Flavonoids conjugated forms

The yield of flavonoids in the urine is dramatically dependent on the flavonoid under examination. For quercetin and anthocyanins, it is less than 1.5%, for isofla-vones it is 2 to 20% and for catechins it is -5% (reviewed in Scalbert and Williamson ). Flavonoids are found in urine as conjugated forms for example, rat urinary catechins are (+)-catechin-5-0-P-glucuronide and (-)-epicatechin-5-0-P-glucuronide. Generally, renal excretion is not a major pathway for intact flavonoids and the urinary content of flavonoids cannot be used as a biomarker of bioavailability or dietary intake. 

Different conjugated forms of flavonoids are also found in plants. However, the complex flavonoids are not ubiquitously present in all plant species. Biflavonoids (anacarduflavanone, licoagrone, cordigone, leucaediflavone, delicaflavone, licoagrodin, and bicaryone A, B, C, and D are unusual biflavonoids), triflavonoids (aulacomniumtriluteolin), tetraflavonoids (lophiroflavan A, taiwanhomoflavone C), pentaflavonoids (ochnachalcone), and hexaflavonoids (azobechalcone) [7] have been reported from different plant extracts. 

Flavonoids can be determined quantitatively by direct (in glycoside or conjugated form) or indirect (after hydrolysis) analysis. However, sample preparation (e.g., particle size) and solvents used in extraction steps can significantly affect the results [99]. Method development for quantitation is often validated in terms of selectivity, accuracy, precision, recovery, calibration curve, and reproducibility. Biological sample methods have to comply with the Food and Drug Administration (FDA) guidelines for validation of bioanalytical method [100]. 

Three generalizations may be made specifically about flavonoid patterns in wood The compounds mainly occur free and not in conjugated form as glycoside reduced flavonoids - e.g. flavanones and flavanonols - predominate and several biosynthetically related structures representing different flavonoid classes commonly co-occur. An indication of the main structural classes found in wood is seen in Fig. 7.5.1. These classes are, of course, closely related biosynthetically, as shown in the figure. Compounds of the most common hydrox-ylation pattern are illustrated, but other series of hydroxylated flavonoids (e.g. 5-deoxy compounds) may also be found. 

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