Colonic lysozymes of rabbit (Japanese white): recent divergence and functional conversion

Lysozyme was extracted from the feces of rabbit (Japanese White) with 2.5% acetic acid and purified by ion-exchange chromatography. Subsequent ion-exchange HPLC at pH 4.0 revealed the presence of two isozymes, namely rabbit colonic lysozymes 1 and 2. The amino acid sequences of these lysozymes were determined. The colonic lysozymes 1 and 2 showed 98% identity with each other and 94 and 95% identities with rabbit kidney lysozyme, respectively. The very high identities between kidney and colonic lysozymes indicate that the colonic isozymes diverged from the conventional kidney lysozyme very recently, probably after the divergence of rabbit from other rodents, accompanying the gene duplication. Despite the small changes in the sequences, the enzymatic properties of colonic lysozyme differ from those of the kidney lysozyme. The activity of the colonic lysozyme against Micrococcus luteus cells showed a narrow and acidic pH optimum, in contrast to the wide and high pH optimum of the kidney lysozyme. Changes in the enzymatic properties are analogous to those of the ruminant stomach lysozymes and may implicate adaptive evolution in the functional conversion of rabbit colonic lysozymes in gut.