Gastric Lysozymes

Lysozymes are a group of enzymes that catalyze the hydrolysis of specific glycosidic bonds in mucopolysaccharides that constitute some bacterial cell walls. Many mammals, including man, have moderate to high levels of lysozyme in secretions (tears and saliva), white blood cells and tissue macrophages. In these settings, lysosome is thought to be an important antibacterial defense and to have originally evolved for that purpose.

Ruminants are herbivorous animals that ferment foodstuffs prior to their entry into the glandular stomach (so-called "foregut fermenters"). As part of their digestive process, huge numbers of fermentative bacteria flow into the stomach; those bacteria represent the animal's major source of dietary protein and must be efficiently digested. In contrast to most other animals, ruminants secrete large amounts of lysozyme into the lumen of their true stomachs (the abomasum), a trick that appears to have evolved to facilitate digestion of bacteria coming from a fermentative foregut. Interestingly, they have very low levels of lysozyme in other secretions.

Cattle have at least 10 lysozyme genes, of which at least four are expressed in the stomach. In comparison to lysozymes from other species and bovine lysozymes not expressed in stomach, the gastric lysozymes from cows have two characteristics consistent with an important role in gastric digestion:

• They are most active at low pH, in contrast to other lysozymes which function best at a more basic pH and have a wider pH optima.
• They are unusually resistant to cleavage by pepsin, which undoubtedly prolongs their survival by minimizing proteolysis in the lumen of the stomach.

It thus appears that evolution of foregut fermentation was accompanied by recruitment of lysozyme as a lytic digestive enzyme, and that selection of this enzyme to act in stomach fluid has driven its molecular evolution. It also turns out that colubine monkeys such as languars, which are also foregut fermenters, also show high levels of expression of stomach-type lysozyme. Thus, gastric lysozymes are also a good example of convergent evolution of proteins in distantly related species (cows and languars) that happen to share a digestive strategy.

References and Reviews

• Irwin DM: Evolution of the bovine lysozyme gene family: changes in gene expression and reversion of function. J Molec Evolution 41:299, 1995.
• Irwin DM, Wilson AC: Concerted evolution of ruminant stomach lysozymes. Characterization of lysozyme cDNA clones from sheep and deer. J Biol Chem 265:4944, 1990.

Send comments to Richard.Bowen@colostate.edu