Microbial Metabolism. Part 12.

Abstract

Flavonoids which constitute an important part of human diet are associated with many beneficial biological activities, 2) though some may exhibit mutagenic and/or prooxidant properties.3) They are known to interact with the cytochrome P450 drug-metabolizing enzymes found in liver and other tissues including skin. The capability of flavonoids to induce biosynthesis of Cytochromes P450 (CYPs) and to modulate their activity could lead to loss of biological activity of a drug or to increase its concentration in the plasma resulting in an overdose.3) They may also activate or inhibit CYPs involved in the process of chemical carcinogenesis. It has been observed that Flavonoids with hydroxyl groups tend to inhibit these enzymes, whereas those without seem to stimulate them.4) The CYPs in turn can bring about transformation of flavonoids to metabolites which could interact with them and modify their activities. This underlines the importance of determining the metabolic pathways of flavonoids which are being consumed in measurable amounts by man. There are two major sites of flavonoid metabolism.3) They are the liver and the colon microflora. Several in vivo and in vitro experiments were conducted to identify the mammalian metabolites of different classes of flavonoids. Based on these observations various suggestions were made with regards to their metabolic pathways. The results of one such experiment involving the metabolism of 16 naturally occurring flavonoids by rat liver microsomes suggested that the B-ring was the main structural moiety that was subjected to biotransformation.5) The number and the position of the hydroxyl and methoxy groups in the B-ring seemed to determine the extent of metabolism. The non polar flavonoids were metabolized more extensively than the polar ones. The absence of hydroxyl groups in the B-ring or the presence of one such group at C-4Ј position resulted in 3Ј and 4Ј dihydroxylated (catechol) products. It was suggested that the semiquinones or quinones that could result from the oxidation of catechol may be a source of mutagenic free radicals. However, it has been demonstrated that partial detoxification takes place by O-methylation of the catechol moiety. 5,6) Flavanones, as compared to other flavonoids are less abundant in nature. Propolis, honey, citrus fruits and juices are their main dietary sources. 7) Flavanones are associated with many biological properties. The major constituent of citrus fruits, Naringenin for example was shown by in vitro experiments to have antioxidant, antiproliferative and estrogenic activities.7) Since citrus fruits and juices are consumed by man in considerable amounts, investigations have been carried out to evaluate their metabolic parthways. In a transformation study with six flavanones by rat liver microsomes the major metabolic pathway observed was aromatic hydroxylation. However, several previously unreported metabolic pathways including the reduction of the keto group were also discovered. 7) As many as 43 urinary metabolites of flavanone in the ra...

Eleven Microbial Metabolites of 6-Hydroxyflavanone

“…Viewing the reported data, it could be said that microbial transformation efficiency of a given compound could be improved by using a variety of microbial strains. [10][11][12][13]18,27) 1 and its metabolites showed no activity against available test such as antibacterial and antifungal assays. However, weak antileishmanial activity was observed for metabolite 11 when tested against Leishmania donovani.…”

“…[10][11][12][13] It is noteworthy that the number of microbial metabolites formed with flavanones far exceeds those reported for other classes of flavonoids making them difficult subjects for biological evaluations. Identification of potential metabolites by microbial transformation studies could be a source of valuable information for such investigations.…”