44 The Enzymic Synthesis of β-Substituted Alanines in Plants
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The plants produce a number of amino acids that may be regarded as heterocyclic β-sub. alanines. Tryptophan and histidine can be considered as member of this group, but more examples are found among the "non-protein" amino acids synthesized in a more restricted manner by plants. I, II, III, IV and V are plant products that illustrate a range of heterocyclic ring structures. The common identity of these compounds as β-sub. alanines naturally leads to the idea that similar biogenetic pathways may be responsible for their formation in plants. Our recent attempts to demonstrate the reversibility of react. 1 have proved negative: some S-methylcysteine (R=CH_3) disappeared during incubation with VI, but mimosine was not formed. The Leucaena extracts also failed to catalyze transfer of the C_3 moiety from mimosine to indole, pyrazole, uracil, NH_3 or urea: transfer reaction of this kind conceivably could yield the amino acid, try., II, III, α-β-diaminopropionic acid and albizziine, respectively. However, mimosine was produced when 3, 4-dihydroxypyridine (VI) and O-acetylserine were incubated togather with Leucaena Leucocephala extracts: serine or O-phosphoserine could not substitute for the O-acetylserine. pH-activity (7.9-8.0), time course and the effect of B6 on mimosine synthetase are shown in Fig.2, 3 and 4. Mimosine formation was determined chromatographically: paper chromat., amino acid analyzer coupled to a flow monitor system (Fig. 1). During the investigation of a role of O-acetylserine as a intermediate on serine metabolic pathway in high plants, we have demonstrated the formation of β-(pyrazolyl-N)-alanine (II) by an enzyme in watermelon (Citrullus Vulgaris) seedlings from O-acetylserine and pyrazole (Reaction 2), though these are unable to detect any activity with serine and O-phosphoserine. pH represents a fairly sharp optimum for the condensation reaction (Fig.7). When reaction mixtures were incubated at 30℃, β-(pyrazolyl-N)-alanine production was linear for about 1hr. (Fig.8). Formation of II occurred from pyrazole and serine in the presence of the treated watermelon extract if acetyl CoA were also added, but the rate of reaction was only about 2% of that observed with O-acetylserine and pyrazole. The addition of pyridoxal phosphate to reaction mixtures caused neither stimulation nor inhibition of II formation. However, when 3, 4-dihydroxypyridine was added, II formation was reduced by about 40%. Both mimosine synthetase and β-(pyrazolyl-N)-alanine synthetase in Leucaena Leucocephala and Citrullus Vulgaris seedlings did not require any co-factor such as metal ions.
