Tryptophan Synthetase Activity in Pea Seedling Extracts.

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Abstract
The buds of pea seedlings, both light-grown and etiolated, yielded extracts, which when incubated in the presence of indole, serine, pyridoxal phosphate and buffer, catalyzed the disappearance of indole. Omission of either serine or of pyridoxal phosphate greatly depressed this activity. L-serine appears to be specific for this reaction. The following were inactive: D-serine, glycine, threonine, homoserine, cysteine, alanine and glycolate. The activity is low at pH values below 7.0, starts to rise sharply at about 7.0, and reaches a plateau starting at about pH 8.0-8.5. Indole disappearance progressed at a constantly decreasing rate during the 100-minute incubation period studied. Expanded pea leaves also yielded active extracts, but preparations from stems, root-tips, seeds, pods and cotyledons gave little or no activity. Several other plant extracts were inactive also. This apparent lack of activity appears to be due to the extreme lability of the enzyme in homogenates. Chromatographic evidence, together with spray behavior, indicates that the product of the reaction is tryptophan. Thus, higher plants possess a tryptophan-synthesizing pathway known to exist in microorganisms.