Folate and 10-Formyltetrahydrofolate Dehydrogenase in Human and Rat Retina: Relation to Methanol Toxicity

doi:10.1006/taap.1996.0302

Toxicology and Applied Pharmacology

Volume 141, Issue 2, December 1996, Pages 373-381

https://doi.org/10.1006/taap.1996.0302 Get rights and content

Abstract

Ocular toxicity, a well-recognized outcome of methanol poisoning in humans, correlates with formate accumulation in blood following the ingestion of methanol. Rodents, however, are a species known to be resistant to the toxicity of formate. The present study was designed to determine whether components of folate-dependent formate oxidation, e.g., folate and 10-CHO-H₄-folate dehydrogenase (10-FDH), exist in retina and whether differences in these components might explain species-determined susceptibility to methanol intoxication. Total folate levels were determined in human and rat retinal tissues and were found to be much lower than the levels in liver. However, folate levels in human retina were only 14% of those determined in rat retina. Western blot analysis was used to quantify the amount of cytosolic and mitochondrial 10-FDH in retinal tissues. Results of these studies showed that comparable amounts of this enzyme were present in both cellular compartments in each species. However, the amount of 10-FDH in human retina was approximately three times the amount found in rat retina. In order to determine the cell type(s) within the retina that contained 10-FDH, immunohistochemical staining for 10-FDH was carried out. Confocal microscopic image analysis of human and rat retinae showed colocalization of 10-FDH primarily with Müller cell protein markers [glial fibrillary acidic protein (GFAP), vimentin, and carbonic anhydrase]. Therefore, 10-FDH was found to be preferentially localized in this cell type. Since Müller cells appear to represent the target for formate-induced ocular toxicity, our data suggest that formate oxidation reactions might serve two roles, first a protective role and then a role in methanol-induced toxicity in Müller cells.

References (0)

Cited by (36)

Understanding the postmortem changes in concentrations of methanol in ocular matrices in rabbits
2022, Legal Medicine
Citation Excerpt :
As it has been observed that formic acid constitutes the best indicator when measured 48 h after methanol ingestion and the therapeutic treatment has been started [15]. Two pathways have been suggested for the formation of formic acid: oxidation to carbon dioxide either through the catalase-peroxidative system or the tetrahydrofolic acid (THF) dependent one-carbon pool in the liver and also in the retina [16,17]. Moreover, collection of the samples was carried out after 17 h of drug administration as the preliminary study on autopsy revealed an average time from death to collection of specimens as 17 h.
Death due to abuse and accidental ingestion of methanol is widely known around the globe. The paper presents the postmortem changes in concentrations of methanol in the vitreous humor, aqueous humor, and ocular tissues in a rabbit model in methanol intoxication. Rabbits were intoxicated with methanol at a dose of 6.3 ml/kg through oral gavage. After 3.5 h of methanol administration, the rabbits were sacrificed. Vitreous humor, aqueous humor, and ocular tissues were collected both, perimortem (immediately) and postmortem (17 h post-death). Whole blood and plasma samples were also collected to explore the correlation between levels of methanol in whole blood/plasma and ocular fluids/ocular tissues if any. All the samples were analyzed by Headspace Gas Chromatography. The analysis revealed a decrease in methanol levels at postmortem for all the matrices, except for retina-choroid than its perimortem value. For retina-choroid, no significant change in methanol levels at postmortem was found.
Acute methanol poisonings: Folates administration and visual sequelae
2014, Journal of Applied Biomedicine
Citation Excerpt :
The role of folates in the metabolism of formic acid is well-established. Folates enhance formate metabolism converting it to 10-formyl tetrahydrofolate by the activity of 10-formyl tetrahydrofolate synthase followed by its oxidation to carbon dioxide catalyzed by 10-formyltetrahydrofolate dehydrogenase (McMartin et al., 1977; Johlin et al., 1987; Martinasevic et al., 1996). The presence of a folate derivative enhances formate oxidation by preventing the development of enzyme catalyst deficient metabolic pathways (Black et al., 1985).
During the outbreak of methanol poisonings in the Czech Republic 2012, we studied the clinical effectiveness of folate therapy in preventing visual damage. Data were obtained from a combined prospective and retrospective study on 79 patients: folinic acid was administered in 28, folic acid in 35; 16 patients received no folates. The groups were comparable by age, time to treatment, laboratory findings, symptoms, and treatment. The number of patients with visual sequelae differed neither between the groups treated with folinic/folic acid, nor between the groups with/without folate administration. The patients with visual sequelae were more acidotic and differed in pH, HCO₃⁻, base deficit, anion gap, but not in methanol, ethanol, osmolal gap, formate, and pCO₂. Serum lactate, but not formate differed significantly. The higher serum glucose on admission was in the patients with visual sequelae. Regardless the rationale for folate administration in acute methanol poisoning, its clinical effectiveness in preventing visual damage was not demonstrated in our study. The detoxifying effect of the pathway of tetrahydrofolate-mediated formate conversion is secondary to the formate elimination by haemodialysis. The results of our study cannot promote folinic acid as more efficient than folic acid, but also cannot discount the possible utility of adjunct folate therapy.
Proteomic analysis of rat retina after methanol intoxication
2012, Toxicology
Citation Excerpt :
The generation of intoxicated animal model is essential for the study of methanol toxicity. In the methanol metabolic processes, the oxidation of formic acid can be accelerated by 10-formyltetrahydrofolate dehydrogenase (10-FDH) whose concentration in human is only 26% of rat, so the formic acid is prone to accumulate in human body and lead to more serious toxicity (Martinasevic et al., 1996). Previous research has shown that low concentration of N2O inhibits the activity of 10-formyl-H4 folate dehydrogenase.
In order to investigate markers of toxic injury and elucidate the mechanisms underlying methanol intoxication at the protein level, proteomics technology was applied to study variations in retinal protein expression between normal rats and rat models of methanol intoxication.
Seven rats were administered saline and methanol respectively by gavage. After seven days, retinal function was assessed by electroretinography and retinal proteins were extracted and separated by two-dimensional gel electrophoresis. The gels were then stained with AgNO₃ and analyzed with Pdquest software. The differential expression of proteins was analyzed by MALDI-TOF-TOF MS, and related proteins were searched in a protein database.
Twenty-eight spots with significant differences were found, 24 of which were successfully identified. Specifically, there were 14 increased expression proteins, such as aldehyde dehydrogenase, tropomyosin alpha-1 chain, myosin light chain, and crystallin family proteins. There were 10 decreased expression ones, such as glyceraldehyde-3-phosphate dehydrogenase, recoverin, ATP synthase alpha subunit in rats with methanol toxicity.
Retinal function was greatly destroyed upon methanol intoxication. Several key proteins were up- or down-regulated upon induced retinal toxicity, indicating that there are other mechanisms underlying methanol poisoning besides oxidative injury. Together, this data provides insight and knowledge for future studies in this field.
Cellular signaling and factors involved in Müller cell gliosis: Neuroprotective and detrimental effects
2009, Progress in Retinal and Eye Research
Citation Excerpt :
Formate is detoxified to carbon dioxide by a two-step oxidation process that is ATP- and folate-dependent. The sensitivity of the primate retina to methanol/formate toxicity was ascribed to the limited capacity to oxidize formate due to the low amount of retinal folate (Martinasevic et al., 1996). Because the enzymes for formate detoxification are preferentially localized in Müller cells (Martinasevic et al., 1996), they are the primary target for methanol/formate-induced retinal toxicity (Garner et al., 1995).
Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.
Fatal methanol intoxication with different survival times-Morphological findings and postmortem methanol distribution
2008, Forensic Science International
Citation Excerpt :
The slow degradation of formic acid causes accumulation of this toxic acid in human body [13]. Two pathways have been suggested for the formation of formic acid: oxidation to carbon dioxide either through the catalase-peroxidative system or the tetrahydrofolic acid (THF)-dependent one-carbon pool in liver and also retina [11,14]. Thus folic acid can be used to increase formic acid elimination in cases of intoxication.
Three corresponding cases of fatal methanol intoxication with different survival times were investigated ante-mortem and postmortem. Ante-mortem serum methanol concentrations were determined during treatment in hospital for 4 days. Furthermore, postmortem distribution of methanol in various tissues and fluids was measured after autopsy. Morphological and toxicological findings are discussed based on the literature. The morphological findings correlated with the different survival times. The results of the toxicological analyses were partly in keeping with previously published data. Interestingly, very high methanol levels were determined in brain with very low concentrations in femoral venous blood. These results may have implications for postmortem toxicological analysis, brain death diagnosis and organ explanation for transplantation.
Involvement of reduced folate carrier 1 in the inner blood-retinal barrier transport of methyltetrahydrofolate
2008, Drug Metabolism and Pharmacokinetics
The purpose of this study was to elucidate the mechanism of methyltetrahydrofolate (MTF) transport at the inner blood-retinal barrier (inner BRB). The characteristics and function of MTF transport at the inner BRB were examined using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) as an in vitro model of the inner BRB. The [³H]MTF uptake by TR-iBRB2 cells increased with lowering extracellular pH and was Na⁺- and Cl"-independent. The [³H]MTF uptake was concentration-dependent with a K_m of 5.1 μM. This process was inhibited by reduced folate carrier 1 (RFC1) substrates, such as methotrexate and formyltetrahydrofolate, in a concentration-dependent manner with an IC₅₀ of 8.7 and 2.8 μM, respectively, suggesting that RFC1 mediates MTF uptake in TR-iBRB2 cells. Although both RFC1 and proton-coupled folate transporter (PCFT) mRNA, which are pH-sensitive folate transporters, are expressed in TR-iBRB2 cells and isolated rat retinal vascular endothelial cells, the expression level of RFC1 mRNA was 83- and 49-fold greater than that of PCFT, respectively. Taken together, the above findings are consistent with the involvement of RFC1 in the inner BRB transport of MTF.

View all citing articles on Scopus

^☆: This work was supported by a grant from the Nutra Sweet Company.

²: To whom correspondence and reprint requests should be addressed at 2-452 Bowen Science Building, Department of Pharmacology, University of Iowa, Iowa City, IA 52242.

View full text

Regular ArticleFolate and 10-Formyltetrahydrofolate Dehydrogenase in Human and Rat Retina: Relation to Methanol Toxicity☆

Abstract

Regular Article
Folate and 10-Formyltetrahydrofolate Dehydrogenase in Human and Rat Retina: Relation to Methanol Toxicity☆