Biosynthesis of phenylalanine,tyrosine, 3-(3-carboxyphenyl)alanine and 3-(3-carboxy-4-hydroxyphenyl)alanine in higher plants: Examples of the transformation possibilities for chorismic acid

¹⁴C-labelled shikimic acid and double labelled shikimic acid tritiated stereospecifically at C-6 are incorporated into 3-(3-carboxyphenyl)alanine, 3-(3-carboxyl-4-hydroxyphenyl)alanine, phenylalanine, and tyrosine in Resda lutea L., Reseda odoratta L., Iris x Hollandica cv. Prof. Blauw, and Iris x hollandica cv. Wedgwood. The experiments with ¹⁴C-labelled shikimic acid confirm that the aromatic carboxyl groups and rings in 3-(3-carboxyphenyl)-alanine and 3-(3-carboxy-4-hydroxyphenyl)alanine derive from the carboxyl group and ring in shikimic acid whereas the experiments with double labelled shikimic acid demonstrate that the pro-6S-hydrogen atom is retained and the pro-6R-hydrogen atom lost in the biosynthesis of 3-(3-carboxyphenyl)alanine, phenylalanine, and tyrosine in the plants used. ³H was located in the ortho-position in the aromatic rings of phenylalanine and tyrosine but in a position para to the alanine side chain of 3-(3-cabroxyphenyl)alanine. No ³H was found in 3-(3-carboxy-4-hydroxyphenyl)alanine. This supports a derivation of the last two compounds from chorismic acidvia isochorismic acid, isoprephenic acid, and 3′-carboxyphenylpyruvic acid and 3′-carboxy-4′-hydroxyphenylpyruvic acid. The ³H/¹⁴ C ratio in 3-(3-carboxyphenyl)alanine was found higher than in the precursor used. This isotope effect must operate by competition between the pathways from isoprephenic acid to 3′-carboxyphenylpyruvic acid and to 3′-carboxy-4′-hydroxyphenylpyruvic acid. The proposed biosynthetic pathways for the two carboxy-substituted amino acids are in agreement with their distribution patterns in the plant kingdom and suggest that they may derive from minor changes of enzymes involved in the general pathways of aromatic biosynthesis.