A STUDY OF TANNIN PROTEIN INTERACTIONS
This work is an investigation into various aspects of tannin-protein interactions including adverse effects on digestive enzymes, an attempt of structural analysis using selective precipitation with PVP, and their potential industrial utilization. Tannin interaction with digestive enzymes was studied as a function of pH. Complex formation was minimal under pH conditions normally expected for each specific enzyme tested. Trypsinogen-tannin complex formation indicated possible precipitation in the duodenum. In feeding studies with rats, using high tannin sorghum and wattle tannins, the level of enzymatic activity was increased when tannins were present in the diet. The increase was not statistically significant, but indicated that the antinutritional effects of dietary tannins are not a consequence of digestive enzyme inhibition. Distention of the stomach and intestinal tract was observed in rats on a high tannin sorghum diet. The chyme was coarser and association of intestinal material into discrete packets was observed in the duodenum. Various tannin preparations exhibited differences in their interaction with PVP. PVP precipitation by tannins apparently follows a classification scheme based on polymers of catechin and epicatechin. A possible mode of interaction between PVP and tannins is presented, along with evidence suggesting that PVP is not a good model system for tannin-protein interactions. Experimental evidence used to support hydrogen bonding as the predominant mechanism for tannin-protein interaction, as based on polyamide polymers, is therefore questioned. Tannins which do not form a precipitable complex with PVP do interact, but with formation of a soluble complex. The soluble complex is not disrupted by addition of PVP precipitating tannins or catechin. The PVP negative tannins compete with PVP positive tannins for available binding sites and limit PVP precipitation by PVP positive tannins. Formation of an insoluble tannin-protein complex is here suggested to arise by a two-step mechanism. Initially hydrophobic interactions are involved, which may provide for an increase in entropy to allow for spontaneous precipitation. Hydrogen bond formation then occurs to achieve a lattice network which precipitates. To investigate possible industrial utilization of tannin-protein interactions, condensed tannins were immobilized on porous cellulose beads. The results indicate that problems remain to be solved, in particular a modification of the internal structure which limits access of large molecular weight proteins to the tannin binding sites.
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