INHIBITORY EFFECT OF OAK LEAF TANNINS ON HYDROLYSIS OF PROTEINS BY TRYPSIN

Oak leaf tannin forms complexes with both casein and nettle leaf protein at pH 5·0 and the degree of complex formation depends on the ratio of protein to tannin concentrations and on the time of contact between the protein and tannin. When complexed with oak leaf tannin, casein is almost completely protected from hydrolysis by trypsin at pH 7·6. The pH of the mid-gut of larvae of the winter moth, Operophtera brumata, was found to be 9·2 and, at this pH, enzymic hydrolysis of complexes between casein and oak leaf tannins is increasingly inhibited as the proportion of tannin in the initial complex is raised, the inhibition being more marked with condensed than with hydrolysable oak leaf tannin. Reduction in casein digestibility may thus account for the inhibition by oak leaf tannins of larval growth of the winter moth, reared on artificial diets containing casein. When complexed with oak leaf tannins, nettle leaf protein is partially protected from hydrolysis by trypsin at pH 9·2. These observations may help to explain selection in some oak-feeding moth larvae for a high gut pH and for advance of the feeding period to avoid a high tannin content in the host-plant. © 1969.