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Title: Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats: protection by methionine and aggravation by folates

Abstract

Nitrous oxide, which inactivates cobalamin when administered to fruit bats, results in severe neurological impairment leading to ataxia, paralysis and death. This occurs after about 6 weeks in animals depleted of cobalamin by dietary restriction, and after about 10 weeks in cobalamin replete bats. Supplementation of the diet with pteroylglutamic acid caused acceleration of the neurological impairment--the first unequivocal demonstration of aggravation of the neurological lesion in cobalamin deficiency by pteroylglutamic acid. The administration of formyltetrahydropteroylglutamic acid produced similar aggravation of the neurological lesion. Supplementation of the diet with methionine protected the bats from neurological impairment, but failed to prevent death. Methionine supplementation protected against the exacerbating effect of folate, preventing the development of neurological changes. These findings lend support to the hypothesis that the neurological lesion in cobalamin deficiency may be related to a deficiency in the methyl donor S-adenosylmethionine which follows diminished synthesis of methionine.

Authors:
; ;
Publication Date:
Research Org.:
South African Inst. for Medical Research, Johannesburg, South Africa
OSTI Identifier:
6117790
Resource Type:
Journal Article
Resource Relation:
Journal Name: Life Sci.; (United States); Journal Volume: 31:18
Country of Publication:
United States
Language:
English
Subject:
63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTANT EFFECTS ON LIVING ORGS. AND BIOL. MAT.; NERVOUS SYSTEM; INJURIES; NITROUS OXIDE; BIOLOGICAL EFFECTS; VITAMIN B-12; INACTIVATION; BATS; FOLIC ACID; METHIONINE; RESPONSE MODIFYING FACTORS; AMINO ACIDS; ANIMALS; CARBOXYLIC ACIDS; CHALCOGENIDES; DRUGS; HEMATINICS; HEMATOLOGIC AGENTS; HETEROCYCLIC COMPOUNDS; HYDROXY COMPOUNDS; LIPOTROPIC FACTORS; MAMMALS; NITROGEN COMPOUNDS; NITROGEN OXIDES; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; ORGANIC SULFUR COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; PTERIDINES; VERTEBRATES; VITAMIN B GROUP; VITAMINS; 560305* - Chemicals Metabolism & Toxicology- Vertebrates- (-1987)

Citation Formats

van der Westhuyzen, J., Fernandes-Costa, F., and Metz, J. Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats: protection by methionine and aggravation by folates. United States: N. p., 1982. Web. doi:10.1016/0024-3205(82)90039-X.
van der Westhuyzen, J., Fernandes-Costa, F., & Metz, J. Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats: protection by methionine and aggravation by folates. United States. doi:10.1016/0024-3205(82)90039-X.
van der Westhuyzen, J., Fernandes-Costa, F., and Metz, J. Mon . "Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats: protection by methionine and aggravation by folates". United States. doi:10.1016/0024-3205(82)90039-X.
@article{osti_6117790,
title = {Cobalamin inactivation by nitrous oxide produces severe neurological impairment in fruit bats: protection by methionine and aggravation by folates},
author = {van der Westhuyzen, J. and Fernandes-Costa, F. and Metz, J.},
abstractNote = {Nitrous oxide, which inactivates cobalamin when administered to fruit bats, results in severe neurological impairment leading to ataxia, paralysis and death. This occurs after about 6 weeks in animals depleted of cobalamin by dietary restriction, and after about 10 weeks in cobalamin replete bats. Supplementation of the diet with pteroylglutamic acid caused acceleration of the neurological impairment--the first unequivocal demonstration of aggravation of the neurological lesion in cobalamin deficiency by pteroylglutamic acid. The administration of formyltetrahydropteroylglutamic acid produced similar aggravation of the neurological lesion. Supplementation of the diet with methionine protected the bats from neurological impairment, but failed to prevent death. Methionine supplementation protected against the exacerbating effect of folate, preventing the development of neurological changes. These findings lend support to the hypothesis that the neurological lesion in cobalamin deficiency may be related to a deficiency in the methyl donor S-adenosylmethionine which follows diminished synthesis of methionine.},
doi = {10.1016/0024-3205(82)90039-X},
journal = {Life Sci.; (United States)},
number = ,
volume = 31:18,
place = {United States},
year = {Mon Nov 01 00:00:00 EST 1982},
month = {Mon Nov 01 00:00:00 EST 1982}
}
  • It is suggested that mammalian cells have evolved to respond to methionine deficiency since in such circumstances vital methylation reactions are put at risk, due to decreased levels of S-adenosyl-methionine. Decreased cellular homocysteine, as a result of decreased methionine, would also restrict cell division by decreased conversion of plasma 5-CH3-H/sub 4/PteGlu into intracellular polyglutamates. Cobalamin deficiency, either nutritional or due to exposure to the Co(I)cobalamin inactivating agent nitrous oxide, prevents the demethylation of 5-CH3-H/sub 4/PteGlu, which even in the presence of adequate amounts of homocysteine and methionine prevents rapidly proliferating cells from converting enough of the plasma 5-CH3-H/sub 4/ PteGlumore » into folylpolyglutamate forms to permit normal DNA biosynthesis and cell replication. This, together with the trapping of the cellular folate cofactors in the 5-CH3-H/sub 4/PteGlu form, results in megaloblastic changes occurring in tissues such as the marrow. The vital role of the methylation reactions was demonstrated by exposing monkeys to nitrous oxide which inactivated their methionine synthetase. The resultant ataxia and severe demyelination was prevented and diminished by methionine supplementation. When methionine synthetase was similarly inactivated in mice it was shown that while 5-CH3-H/sub 4/PteGlu enters mammalian cells, it is not converted into a polyglutamyl form and subsequently leaves the cell unmetabolised. In similar experiments in rats methionine was found to have only a small effect in restoring folylpolyglutamate biosynthesis. It was found that a decrease in the deoxythymidine salvage pathway by methionine has led others to the mistaken conclusion that methionine has an 'anti-folate' effect in bone marrow, i.e. that it decreases folate availability for thymidylate synthetase.« less
  • Cobalamin-dependent methionine synthase (MetH) is a modular protein that catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine to produce methionine and tetrahydrofolate. The cobalamin cofactor, which serves as both acceptor and donor of the methyl group, is oxidized once every {approx}2,000 catalytic cycles and must be reactivated by the uptake of an electron from reduced flavodoxin and a methyl group from S-adenosyl-L-methionine (AdoMet). Previous structures of a C-terminal fragment of MetH (MetH{sup CT}) revealed a reactivation conformation that juxtaposes the cobalamin- and AdoMet-binding domains. Here we describe 2 structures of a disulfide stabilized MetH{sup CT} ({sub s-s}MetH{supmore » CT}) that offer further insight into the reactivation of MetH. The structure of {sub s-s}MetH{sup CT} with cob(II)alamin and S-adenosyl-L-homocysteine represents the enzyme in the reactivation step preceding electron transfer from flavodoxin. The structure supports earlier suggestions that the enzyme acts to lower the reduction potential of the Co(II)/Co(I) couple by elongating the bond between the cobalt and its upper axial water ligand, effectively making the cobalt 4-coordinate, and illuminates the role of Tyr-1139 in the stabilization of this 4-coordinate state. The structure of {sub s-s}MetH{sub CT} with aquocobalamin may represent a transient state at the end of reactivation as the newly remethylated 5-coordinate methylcobalamin returns to the 6-coordinate state, triggering the rearrangement to a catalytic conformation.« less
  • B{sub 12}-dependent methionine synthase (MetH) from Escherichia coli is a large modular protein that is alternately methylated by methyltetrahydrofolate to form methylcobalamin and demethylated by homocysteine to form cob(I)alamin. Major domain rearrangements are required to allow cobalamin to react with three different substrates: homocysteine, methyltetrahydrofolate, and S-adenosyl-l-methionine (AdoMet). These same rearrangements appear to preclude crystallization of the wild-type enzyme. Disulfide cross-linking was used to lock a C-terminal fragment of the enzyme into a unique conformation. Cysteine point mutations were introduced at Ile-690 and Gly-743. These cysteine residues span the cap and the cobalamin-binding module and form a cross-link that reducesmore » the conformational space accessed by the enzyme, facilitating protein crystallization. Here, we describe an x-ray structure of the mutant fragment in the reactivation conformation; this conformation enables the transfer of a methyl group from AdoMet to the cobalamin cofactor. In the structure, the axial ligand to the cobalamin, His-759, dissociates from the cobalamin and forms intermodular contacts with residues in the AdoMet-binding module. This unanticipated intermodular interaction is expected to play a major role in controlling the distribution of conformers required for the catalytic and the reactivation cycles of the enzyme.« less
  • Cobalamin-independent methionine synthase (MetE) catalyzes the transfer of a methyl group from methyltetrahydrofolate to L-homocysteine (Hcy) without using an intermediate methyl carrier. Although MetE displays no detectable sequence homology with cobalamin-dependent methionine synthase (MetH), both enzymes require zinc for activation and binding of Hcy. Crystallographic analyses of MetE from T. maritima reveal an unusual dual-barrel structure in which the active site lies between the tops of the two ({beta}{alpha}){sub 8} barrels. The fold of the N-terminal barrel confirms that it has evolved from the C-terminal polypeptide by gene duplication; comparisons of the barrels provide an intriguing example of homologous domainmore » evolution in which binding sites are obliterated. The C-terminal barrel incorporates the zinc ion that binds and activates Hcy. The zinc-binding site in MetE is distinguished from the (Cys){sub 3}Zn site in the related enzymes, MetH and betaine-homocysteine methyltransferase, by its position in the barrel and by the metal ligands, which are histidine, cysteine, glutamate, and cysteine in the resting form of MetE. Hcy associates at the face of the metal opposite glutamate, which moves away from the zinc in the binary E {center_dot} Hcy complex. The folate substrate is not intimately associated with the N-terminal barrel; instead, elements from both barrels contribute binding determinants in a binary complex in which the folate substrate is incorrectly oriented for methyl transfer. Atypical locations of the Hcy and folate sites in the C-terminal barrel presumably permit direct interaction of the substrates in a ternary complex. Structures of the binary substrate complexes imply that rearrangement of folate, perhaps accompanied by domain rearrangement, must occur before formation of a ternary complex that is competent for methyl transfer.« less