STUDIES OF LIGNIN BIOSYNTHESIS USING ISOTOPIC CARBON: VII. THE ROLE OF p-HYDROXYPHENYLPYRUVIC ACID

L-Phenylalanine-C14 and p-hydroxyphenylpyruvic acid-3-C14 were administered to wheat, buckwheat, and salvia (Salvia splendens Sello), and after 24 hours the "lignin aldehydes" (p-hydroxybenzaldehyde, vanillin, and syringaldehyde) were isolated following alkaline nitrobenzene oxidation of the plant fraction insoluble in ethanol–benzene and water. Although all species converted phenylalanine readily to lignin, only wheat could form the syringyl and guaiacyl parts of lignin efficiently from p-hydroxyphenylpyruvic acid. p-Hydroxybenzaldehyde recovered from all species was heavily labelled after feeding both compounds. The L-tyrosine from acid hydrolysis and the p-hydroxybenzaldehyde from nitrobenzene oxidation, isolated from buckwheat activated by feeding p-hydroxyphenylpyruvic acid-3-C14, had the same molar specific activity, and it is considered probable that most if not all of the p-hydroxybenzaldehyde in all these species was derived from protein tyrosine in the extracted residue. When p-hydroxyphenyllactic acid-3-C14 was metabolized by wheat all the aldehydes were labelled, but none possessed measurable radioactivity when this compound was administered to buckwheat. Consequently, neither p-hydroxyphenylpyruvic nor p-hydroxyphenyllactic acid is a general intermediate in lignification, and differences noted here and in previous papers between grasses and non-grasses probably result from the unique ability of grasses to convert p-hydroxyphenyllactic acid to p-hydroxycinnamic acid. This idea is supported by the ability of wheat to form p-hydroxycinnamic and ferulic acids readily from both phenylalanine and tyrosine, whereas only phenylalanine is a good precursor of these acids in salvia. A scheme is presented showing the metabolic interconversions of phenylpropanoid acids and related compounds leading to lignin.