Stomach lysozymes of ruminants. I. Distribution and catalytic properties.

A major regulatory shift affecting the expression of lysozyme c may have been involved in the origin of two groups of mammals whose nutrition depends on foregut bacteria. A survey of 23 mammalian species reveals that the lysozyme c activity per g of stomach mucosa is many times higher for ruminants and a leaf-eating monkey than for animals lacking a foregut. The implication is that stomach lysozyme c functions as a major digestive enzyme in ruminant-like mammals, helping to make those bacterial which enter the stomach from the foregut available for hydrolysis by conventional digestive enzymes. The high level of stomach lysozyme is due to more enzyme molecules rather than to an increase in the activity of each molecule. This was shown for the cow by purifying the three, non-allelic lysozymes c that account for the lysozyme activity in gastric mucosa and measuring their specific activities and for other foregut fermenters by immunological titration. Lysozyme appears in the stomach mucosa before birth and reaches adult levels before weaning. Other tissues tested from cattle lack lysozyme c and may instead have low levels of another lysozyme that could belong to the g class, the first indication that lysozyme g may be present in mammals. The lysozymes of eight ruminants, four Old World monkeys, and 12 other animals were compared as regards the ability to lyse bacterial cells under various conditions and to resist inactivation by pepsin. There are differences among these species in the dependence of the rate of bacterial lysis on time, pH, and ionic strength. Although not every lysozyme was tested in all of these catalytic respects, there were no exceptions to the following generalizations. First, at ionic strengths above 0.1 and pH values above 5, the rate of lysis by ruminant and monkey lysozymes c rose with the time of reaction, whereas the rate was more nearly constant for the other animal lysozymes. Second, the lytic activity at neutral pH is lower than at pH 5 for the ruminant and monkey lysozymes c when the ionic strength is over 0.1; by contrast, for other lysozymes c under these conditions the activity at neutral pH is about as high as at pH 5. This latter property, which may be viewed as an adaptation for functioning as a digestive enzyme in the stomach, can be explained in part by differences in electrostatic interactions between lysozyme and the substrate due to the relatively non-basic nature of ruminant and monkey lysozymes compared to other lysozymes c.(ABSTRACT TRUNCATED AT 400 WORDS)