In studies carried out on samples growing on diverse host trees, collected from diverse geographic locations, tyramine is present in leaves and stems (Vazquez y Novo et al, 1989). The detected concentration in most samples remains below 10 mg% but roughly 10% yield over 100 mg%. Plants parasiting Geoffroea decorticans (H. et Arn.) Burkart (Fabaceae) are the ones presenting the greatest amounts of tyramine, ranging from 120 to 360 mg per 100 g of dried material.
In all plant samples, regardless of the host tree, the phytochemical study of the flavonoids has disclosed the presence of quercetin as the only flavonol (Graziano et al., 1967; Wagner, 1993). Quercetin occurs free and monoglycosylated with xylose, rhamnose and arabinose at the hydroxyl group in position 3 of the flavonol skeleton (Wagner, 1993; Fernández et al., 1998).
Leucoanthocyanidins, catechin-4-b-ol and proanthocyanidins with variable degrees of polymerization (dimers, oligomers and polymers of catechin and epicatechin) which yield cyanidin after acid treatment are also present (Wagner, 1993; Fernández et al., 1998) (Figure 4).
In L. cuneifolia the precursor dihydroquercetin may follow one of two metabolic pathways: (1) The enzyme flavonol synthase oxidises dihydroquercetin leading to quercetin, part of which accumulates, while most is glycosilated in the hydroxyl group in the C3 by the uridine diphosphate-sugar-flavonoid-3-O-glycosyl-transferase. (2) The alternative pathway is activated by NADPH-dependent 3-hydroxyflavanone-4-reductase, which reduces the carbonyl group rendering leucocyanidin. This latter compound may be transformed into flavan-3-ol (catechin or epicatechin) by 3,4 cys-diol-reductase. Both leucocyanidin and flavan-3-ol may be condensed originating dimers, oligomers and polymers by the proanthocyanidin synthase enzymatic complex (Stafford, 1990).
Flavonoid synthesis in L. cuneifolia is more simple than in V. album where biosynthesis is more diversified since in the latter S-adenosyl-L methionine-X-O-methylase (SAM) gives rise to the methylated flavonoids not detected in the Argentine mistletoe (Figure 5).
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