S-methylmethionine conversion to dimethylsulfoniopropionate: evidence for an unusual transamination reaction

• Published in: Plant physiology (Bethesda) Vol. 115; no. 4; pp. 1541 - 1548
• Main Authors: Rhodes, D, Gage, D.A, Cooper, A.J.L, Hanson, A.D
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Physiologists 01.12.1997
• Subjects: ACIDE GLUTAMIQUE; Acidic amino acids; ACIDO GLUTAMICO; ACTIVIDAD CATALITICA; ACTIVITE CATALYTIQUE; Amides; Amino acids; AMINO COMPOUNDS; AMINOTRANSFERASAS; AMINOTRANSFERASE; AMINOTRANSFERASES; Ammonia; Betaines; BIOCHEMICAL PATHWAYS; Biochemistry and Enzymology; Biological and medical sciences; BIOSINTESIS; BIOSYNTHESE; BIOSYNTHESIS; CARBOXY-LYASES; CATALYTIC ACTIVITY; COMPOSE AMINE; COMPOSITAE; COMPUESTOS DE AMINA; DEAMINATION; Decarboxylation; DESAMINACION; DESAMINATION; Enzymes; FEUILLE; Fundamental and applied biological sciences. Psychology; GLUTAMATE-AMMONIA LIGASE; GLUTAMATES; GLUTAMIC ACID; GLUTAMINA; GLUTAMINE; HOJAS; ISOTOPE RADIOACTIF; LEAVES; LIASAS; LIGASAS; LIGASE; LIGASES; LYASE; LYASES; Metabolism; METABOLITE; METABOLITES; METABOLITOS; Neutral amino acids; Nitrogen metabolism; Oxidases; OXIDOREDUCTASES; OXIDORREDUCTASAS; OXYDOREDUCTASE; Plant physiology and development; Plants; PRECURSORS; PROPIONATE; PROPIONATES; PROPIONATOS; RADIOACTIVE TRACERS; RADIOISOTOPES; RADIOISOTOPOS; TRANSAMINACION; TRANSAMINATION; VIA BIOQUIMICA DEL METABOLISMO; VOIE BIOCHIMIQUE DU METABOLISME; WEDELIA; Sulfur compounds; Methionine; Osmoregulation; Propionic acid derivatives; Biosynthesis; Plant leaf; Compositae; Metabolism; Transamination; Dicotyledones; Angiospermae; Plant tissue disc; Spermatophyta
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.115.4.1541

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