC+/VE+, VC+/VE-, VC-/VE+ and VC-/VE-), fed diets with or without 500 mg/kg VE and given water with or without 1·5 g/l VC ad libitum. Then, VC and VE concentrations in the plasma and various tissues were determined. Further, gene expression levels of transporters associated with VC and VE, such as α-tocopherol transfer protein (α-TTP) and sodium-dependent vitamin C transporters (SVCTs), were examined. These results showed that the VE levels in the VC-depleted (VC-/VE+) group were significantly lower than those in the VC+/VE+ group in the liver and heart; the VC levels in the VE-depleted (VC+/VE-) group were significantly lower than those in the VC+/VE+ group in the kidneys. The α-TTP gene expression in the liver and kidneys was decreased by VC and/or VE depletion. Moreover, SVCT1 gene expression in the liver was decreased by both VC and VE depletion. In conclusion, these results indicate that VC spares VE mainly in the liver and heart and that VE spares VC in the kidneys of SMP30-KO mice. Thus, interaction between VC and VE is likely to be tissue specific.

Ascorbate Is a Primary Antioxidant in Mammals

Ascorbate (vitamin C in primates) functions as a cofactor for a number of enzymatic reactions represented by prolyl hydroxylases and as an antioxidant due to its ability to donate electrons, which is mostly accomplished through non-enzymatic reaction in mammals. Ascorbate directly reacts with radical species and is converted to ascorbyl radical followed by dehydroascorbate. Ambiguities in physiological relevance of ascorbate observed during in vivo situations could be attributed in part to presence of other redox systems and the pro-oxidant properties of ascorbate. Most mammals are able to synthesize ascorbate from glucose, which is also considered to be an obstacle to verify its action. In addition to animals with natural deficiency in the ascorbate synthesis, such as guinea pigs and ODS rats, three strains of mice with genetic removal of the responsive genes (GULO, RGN, or AKR1A) for the ascorbate synthesis have been established and are being used to investigate the physiological roles of ascorbate. Studies using these mice, along with ascorbate transporter (SVCT)-deficient mice, largely support its ability in protection against oxidative insults. While combined actions of ascorbate in regulating epigenetics and antioxidation appear to effectively prevent cancer development, pharmacological doses of ascorbate and dehydroascorbate may exert tumoricidal activity through redox-dependent mechanisms.

Reduced Plasma Ascorbate and Increased Proportion of Dehydroascorbic Acid Levels in Patients Undergoing Hemodialysis

Ascorbate functions as an electron donor and scavenges free radicals. Dehydroascorbic acid (DHA), the oxidized form of ascorbate, is generated as a result of these reactions. While low plasma ascorbate levels have been reported in hemodialysis patients worldwide, no studies have measured DHA because it is not generalized. In this study, we aimed to clarify whether plasma ascorbate levels are low in dialysis patients and whether plasma ascorbate levels fluctuate before and after dialysis. Moreover, we applied our previously established method to measure the plasma ascorbate and DHA levels in chronic kidney disease (CKD) stage G3–G5 non-hemodialysis-dependent patients, and pre- and post-dialysis plasma ascorbate and DHA levels in CKD stage G5D hemodialysis patients. The sample size was calculated using G-power software. The pre-dialysis plasma total ascorbate levels, including DHA, were significantly (56%) lower in hemodialysis patients than in non-hemodialysis-dependent CKD patients. After dialysis, there was a 40% reduction in the plasma total ascorbate levels. Hemodialysis increased the post-dialysis plasma proportions of DHA from 37% to 55%. The study results demonstrated lower plasma total ascorbate levels in hemodialysis patients compared with in non-hemodialysis-dependent CKD patients; these low levels in hemodialysis patients were further reduced by hemodialysis and increased DHA proportion.

Vitamin C Deficiency in the Young Brain-Findings from Experimental Animal Models

Severe and long-term vitamin C deficiency can lead to fatal scurvy, which is fortunately considered rare today. However, a moderate state of vitamin C (vitC) deficiency (hypovitaminosis C)-defined as a plasma concentration below 23 μM-is estimated to affect up to 10% of the population in the Western world, albeit clinical hallmarks in addition to scurvy have not been linked to vitC deficiency. The brain maintains a high vitC content and uniquely high levels during deficiency, supporting vitC's importance in the brain. Actions include both antioxidant and co-factor functions, rendering vitamin C deficiency likely to affect several targets in the brain, and it could be particularly significant during development where a high cellular metabolism and an immature antioxidant system might increase sensitivity. However, investigations of a non-scorbutic state of vitC deficiency and effects on the developing young brain are scarce. This narrative review provides a comprehensive overview of the complex mechanisms that regulate vitC homeostasis in vivo and in the brain in particular. Functions of vitC in the brain and the potential consequences of deficiency during brain development are highlighted, based primarily on findings from experimental animal models. Perspectives for future investigations of vitC are outlined.

Vitamin C-Sources, Physiological Role, Kinetics, Deficiency, Use, Toxicity, and Determination

Vitamin C (L-ascorbic acid) has been known as an antioxidant for most people. However, its physiological role is much larger and encompasses very different processes ranging from facilitation of iron absorption through involvement in hormones and carnitine synthesis for important roles in epigenetic processes. Contrarily, high doses act as a pro-oxidant than an anti-oxidant. This may also be the reason why plasma levels are meticulously regulated on the level of absorption and excretion in the kidney. Interestingly, most cells contain vitamin C in millimolar concentrations, which is much higher than its plasma concentrations, and compared to other vitamins. The role of vitamin C is well demonstrated by miscellaneous symptoms of its absence—scurvy. The only clinically well-documented indication for vitamin C is scurvy. The effects of vitamin C administration on cancer, cardiovascular diseases, and infections are rather minor or even debatable in the general population. Vitamin C is relatively safe, but caution should be given to the administration of high doses, which can cause overt side effects in some susceptible patients (e.g., oxalate renal stones). Lastly, analytical methods for its determination with advantages and pitfalls are also discussed in this review.

Nuclear and Cytoplasmic Functions of Vitamin C

Vitamin C (ascorbic acid) is a water-soluble antioxidant and a cofactor for a large number of enzymes. It is present in all tissues and especially abundant in corneal epithelium, stem cells, and neurons. Although similar to thiols in its ability to react with many reactive oxygen species (ROS), ascorbate is much better (>100× faster) than glutathione at scavenging of primary ROS (superoxide radical and singlet oxygen). Ascorbate appears to be especially important for elimination of O₂^•– in the nucleus which contains little or no SOD activity. Cofactor functions of ascorbate involve the maintenance of activity of Fe(II)/2-oxoglutarate-dependent dioxygenases via reduction of Fe(III). The most prominent activity of ascorbate-dependent dioxygenases in the cytoplasm is hydroxylation of prolines in proteins involved in the formation of extracellular matrix and regulation of metabolism and hypoxia responses. In the nucleus, ascorbate is important for oxidative demethylation of 5-methylcytosine in DNA (by TET proteins) and removal of methyl groups from histone lysines (by JmjC demethylases). Differentiation and other cellular reprograming processes involving DNA demethylation are especially sensitive to ascorbate insufficiency. High doses of vitamin C alone or in combinations with drugs produced cancer-suppressive effects which involved redox, immune, and epigenetic mechanisms. Solutions to vitamin C deficiency in cultured cells are discussed to improve the physiological relevance of in vitro models. An abundance of vitamin C in rodents limits their ability to fully recapitulate human sensitivity to adverse health effects of malnutrition and xenobiotics, including neurotoxicity, lung injury, and intergenerational and other epigenetic effects.

Strategies to Combat Heat Stress in Broiler Chickens: Unveiling the Roles of Selenium, Vitamin E and Vitamin C

Heat stress compromises efficient poultry production by impairing growth performance and increasing mortality. Mechanisms to dissipate excess heat divert energy from efficient production. This includes increased energy expenditure for respiration, oxidative stress and micronutrient absorption. The fortification of diets with particular feed additives has been known as one of the most important approaches to minimize the negative impacts of heat stress on broiler production. In this context, the promising functional feed additives appeared to be selenium and vitamins E and C. The fortification of broiler diets with these feed additives has been proven to enhance the function of vital organs, immune system response and growth performance of broilers under heat stress. The current review highlights recent successful experiences in the alleviation of heat stress symptoms in broilers using the above-mentioned additives. Selenium and vitamins E and C enhanced production performance in broiler chickens challenged with acute heat stress. The combination of these additives, by employing multiple mechanisms and through synergistic effects, improves heat stress symptoms more efficiently than their individual forms. Emerging literature reveals that selenium and vitamins E and C are involved in close interactions to protect proteins and lipids from oxidative damage and boost immune system function.

Dietary ascorbic acid restriction in GNL/SMP30-knockout mice unveils the role of ascorbic acid in regulation of somatic and visceral pain sensitivity

Ca_v3.2 T-type Ca²⁺ channels are expressed in the primary afferents and play a pronociceptive role. The activity of Ca_v3.2 is enhanced by H₂S, a gasotransmitter, and suppressed by ascorbic acid (vitamin C) through metal-catalyzed oxidation of the Zn²⁺-binding His¹⁹¹ in Ca_v3.2. Since rodents, but not humans, are capable of synthesizing ascorbic acid, the present study examined the role of ascorbic acid in nociceptive processing, using the mice lacking GNL/SMP30, an enzyme essential for ascorbic acid biosynthesis. Intraplantar and intracolonic administration of NaHS, an H₂S donor, caused somatic allodynia and referred hyperalgesia, respectively, and repeated treatment with paclitaxel produced neuropathic allodynia in wild-type mice, all of which were suppressed by ascorbic acid or T-type Ca²⁺ channel blockers. Dietary ascorbic acid restriction caused dramatic decreases in plasma and tissue ascorbic acid levels in GNL/SMP30-knockout, but not wild-type, mice. The ascorbic acid restriction enhanced the somatic and visceral hypersensitivity following intraplantar and intracolonic NaHS, respectively, and paclitaxel-induced neuropathy in GNL/SMP30-knockout mice, while it had no such effect in wild-type mice. Together, our data unveil the critical role of ascorbic acid in regulating somatic and visceral pain sensitivity and support accumulating clinical evidence for the usefulness of ascorbic acid in pain management.

Vitamin C is not the Missing Link Between Cigarette Smoking and Spinal Pain

STUDY DESIGN

A nationwide cross-sectional study.

OBJECTIVES

To measure the associations between cigarette smoking (defined as serum cotinine concentration >15 ng/mL) and the 3-month prevalence of spinal pain (neck pain, low back pain, low back pain with pain below knee, and self-reported diagnosis of arthritis/rheumatism) and related limitations, and to verify whether these associations are mediated by serum concentrations of vitamin C.

SUMMARY OF BACKGROUND DATA

Cigarette smoking has been consistently associated with back pain, but this association has never been explained. Because vitamin C has recently been reported to be associated with spinal pain and related functional limitations, and the metabolism of vitamin C differs between smokers and nonsmokers, we hypothesized that the prevalence of spinal pain and related limitations might be greater among smokers because they are more susceptible to be in a state of hypovitaminosis C.

METHODS

We conducted secondary analyses of National Health and Nutrition Examination Survey (NHANES) 2003 to 2004 data on 4438 individuals aged ≥20 years.

RESULTS

Serum concentrations of vitamin C and cotinine were strongly and inversely correlated (r = -0.35, P < 0.0001). Smoking was statistically associated with the prevalence of neck pain [adjusted odds ratio: aOR: 1.25; 95% confidence interval (95% CI): 1.06-1.47], low back pain (aOR: 1.20; 95% CI: 1.04-1.39), and low back pain with pain below knee (aOR: 1.58; 95% CI: 1.13-2.22) and related limitations, with a dose-response relationship (P < 0.05). However, the associations between smoking and spinal pain were not mediated by concentrations of vitamin C.

CONCLUSION

These results confirm the relationship between smoking and spinal pain, but they do not support a mediating effect of vitamin C on this relationship.

LEVEL OF EVIDENCE

2.

Serum vitamin C and spinal pain: a nationwide study

Back pain brings about one of the heaviest burden of disease. Despite much research, this condition remains poorly understood, and effective treatments are frustratingly elusive. Thus, researchers in the field need to consider new hypotheses. Vitamin C (ascorbic acid) is an essential cofactor for collagen crosslinks, a key determinant of ligament, tendon, and bone quality. Recent studies have reported high frequency of hypovitaminosis C in the general population. We hypothesized that lack of vitamin C contributes to poor collagen properties and back pain. We conducted this study to examine the associations between serum concentration of vitamin C and the prevalence of spinal pain and related functional limitations in the adult general population. This study used nationwide cross-sectional data from the U.S. National Health and Nutrition Examination Survey (NHANES) 2003-2004. Data were available for 4742 individuals aged ≥20 years. Suboptimal serum vitamin C concentrations were associated with the prevalence of neck pain (adjusted odds ratio [aOR]: 1.5; 95% confidence interval [CI]: 1.2-2.0), low back pain (aOR: 1.3; 95% CI: 1.0-1.6), and low back pain with pain below knee (aOR: 1.3; 95% CI: 1.0-1.9) in the past 3 months, self-reported diagnosis of arthritis/rheumatism (aOR: 1.4; 95% CI: 1.2-1.7), and related functional limitations' score (adjusted difference of means [aB]: 0.03; 95% CI: 0.00-0.05). The prevalence of hypovitaminosis C in the general population is high. Our study shows associations between vitamin C and spinal pain that warrant further investigation to determine the possible importance of vitamin C in the treatment of back pain patients.

Emerging Evidence on Neutrophil Motility Supporting Its Usefulness to Define Vitamin C Intake Requirements

Establishing intake recommendations for vitamin C remains a challenge, as no suitable functional parameter has yet been agreed upon. In this report, we review the emerging evidence on neutrophil motility as a possible marker of vitamin C requirements and put the results in perspective with other approaches. A recent in vitro study showed that adequate levels of vitamin C were needed for this function to work optimally when measured as chemotaxis and chemokinesis. In a human study, neutrophil motility was optimal at intakes ≥250 mg/day. Interestingly, a Cochrane review showed a significant reduction in the duration of episodes of common cold with regular vitamin C intakes in a similar range. Additionally, it was shown that at a plasma level of 75 µmol/L, which is reached with vitamin C intakes ≥200 mg/day, incidences of cardiovascular disease were lowest. This evidence would suggest that daily intakes of 200 mg vitamin C might be advisable for the general adult population, which can be achieved by means of a diverse diet. However, additional studies are warranted to investigate the usefulness of neutrophil motility as a marker of vitamin C requirements.

Industrial robustness linked to the gluconolactonase from Zymomonas mobilis

The physiological characteristics and the potential gluconolactone production of the gluconolactonase-deficient strain, Zymomonas mobilis ZM4 gnlΔ, were investigated via growth inhibitory assay and biotransformation of glucose and fructose into gluconolactone and sorbitol, respectively. The results of ethanol fermentation studies performed in the presence of high concentration of glucose (>200 g l^-1) under fermentative or aerobic conditions indicated that a significant reduction of volumetric ethanol productivity from the strain of ZM4 gnlΔ was noticeable due to the reduced rates of specific growth, sugar uptake, and biomass yield as compared with those of the parental strain ZM4. The biotransformation prepared at pH 6.0 using the permeabilized cell indicated that gluconic acid from ZM4 gnlΔ was still produced as a major product (67 g l^-1) together with sorbitol (65 g l^-1) rather than gluconolactone after 24 h. Only small amount of gluconolactone was transiently overproduced up to 9 g l^-1, but at the end of biotransformation, all gluconolactone were oxidized into gluconic acid. This indicated that autolysis of gluconolactone at the pH led to such results despite under gluconolactonase inactivation conditions. The physiological characteristics of ZM4 gnlΔ was further investigated under various stress conditions, including suboptimal pH (3.5~6.0), temperature (25~40 °C), and presence of growth inhibitory molecules including hydrogen peroxide, ethanol, acetic acid, furfural, and so forth. The results indicated that ZM4 gnlΔ was more susceptible at high glucose concentration, low pH of 3.5, and high temperature of 40 °C and in the presence of 4 mM H₂O₂ comparing with ZM4. Therefore, the results were evident that gluconolactonase in Z. mobilis contributed to industrial robustness and anti-stress regulation.

Bone Degeneration and Its Recovery in SMP30/GNL-Knockout Mice

Senescence marker protein-30 (SMP30) decreases androgen-independently with aging and is a lactone-hydrolyzing enzyme gluconolactonase (GNL) that is involved in vitamin C biosynthesis. In the present study, bone properties of SMP30/GNL knockout (KO) mice with deficiency in vitamin C synthesis were investigated to reveal the effects of SMP30/GNL and exogenous vitamin C supplementation on bone formation. Mineral content (BMC) and mineral density (BMD) of the mandible and femur of SMP30/GNL KO and wild-type mice at 2 and 3 months of age with or without vitamin C supplementation were measured by dual-energy X-ray absorptiometry. Body and bone weight of both age groups decreased and became significantly lower than those of wild-type mice. The bones of SMP30/GNL KO mice were rough and porous, with BMC and BMD significantly below wild-type. Oral supplementation with vitamin C eliminated differences in body weight, bone weight, BMC, and BMD between SMP30/GNL KO and wild-type mice at each age. These results indicate that bone degeneration in SMP30/GNL KO mice was caused by lack of vitamin C, and that this mouse strain is an appropriate model for bone metabolism in humans, which have no ability to synthesize vitamin C.

Metabolite profile of a mouse model of Charcot-Marie-Tooth type 2D neuropathy: implications for disease mechanisms and interventions

Charcot–Marie–Tooth disease encompasses a genetically heterogeneous class of heritable polyneuropathies that result in axonal degeneration in the peripheral nervous system. Charcot–Marie–Tooth type 2D neuropathy (CMT2D) is caused by dominant mutations in glycyl tRNA synthetase (GARS). Mutations in the mouse Gars gene result in a genetically and phenotypically valid animal model of CMT2D. How mutations in GARS lead to peripheral neuropathy remains controversial. To identify putative disease mechanisms, we compared metabolites isolated from the spinal cord of Gars mutant mice and their littermate controls. A profile of altered metabolites that distinguish the affected and unaffected tissue was determined. Ascorbic acid was decreased fourfold in the spinal cord of CMT2D mice, but was not altered in serum. Carnitine and its derivatives were also significantly reduced in spinal cord tissue of mutant mice, whereas glycine was elevated. Dietary supplementation with acetyl-L-carnitine improved gross motor performance of CMT2D mice, but neither acetyl-L-carnitine nor glycine supplementation altered the parameters directly assessing neuropathy. Other metabolite changes suggestive of liver and kidney dysfunction in the CMT2D mice were validated using clinical blood chemistry. These effects were not secondary to the neuromuscular phenotype, as determined by comparison with another, genetically unrelated mouse strain with similar neuromuscular dysfunction. However, these changes do not seem to be causative or consistent metabolites of CMT2D, because they were not observed in a second mouse Gars allele or in serum samples from CMT2D patients. Therefore, the metabolite ‘fingerprint’ we have identified for CMT2D improves our understanding of cellular biochemical changes associated with GARS mutations, but identification of efficacious treatment strategies and elucidation of the disease mechanism will require additional studies.

Summary: A metabolomics analysis of a mouse model of Charcot–Marie–Tooth type 2D neuropathy revealed a clear distinction between mutant and control samples, and the therapeutic potential of a subset of these changes was explored.

Vitamin C prevents cigarette smoke-induced pulmonary emphysema in mice and provides pulmonary restoration

Vitamin C (VC) is a potent antioxidant and is essential for collagen synthesis. We investigated whether VC treatment prevents and cures smoke-induced emphysema in senescence marker protein-30 knockout (SMP30-KO) mice, which cannot synthesize VC. Two smoke-exposure experiments using SMP30-KO mice were conducted. In the first one (a preventive study), 4-month-old mice received minimal VC (0.0375 g/l) [VC(L)] or physiologically sufficient VC (1.5 g/l) [VC(S)] and exposed to cigarette smoke or smoke-free air for 2 months. Pulmonary evaluations followed when the mice were 6 months of age. The second study began after the establishment of smoke-induced emphysema (a treatment study). These mice no longer underwent smoke exposure but received VC(S) or VC(L) treatment for 2 months. Morphometric analysis was performed, and measurements of oxidative stress, collagen synthesis, and vascular endothelial growth factor in the lungs were evaluated. Chronic smoke exposure caused emphysema (29.6% increases of mean linear intercepts [MLI] and 106.5% increases of destructive index compared with the air-only group) in 6-month-old SMP30-KO mice, and this emphysema closely resembled human chronic obstructive pulmonary disease. Smoke-induced emphysema persisted in the VC(L) group after smoking cessation, whereas VC treatment provided pulmonary restoration (18.5% decrease of MLI and 41.3% decrease of destructive index compared with VC(L) group). VC treatment diminished oxidative stress, increased collagen synthesis, and improved vascular endothelial growth factor levels in the lungs. Our results suggest that VC not only prevents smoke-induced emphysema in SMP30-KO mice but also restores emphysematous lungs. Therefore, VC may provide a new therapeutic strategy for treating chronic obstructive pulmonary disease in humans.

Behavioral and monoamine changes following severe vitamin C deficiency

Severe vitamin C deficiency (ascorbic acid; AA) was induced in gulo-/- mice incapable of synthesizing their own AA. A number of behavioral measures were studied before and during the deprivation period, including a scorbutic period, during which weight loss was observed in the mice. Mice were then resuscitated with AA supplements. During the scorbutic period, gulo-/- mice showed decreased voluntary locomotor activity, diminished physical strength, and increased preference for a highly palatable sucrose reward. These behaviors all returned to control levels following resuscitation. Altered trial times in subordinate mice in the tube test for social dominance in the AA-deprived mice persisted following resuscitation and may signify a depressive-like behavior in these mice. Biochemical analyses were undertaken following a second deprivation period. AA deficiency was accompanied by decreased blood glucose levels, oxidative damage to lipids and proteins in the cortex, and decreases in dopamine and serotonin metabolites in both the cortex and striatum. Given the reasonably high proportions of the population that do not consume sufficient AA in the diet, these data have important implications for physical and psychological function in the general population.

Senescence Marker Protein 30: Functional and Structural Insights to its Unknown Physiological Function

Senescence marker protein 30 (SMP30) is a multifunctional protein involved in cellular Ca(2+) homeostasis and the biosynthesis of ascorbate in non-primate mammals. The primary structure of the protein is highly conserved among vertebrates, suggesting the existence of a significant physiological function common to all mammals, including primates. Enzymatic activities of SMP30 include aldonolactone and organophosphate hydrolysis. Protective effects against apoptosis and oxidative stress have been reported. X-ray crystallography revealed that SMP30 is a six-bladed β-propeller with structural similarity to paraoxonase 1, another protein with lactonase and organophosphate hydrolase activities. SMP30 has recently been tied to several physiological conditions including osteoporosis, liver fibrosis, diabetes, and cancer. This review aims to describe the recent advances made toward understanding the connection between molecular structure, enzymatic activity and physiological function of this highly conserved, multifaceted protein.

Pancreatic insulin release in vitamin C-deficient senescence marker protein-30/gluconolactonase knockout mice

We recently identified senescence marker protein-30 as the lactone-hydrolyzing enzyme gluconolactonase, which is involved in vitamin C biosynthesis. In this study, we investigated the effects of vitamin C on insulin secretion from pancreatic β-cells using senescence marker protein-30/gluconolactonase knockout mice. In intraperitoneal glucose tolerance tests, vitamin C-deficient senescence marker protein-30/gluconolactonase knockout mice demonstrated impaired glucose tolerance with significantly lower blood insulin levels at 30 and 120 min post-challenge than in wild type mice (p<0.01-0.05). In contrast, vitamin C-sufficient senescence marker protein-30/gluconolactonase knockout mice demonstrated significantly higher blood glucose and lower insulin only at the 30 min post-challenge time point (p<0.05). Senescence marker protein-30/gluconolactonase knockout mice showed enhanced insulin sensitivity regardless of vitamin C status. Static incubation of islets revealed that 20 mM glucose-stimulated insulin secretion and islet ATP production were significantly decreased at 60 min only in vitamin C-deficient SMP30/GNL knockout mice relative to wild type mice (p<0.05). These results indicate that the site of vitamin C action lies between glycolysis and mitochondrial oxidative phosphorylation, while SMP30 deficiency itself impairs the distal portion of insulin secretion pathway.

Lack of grafted liver rejuvenation in adult-to-pediatric liver transplantation

BACKGROUND

A grafted donor liver should grow and survive under the different conditions presented by a liver transplantation recipient. It has remained unclear, however, whether the age of a grafted liver can be modulated by recipient factors.

AIMS

This study investigated whether a grafted aged donor liver can be rejuvenated in a pediatric recipient.

METHODS

Of 119 living donor liver transplants, ten pairs were adult-to-pediatric combinations. Senescence marker protein-30 (SMP-30), which is a protein that is remarkably reduced upon aging, was used as a senescence marker. Immunohistochemical staining for SMP-30 was performed in biopsy specimen after living donor liver transplantation (LDLT). Re-expression of SMP-30 was investigated in a biopsied adult liver (n = 6) that had been transplanted in a pediatric recipient.

RESULTS

A remarkable expression of SMP-30 was seen in a control pediatric normal liver in comparison with that in an aged adult donor biopsy. Re-expression or an increase in SMP-30 was not observed in the liver of any pediatric recipient who had received an adult liver.

CONCLUSION

An adult grafted liver does not appear to rejuvenate in a pediatric recipient.

Structural studies on human 2-oxoglutarate dependent oxygenases

2-Oxoglutarate and ferrous iron-dependent oxygenases have emerged as an important family of human enzymes that catalyse hydroxylations and related demethylation reactions. Their substrates in humans include proteins, nucleic acids, lipids and small molecules. They play roles in collagen biosynthesis, hypoxic sensing, regulation of gene expression and lipid biosynthesis/metabolism. Structural analyses, principally employing crystallography, have revealed that all of these oxygenases possess a double-stranded β-helix core fold that supports a highly conserved triad of iron binding residues and a less well conserved 2-oxoglutarate co-substrate binding site. The 2-oxoglutarate binds to the iron in a bidentate manner via its 1-carboxylate and 2-oxo groups. The primary substrate binding elements are more variable and can involve mobile elements.

Physiological and biochemical aspects of hydroxylations and demethylations catalyzed by human 2-oxoglutarate oxygenases

Pioneering work in the 1960s defined prolyl and lysyl hydroxylations as physiologically important oxygenase-catalyzed modifications in collagen biosynthesis; subsequent studies demonstrated that extracellular epidermal growth factor-like domains were hydroxylated at aspartyl and asparaginyl residues. More recent work on the hypoxia-sensing mechanism in animals has shown that prolyl and asparaginyl hydroxylation of the hypoxia-inducible transcription factor play central roles in sensing hypoxia, by regulating protein-protein interactions in an oxygen-dependent manner. The collective results imply that protein hydroxylation is more common than previously perceived. Most protein hydroxylases employ Fe(II) as a cofactor, and 2-oxoglutarate and oxygen as co-substrates. Related enzymes catalyze the demethylation of N(ɛ)-methyl lysine residues in histones and of N-methylated nucleic acids, as well as hydroxylation of 5-methyl cytosine in DNA and 5-methoxycarbonylmethyluridine at the wobble position of tRNA. The combination of new molecular biological and analytical techniques is likely to reveal further roles for oxygenase-mediated modifications to biomacromolecules.

Complete lack of vitamin C intake generates pulmonary emphysema in senescence marker protein-30 knockout mice

Vitamin C (VC) is a potent antioxidant and plays an essential role in collagen synthesis. As we previously reported, senescence marker protein-30 (SMP30) knockout (KO) mice cannot synthesize VC due to the genetic disruption of gluconolactonase (i.e., SMP30). Here, we utilized SMP30 KO mice deprived of VC and found that VC depletion caused pulmonary emphysema due to oxidative stress and a decrease of collagen synthesis by the third month of age. We grew SMP30 KO mice and wild-type (WT) mice on VC-free chow and either VC water [VC(+)] or plain water [VC(−)] after weaning at 4 wk of age. Morphometric findings and reactive oxygen species (ROS) in the lungs were evaluated at 3 mo of age. No VC was detected in the lungs of SMP30 KO VC(−) mice, but their ROS increased 50.9% over that of the VC(+) group. Moreover, their collagen content in the lungs markedly decreased, and their collagen I mRNA decreased 82.2% compared with that of the WT VC(−) group. In the SMP30 KO VC(−) mice, emphysema developed [21.6% increase of mean linear intercepts (MLI) and 42.7% increase of destructive index compared with VC(+) groups], and the levels of sirtuin 1 (Sirt1) decreased 16.8%. However, VC intake increased the MLI 16.2% and thiobarbituric acid reactive substances 22.2% in WT mice, suggesting that an excess of VC can generate oxidative stress and may be harmful during this period of lung development. These results suggest that VC plays an important role in lung development through affecting oxidant-antioxidant balance and collagen synthesis.

Vitamin C transporter Slc23a1 links renal reabsorption, vitamin C tissue accumulation, and perinatal survival in mice

Levels of the necessary nutrient vitamin C (ascorbate) are tightly regulated by intestinal absorption, tissue accumulation, and renal reabsorption and excretion. Ascorbate levels are controlled in part by regulation of transport through at least 2 sodium-dependent transporters: Slc23a1 and Slc23a2 (also known as Svct1 and Svct2, respectively). Previous work indicates that Slc23a2 is essential for viability in mice, but the roles of Slc23a1 for viability and in adult physiology have not been determined. To investigate the contributions of Slc23a1 to plasma and tissue ascorbate concentrations in vivo, we generated Slc23a1-/- mice. Compared with wild-type mice, Slc23a1-/- mice increased ascorbate fractional excretion up to 18-fold. Hepatic portal ascorbate accumulation was nearly abolished, whereas intestinal absorption was marginally affected. Both heterozygous and knockout pups born to Slc23a1-/- dams exhibited approximately 45% perinatal mortality, and this was associated with lower plasma ascorbate concentrations in dams and pups. Perinatal mortality of Slc23a1-/- pups born to Slc23a1-/- dams was prevented by ascorbate supplementation during pregnancy. Taken together, these data indicate that ascorbate provided by the dam influenced perinatal survival. Although Slc23a1-/- mice lost as much as 70% of their ascorbate body stores in urine daily, we observed an unanticipated compensatory increase in ascorbate synthesis. These findings indicate a key role for Slc23a1 in renal ascorbate absorption and perinatal survival and reveal regulation of vitamin C biosynthesis in mice.

Senescence marker protein-30/gluconolactonase deletion worsens glucose tolerance through impairment of acute insulin secretion

Senescence marker protein-30 (SMP30) is an androgen-independent factor that decreases with age. We recently identified SMP30 as the lactone-hydrolyzing enzyme gluconolactonase (GNL), which is involved in vitamin C biosynthesis in animal species. To examine whether the age-related decrease in SMP30/GNL has effects on glucose homeostasis, we used SMP30/GNL knockout (KO) mice treated with L-ascorbic acid. In an ip glucose tolerance test at 15 wk of age, blood glucose levels in SMP30/GNL KO mice were significantly increased by 25% at 30 min after glucose administration compared with wild-type (WT) mice. Insulin levels in SMP30/GNL KO mice were significantly decreased by 37% at 30 min after glucose compared with WT mice. Interestingly, an insulin tolerance test showed a greater glucose-lowering effect in SMP30/GNL KO mice. High-fat diet feeding severely worsened glucose tolerance in both WT and SMP30/GNL KO mice. Morphometric analysis revealed no differences in the degree of high-fat diet-induced compensatory increase in beta-cell mass and proliferation. In the static incubation study of islets, insulin secretion in response to 20 mm glucose or KCl was significantly decreased in SMP30/GNL KO mice. On the other hand, islet ATP content at 20 mm in SMP30/GNL KO mice was similar to that in WT mice. Collectively, these data indicate that impairment of the early phase of insulin secretion due to dysfunction of the distal portion of the secretion pathway underlies glucose intolerance in SMP30/GNL KO mice. Decreased SMP30/GNL may contribute to the worsening of glucose tolerance that occurs in normal aging.

Effect of vitamin C depletion on age-related hearing loss in SMP30/GNL knockout mice

Using senescence marker protein 30 (SMP30)/gluconolactonase (GNL) knockout (KO) mice, which cannot synthesize vitamin C (VC), we examined whether modulating VC level affects age-related hearing loss (AHL). KO and wild-type (WT) C57BL/6 mice were given water containing 1.5 g/L VC [VC(+)] or 37.5mg/L VC [VC(-)]. At 10 months of age, KO VC(-) mice showed significant reduction in VC level in the inner ear, plasma, and liver, increase in auditory brainstem response (ABR) thresholds, and decrease in the number of spiral ganglion cells compared to WT VC(-), WT VC(+), and KO VC(+) mice. There were no differences in VC level in the inner ear, ABR thresholds, or the number of spiral ganglion cells among WT VC(-), WT VC(+), and KO VC(+) mice. These findings suggest that VC depletion can accelerate AHL but that supplementing VC may not increase VC level in the inner ear or slow AHL in mice.

Hydrogen-rich pure water prevents superoxide formation in brain slices of vitamin C-depleted SMP30/GNL knockout mice

Hydrogen is an established anti-oxidant that prevents acute oxidative stress. To clarify the mechanism of hydrogen's effect in the brain, we administered hydrogen-rich pure water (H(2)) to senescence marker protein-30 (SMP30)/gluconolactonase (GNL) knockout (KO) mice, which cannot synthesize vitamin C (VC), also a well-known anti-oxidant. These KO mice were divided into three groups; recipients of H(2), VC, or pure water (H(2)O), administered for 33 days. VC levels in H(2) and H(2)O groups were <6% of those in the VC group. Subsequently, superoxide formation during hypoxia-reoxygenation treatment of brain slices from these groups was estimated by a real-time biography imaging system, which models living brain tissues, with Lucigenin used as chemiluminescence probe for superoxide. A significant 27.2% less superoxide formed in the H(2) group subjected to ischemia-reperfusion than in the H(2)O group. Thus hydrogen-rich pure water acts as an anti-oxidant in the brain slices and prevents superoxide formation.