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PLANT PHYSIOLOGY , Vol 115, Issue 4 1541-1548, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
S-Methylmethionine Conversion to Dimethylsulfoniopropionate: Evidence for an Unusual Transamination Reaction
D. Rhodes, D. A. Gage, AJL. Cooper and A. D. Hanson
Department of Horticulture, Purdue University, West Lafayette, Indiana 47907 (D.R.)
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.
This article has been cited by other articles:
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