(6,52,65,76,77). In several cell suspension cultures, activity of phenylalanine ammonia-lyase (PAL) was correlated to RA (6,77). Basic characteristics of this enzyme were determined in protein preparations from suspension cell suspension cultures of Coleus blumei (78). The reaction, following formation of cinnamic acid, is the hydroxylation in the position 4 to 4-coumaric acid by cytochrome P450 monooxygenase cinnamate 4-hydroxylase (74). Generally, coenzyme A thioester, or glucose or chlorogenic acid have been shown to serve as donors of hydroxycinnamic acid moieties (75). Further, using A. officinalis cell suspension cultures, it was reported tyrosine amino transferase catalyzes the first step of the transformation of tyrosine to 3,4-dihydroxyphenyllactic acid. Several isoforms of tyrosine aminotransferase were found to be active in cell suspension cultures of A. officinalis (76,77). Prephenate aminotransferase in A. officinalis cell suspension cultures was found to be important, and its activity was affected by 3,4-dihydroxyphenyllactic acid (79). Other enzymes of late steps in the RA biosynthesis pathway, like hydroxyphenylpyruvate reductase and RA synthase (hydroxycinnamoyl-CoA, hydroxyphenyllactate, hydroxycinnamoyl transferase), were isolated and characterized in cell suspension cultures of C. blumei (80-82). Under the release of coenzyme A, the ester linkage is formed between carboxyl group of 4-coumaric acid and the aliphatic hydroxyl group of 4-coumaric acid and the aliphatic hydroxyl group of 4-hydroxyphenyllactate (75). Other studies have isolated microsomal hydroxylase, later confirmed as cytochrome P450 monooxygenases (74,83), whose activities introduce hydroxyl groups at position 3 and 3'-hydroxyphenyllactate to give rise to the aromatic rings of ester 4-coumaroyl-4'-hydroxyphenyllactate to give rise to RA (52). A number of cDNAs encoding cytochrome P450s, which can hydroxylate 4-coumaric acid or 4-coumaroyl moiety in an ester, have been isolated (84-86). This enzyme isolation led to the proposal that the complete biosynthetic pathway for RA biosynthesis originates from phenylalanine and tyrosine (52,75).
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