The amino acids phenylalanine and tyrosine have been shown to be precursors of RA biosynthesis (52,73-75). Phenylalanine is transformed to an activated hydroxycinnamic acid by the enzymes of the general phenylpropanoid pathway, which are already well known for biosynthesis of flavonoid or lignin (75). Using radioactive phenylalanine and tyrosine, it was established that these two amino acids are incorporated into caffeic acid and 3,4-dihydroxyphenyllactic acid moieties, respectively (73). Steps in RA biosynthesis originating from phenylalanine and tyrosine have been characterized (Figure 8.2)
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