Effects of surfactants, pH, and certain cations on precipitation of proteins by tannins.

Tannic acid and pin oak tannins precipitate large amounts of the abundant leaf protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC), over a wide pH range (6.15-9.30) in the presence of sodium, potassium, magnesium, and calcium ions at concentrations comparable to those reported in the gut fluids of lepidopteran herbivores. The presence of lysolecithin, a surfactant known to be present in the gut fluids of some insects, significantly reduces the amount of RuBPC precipitated under these conditions. We conclude that high detergency is far more effective than high alkalinity in countering the potential protein-precipitating properties of tannins. We further conclude that tannins do not deserve the status they were once accorded as general, all-purpose, dose-dependent, antidigestive defensive chemicals. We also describe the application of the Schaffner-Weissman protein assay for studying the protein-precipitating capacity of plant extracts. This method is far superior to the one we have used in our earlier studies.