RESUMEN
Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.
Asunto(s)
Deficiencia de Vitamina B 12/genética , Vitamina B 12/metabolismo , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/genética , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Acilcoenzima A/genética , Acilcoenzima A/fisiología , Humanos , Vitamina B 12/química , Vitamina B 12/farmacocinética , Deficiencia de Vitamina B 12/metabolismoRESUMEN
Vitamin B12 is produced only by prokaryotes and utilized by animals as an essential micronutrient. Genetic complementation analysis of cell lines from patients indicated that at least eight gene products are involved in intracellular B12 metabolism and utilization. We have investigated bacterial adenosylcobalamin-dependent enzymes and elucidated their structure-based fine mechanisms. They tend to undergo mechanism-based inactivation during catalysis, because they use highly reactive radicals for catalyzing chemically difficult reactions. We have discovered molecular chaperone-like reactivating factors for these enzymes that release a damaged cofactor forming apoenzyme. Methylcobalamin-dependent methionine synthase also undergoes inactivation, because it utilizes cob (I) alamin, a super nucleophile, for catalysis. Methionine synthase reductase is a reactivating partner for this enzyme. Recent studies suggested that activity-maintaining systems for B12 enzymes are present in animal cells as well, and thus hints for designing therapeutic agents for B12-related metabolic disorders might be obtained from the investigations of microbial B12 metabolism.
Asunto(s)
5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Metilmalonil-CoA Mutasa/fisiología , Vitamina B 12/metabolismo , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/química , Animales , Apoenzimas , Catálisis , Ferredoxina-NADP Reductasa/química , Ferredoxina-NADP Reductasa/fisiología , Humanos , Metilmalonil-CoA Mutasa/química , Chaperonas Moleculares , Deficiencia de Vitamina B 12RESUMEN
Homocysteine, a sulfur amino acid, is an intermediate metabolite of methionine. In 1969, McCully reported autopsy evidence of extensive arterial thrombosis and atherosclerosis in children with elevated plasma homocysteine concentrations and homocystinuria. On the basis of this observation, he proposed that elevated plasma homocysteine (hyperhomocysteinemia) can cause atherosclerotic vascular disease. Hyperhomocysteinemia is now well established as an independent risk factor for atherosclerotic vascular disease. Mild hyperhomocysteinemia is quite prevalent in the general population. It can be caused by genetic defects in the enzymes involved in homocysteine metabolism or nutritional deficiencies in vitamin cofactors, certain medications or renal disease. An increase of 5 micromol per liter in the plasma homocysteine concentration raises the risk of coronary artery disease by as much as an increase of 20 mg per deciliter in the cholesterol concentration. In this article, we review the biochemical, experimental and clinical studies on hyperhomocysteinemia, with emphasis on the metabolism and pharmacokinetics of homocysteine.
Asunto(s)
Enfermedades Cardiovasculares/etiología , Homocisteína/metabolismo , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Animales , Betaína-Homocisteína S-Metiltransferasa/fisiología , Deuterio , Cromatografía de Gases y Espectrometría de Masas , Gota , Homocisteína/sangre , Humanos , Hiperhomocisteinemia , Marcaje Isotópico , Metionina/metabolismo , Oxidación-Reducción , Factores de Riesgo , Albúmina Sérica , Vitamina B 12/fisiologíaRESUMEN
This paper describes (i) the expression profile of the methionine synthase gene (MET6) in the human pathogenic fungus Cryptococcus neoformans and (ii) the phenotypes of a C. neoformans met6 mutant. In contrast to the MET3 gene, which showed no significant change in expression in any environmental condition tested, the MET6 gene showed a substantial induction in response to methionine and a dramatic transcriptional induction in response to homocysteine. Like a met3 mutant, the met6 mutant was a methionine auxotroph. However, relative to a met3 mutant, the met6 mutant grew very slowly and was less heat-shock resistant. In contrast to a met3 mutant, the met6 mutant lost viability when starved of methionine, and it was deficient in capsule formation. Like a met3 mutant, the met6 mutant was avirulent. In contrast to a met3 mutant, the met6 mutant was hypersensitive to fluconazole and to the calcineurin inhibitors FK506 and cyclosporin A. A synergistic fungicidal effect was also found between each of these drugs and met6. The phenotypic differences between the met3 and met6 mutants may be due to the accumulation in met6 mutants of homocysteine, a toxic metabolic intermediate that inhibits sterol biosynthesis.
Asunto(s)
5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/genética , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Cryptococcus neoformans/enzimología , Cryptococcus neoformans/patogenicidad , Factores de Virulencia/genética , Adaptación Fisiológica , Animales , Antifúngicos/farmacología , Criptococosis/microbiología , Cryptococcus neoformans/crecimiento & desarrollo , Ciclosporina/farmacología , Fluconazol/farmacología , Eliminación de Gen , Regulación Fúngica de la Expresión Génica , Genes Fúngicos , Homocisteína/metabolismo , Calor , Melaninas/biosíntesis , Metionina/metabolismo , Ratones , Pruebas de Sensibilidad Microbiana , Mutación , ARN de Hongos/análisis , ARN de Hongos/aislamiento & purificación , ARN Mensajero/análisis , ARN Mensajero/aislamiento & purificación , Tacrolimus/farmacología , Transcripción Genética , Virulencia/genéticaRESUMEN
The intracellular ratio between methionine and its activated form S-adenosylmethionine (AdoMet) is of crucial importance for the one-carbon metabolism. AdoMet recycling into methionine was believed to be largely achieved through the methyl and the thiomethyladenosine cycles. We show here that in yeast, AdoMet recycling actually occurs mainly through the direct AdoMet-dependent remethylation of homocysteine. Compelling evidences supporting this result were obtained owing to the identification and functional characterization of two new genes, SAM4 and MHT1, that encode the yeast AdoMet-homocysteine methyltransferase and S-methylmethionine-homocysteine methyltransferase, respectively. Homologs of the Sam4 and Mht1 proteins exist in other eucaryotes, indicating that such enzymes would be universal and not restricted to the bacterial or fungal kingdoms. New pathways for AdoMet or S-methylmethionine-dependent methionine synthesis are presented.
Asunto(s)
Proteínas de la Membrana , Metionina/biosíntesis , S-Adenosilmetionina/metabolismo , Proteínas de Saccharomyces cerevisiae , Levaduras/metabolismo , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Proteínas Adaptadoras Transductoras de Señales , Secuencia de Aminoácidos , Desoxiadenosinas/metabolismo , Proteínas Fúngicas/química , Proteínas Fúngicas/genética , Regulación de la Expresión Génica , Homocisteína S-Metiltransferasa/fisiología , Datos de Secuencia Molecular , Proteínas de Saccharomyces cerevisiae/fisiología , Tionucleósidos/metabolismo , Vitamina B 12/farmacología , Vitamina U/metabolismoAsunto(s)
5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Cistationina betasintasa/fisiología , Ácido Fólico/uso terapéutico , Homocisteína/sangre , Defectos del Tubo Neural/etiología , Oxidorreductasas actuantes sobre Donantes de Grupo CH-NH/fisiología , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/deficiencia , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/genética , Cistationina betasintasa/deficiencia , Cistationina betasintasa/genética , Femenino , Ácido Fólico/administración & dosificación , Ácido Fólico/metabolismo , Humanos , Hiperhomocisteinemia/complicaciones , Recién Nacido , Metilenotetrahidrofolato Reductasa (NADPH2) , Defectos del Tubo Neural/prevención & control , Necesidades Nutricionales , Oxidorreductasas actuantes sobre Donantes de Grupo CH-NH/deficiencia , Oxidorreductasas actuantes sobre Donantes de Grupo CH-NH/genética , Embarazo , Complicaciones del Embarazo/sangre , Vitamina B 12/fisiologíaRESUMEN
BACKGROUND: Elevated plasma total homocysteine, an independent risk factor for cardiovascular disease, is commonly observed in renal patients. We have previously shown that the kidney is a major site for the removal of plasma homocysteine in the rat. The present investigation was performed to further characterize the capacity of the kidney to handle acute elevations in plasma homocysteine concentrations. METHODS: Acute hyperhomocysteinemic conditions (4- to 7-fold > controls) in rats were produced by either a primed-continuous infusion of L-homocysteine or exposure to 80:20% nitrous oxide:oxygen, which results in the inhibition of methionine synthase. RESULTS: At physiological homocysteine concentrations, approximately 15% of the arterial plasma homocysteine was removed on passage through the kidney. Renal homocysteine uptake was approximately 85% of the filtered load. The urinary excretion of homocysteine was negligible (<2%). During acute hyperhomocysteinemia produced by the infusion of L-homocysteine, renal homocysteine uptake was increased fourfold and was equivalent to 50% of the infused dose, while urinary excretion remained negligible. Renal homocysteine uptake during nitrous oxide-induced hyperhomocysteinemia increased threefold, with urinary excretion remaining negligible. CONCLUSIONS: These results provide strong evidence that the kidney has a significant capacity for metabolizing acute elevations in plasma homocysteine, and support a very limited role for the re-methylation pathway in renal homocysteine metabolism.
Asunto(s)
Homocisteína/metabolismo , Riñón/metabolismo , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Enfermedad Aguda , Animales , Hemodinámica , Homocisteína/sangre , Masculino , Metilación , Ratas , Ratas Sprague-DawleyAsunto(s)
5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/efectos de los fármacos , Encéfalo/efectos de los fármacos , Etanol/toxicidad , Metionina Adenosiltransferasa/efectos de los fármacos , 5-Metiltetrahidrofolato-Homocisteína S-Metiltransferasa/fisiología , Animales , Encéfalo/enzimología , Femenino , Metionina Adenosiltransferasa/fisiología , Ratas , Ratas Sprague-DawleyRESUMEN
The excretion of methylmalonic acid was measured and deoxyuridine suppression test (dU-test) was performed in 5 patients before and after 24 h of ventilation with nitrous oxide. The mean urinary 2-methylmalonic acid was increased from 90 to 320 mumol/24 h and the dU-tests became abnormal. These findings indicate that the 2 vitamin B-12 dependent enzymes, methylmalonyl-CoA-mutase and methione synthetase, are depressed by nitrous oxide.