Proanthocyanidine oxidase

Leucoanthocyanidin reductase (LAR) converted leucoanthocyanidins (flavan-3,4-diols, 37) to flavan-3-ols (40). The flavan-3-ols (40) were converted to condensed tannins (proanthocyanidins, PA, 44) by condensing enzyme (CE). Following this condensed tannins (proanthocyanidins, PA, 44) finally yielded their oxidized tannins (oxidized proanthocyanins, 45) by proanthocyanidine oxidase (PRO) (Fig. 9) [23,24]. 

Indirect evidence for the intermediacy of a /)-quinone methide of type (213) in the oxidative conversion of B- into A-type proanthocyanidins came from the oxidation of epigallocatechin (216) with the homogenate of banana fruit flesh polyphenol oxidase. " Besides racemization at C-2, the oxidative conversion also gave retro-ct-hydroxydihydrochalcone (219) (Scheme 24), presumably via initial oxidation of (216) to the B-ring quinone methide (217). Hydration gave the unstable hemiacetal (218) that would equilibrate with the 1,3-diarylketone (219). It was also shown that laccase (EC 1.10.3.2) catalyzed the conversion of procyanidin B-2 (5) into procyanidin A-2 (215). 

The antioxidant activity of alizarin was established in four different assays (1) suppression of light emission in the p-iodophenol enhanced chemiluminescent assay, (2) scavenging of superoxide anion (02 -) in a hypoxanthine-xanthine oxidase system, (3) protection of rat liver microsomes from lipid peroxidation by ADP/iron(II) ions, and (4) protection of bromobenzene-intoxicated mice from liver injury in vivo [141]. Alizarin was compared with Trolox (water soluble vitamin E), the flavonoid baicalin and green tea proanthocyanidins. In assay (1) the activity of alizarin was 76% of that of Trolox. In assay (2) the inhibition of 02 -induced chemiluminescence was 40%, 32%, 23% and 14% for Trolox, alizarin, green tea polyphenols and baicalin respectively. Alizarin was not significantly active in the lipid peroxidation assay but after baicalin the most active compound in the in vivo assay. This shows again the difficulty in the evaluation of antioxidant activity and the differences between in vitro and in vivo assays [141]. 

Phenolic compounds isolated from grape seed extract (catechin and proanthocyanidin B4) Cardiac H9C2 cells Reduction in xanthine oxidase (XO)/xanthine-induced intracellular reactive oxygen species (ROS) accumulation and cardiac cell apoptosis [149]... 

Senescence in leaf tissue leads to a general desiccation of the organ and consequent breakdown of the cells. Possibly proanthocyanidins are further polymerized by the native oxidase enzymes of the leaf, and hence rendered insoluble. 

Hathway has shown that oak bark contains large concentrations of oxidase enzymes and demonstrated their effect on the flavan-3-ols which occur in oak bark (59). Ahn and Gstirner have reported the presence of oxidatively coupled flavan-3-ol dimers in oak bark, in addition to the normal proanthocyanidin dimers (2). Such products are consistent with the presence of oxidase/peroxidase enzymes. Such secondary processes would explain the very high dispersitivities observed for Firms sylvestris tannins, compared with values for tannins isolated from living plant tissue (R. Marutzky and L. J. Porter, unpublished observations). It would also account for the extremely high MW tannin-like material isolated by Yazaki and Hillis from Pinus radiata bark methanol extracts by ultrafiltration (145). 

It is still not known whether the condensation of monomers proceeds by enzymatic or non-enzymatic mechanisms, or by both mechanisms. The main building units are (ra s-2,3-flavan-3-ols, that is (2i ,3S)-isomers and cis-2,3-flavan-3-ols that are (2J ,3J )-isomers. The majority of proanthocyanidins arise from (+)-catechin, which is the lower unit and (-)-epicatechin, which is the upper unit in proanthocyanidins. It is assumed that the formation of the upper units involves quinones or carbocations arising from leucoanthocyanidins, anthocyanidins and flavan-3-ols and perhaps some enzymes, such as leucoanthocyanidin reductases and polyphenol oxidases. The expected mechanism of formation of these proanthocyanidin units is outlined in Figure 8.95. 

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