EVIDENCE FOR FUNCTIONAL HETEROGENEITY BOTH BETWEEN AND WITHIN FOUR SOURCES OF CONDENSED TANNIN (PROLINE-RICH PROTEINS, GLYCOPROTEINS, IODINATION, COMPETITIVE BINDING ASSAY)
Abstract
Condensed tannins are polymers of flavan-3-ols that are produced by many plants in a wide variety of tissues. The ability of these compounds to actively precipitate proteins has been linked to nutritional deficiencies in many animals. However the functions of tannins remains obscure because they are difficult to isolate or assay. It is also unknown to what extent tannins differ in their protein binding and precipitating activities. Four purified tannins (quebracho, wattle, pinto bean and sorghum) were compared by chemical assays and astringency towards ('14)C -BSA. Quebracho and wattle tannins were much less astringent and had longer chain lengths than sorghum or pinto bean tannins. Quebracho tannin was fractionated by purifying precipitable tannin from protein/tannin complexes. Tannin precipitated by high affinity protein (gelatin) was more astringent than tannin precipitated by low affinity protein (hen egg ovalbumin) or prepared by chromatography on Sephadex LH-20. Astringency seems to be a function of affinity, chain length and solubility of the complex. The relative affinities of quebracho, wattle, pinto bean and sorghum tannins for selected proteins were determined. Generally the tannins had the same rank order of affinity for the protein, implying common mechanisms for interaction. However, quebracho tannin had a very high affinity for salivary proline-rich glycoproteins (PRPs) and pinto bean tannin alone had a measurable affinity for soybean trypsin inhibitor. This suggests that tannin/protein interactions in vivo may be very specific. Protein bound carbohydrate enhanced the binding of PRPs to tannin and conferred specificity on the interactions. Removal of the oligosaccharides eliminated the uniquely high affinity of PRP GP-158 for quebracho tannin. Carbohydrate also increases the solubility of protein/tannin complexes, which may aid the animal in eliminating the complexes. ('125)I -labeled condensed tannin was shown to retain the ability to discriminate between high and low affinity proteins. Labeled tannin was similar to unlabeled tannin in chemical assay and astringency. ('125)I -labeled phenols were isolated from livers and kidneys of rats fed ('125)I -labeled tannin. The techniques described in this thesis should be widely applicable to studying in vivo functions of condensed tannins.
Degree
Ph.D.
Subject Area
Biochemistry
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