Osmoprotectant β-alanine betaine synthesis in the Plumbaginaceae: S-adenosyl- l-methionine dependent N-methylation of β-alanine to its betaine is via N-methyl and N,N-dimethyl β-alanines
2000
Rathinasabapathi, Bala | Sigua, Celia | Ho, Janice | Gage, Douglas A.
β-Alanine betaine is an osmoprotective compound accumulated by most members of the plant family Plumbaginaceae. Leaf and root tissues of Limonium latifolium known to accumulate β-alanine betaine readily convert supplied β-alanine to β-alanine betaine. To identify the intermediates and the enzymes involved in β-alanine betaine synthesis, radiotracer experiments using [ C] formate were employed. These studies demonstrate that β-alanine betaine is synthesized from β-alanine via N-methyl and N,N-dimethyl β-alanines. A rapid and sensitive radiometric assay was developed to measure N-methyltransferase (NMT) activities by using [methyl-¹⁴C] or [methyl-³H] S-adenosyl- l-methionine (AdoMet) as the methyl donor. Leaf extracts from β-alanine betaine accumulators -Armeria maritima, L. latifolium and L. ramosissimum- had detectable NMT activities while none were found in L. perezii, a species that does not accumulate β-alanine betaine. The NMT activities were further characterized from the leaves of L. latifolium. The activities had a pH optimum of 8.0, were soluble and inhibited by S-adenosyl- l-homocysteine. Extractable activities were similar from plants grown under control and salinity stress conditions. Radiolabeling with [ C] l-aspartic acid indicated that, unlike in bacteria, decarboxylation of l-aspartic acid is not the source of β-alanine in the Plumbaginaceae.
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