Effects of acidic and alkaline treatments on tannic acid and its binding property to protein
Osawa, R & Walsh, TP 1993, Journal of Agricultural and Food Chemistry, vol. 41, no. 1, pp. 704-707.
Tannin-protein complexes that must be broken down in the gut of the koala dissociated under highly acidic and highly alkaline conditions in vitro in an environment designed to simulate the marsupial’s stomach.
Tannins are biomolecules that bind to proteins to form chemical complexes and typically exist in eucalypts in high concentrations. The tannin-protein complexes are highly resistant to degrading in the gut of a mammal and consequently disrupt the digestion of dietary proteins, thus koalas require a mechanism for breaking down these complexes. The purpose of this study was to evaluate how changing pH conditions affect tannic acid (TA) and its binding property to the protein bovine serum albumin (BSA) in vitro to investigate how tannins respond to the environment of the koala’s stomach. In highly acidic (pH < 3) and slightly alkaline (pH > 7) environments, the TA-BSA complexes consistently dissociated. The TA that dissociated from the complexes did not hydrolyse in a highly acidic (pH 1 – 3) environment and maintained its binding property to protein; however, in a more extreme environment (pH < 0.5), this property was greatly reduced. The TA only hydrolysed under high pH conditions of > 6.5, at which point the TA released its component gallic acid.
The results of this study are consistent with previous suggestions that pH is a key factor affecting the ability for TA to bind to protein and that the pH range 4 - 6 is ideal for maintaining this property as it is reduced under more acidic or alkaline conditions. This observation is relevant to the digestion of eucalypt leaves in the koala as most mammals have a stomach environment that is highly acidic (pH 1 – 3) and a small intestine environment that is slightly alkaline (pH 7 – 8). If tannin-protein complexes can be degraded in these environments, this would in part facilitate the consumption of large amounts of TA which are the basis of the koala’s diet and provide a means by which the dietary protein fractions can be metabolised. This mechanism is supported by other means including the presence of tannin-protein-complex-degrading bacteria in the koala’s alimentary tract that further support the koala’s unique digestive strategy.
These findings contribute to our understanding of the dietary mechanisms and, more broadly, adaptations that allow the koala to sustain its high-tannin diet.
Summarised by Joanna Horsfall
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