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S-Methylmethionine Conversion to Dimethylsulfoniopropionate: Evidence for an Unusual Transamination Reaction.
MedLine Citation:
PMID:  12223879     Owner:  NLM     Status:  Publisher    
Leaves of Wollastonia biflora (L.) DC. synthesize the osmoprotectant 3-dimethylsulfoniopropionate (DMSP) from methionine via S-methylmethionine (SMM) and 3-dimethylsulfoniopropionaldehyde (DMSP-ald); no other intermediates have been detected. To test whether the amino group of SMM is lost by transamination or deamination, [methyl-2H3,15N]SMM was supplied to leaf discs, and 15N-labeling of amino acids was monitored, along with synthesis of [2H3]DMSP. After short incubations more 15N was incorporated into glutamate than into other amino acids, and the 15N abundance in glutamate exceeded that in the amide group of glutamine (Gln). This is more consistent with transamination than deamination, because deamination would be predicted to give greater labeling of Gln amide N due to reassimilation, via Gln synthetase, of the 15NH4+ released. This prediction was borne out by control experiments with 15NH4Cl. The transamination product of SMM, 4-dimethylsulfonio-2-oxobutyrate (DMSOB), is expected to be extremely unstable. This was confirmed by attempting to synthesize it enzymatically from SMM using L-amino acid oxidase or Gln transaminase K and from 4-methylthio-2-oxobutyrate using methionine S-methyltransferase. In each case, the reaction product decomposed rapidly, releasing dimethylsulfide. The conversion of SMM to DMSP-ald is therefore unlikely to involve a simple transamination that generates free DMSOB. Plausible alternatives are that DMSOB is channeled within a specialized transaminase-decarboxylase complex or that it exists only as the bound intermediate of a single enzyme catalyzing an unusual transamination-decarboxylation reaction.
D. Rhodes; D. A. Gage; AJL. Cooper; A. D. Hanson
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Publication Detail:
Journal Detail:
Title:  Plant physiology     Volume:  115     ISSN:  1532-2548     ISO Abbreviation:  Plant Physiol.     Publication Date:  1997 Dec 
Date Detail:
Created Date:  2002-Sep-11     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0401224     Medline TA:  Plant Physiol     Country:  -    
Other Details:
Languages:  ENG     Pagination:  1541-1548     Citation Subset:  -    
Department of Horticulture, Purdue University, West Lafayette, Indiana 47907 (D.R.).
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