Effect of short-chain acids and glycerol on the proteolytic activity of rumen fluid

The effect of increasing the concentrations of acetic, propionic, butyric, and lactic acids (as sodium salts), and glycerol in the medium by 50, 100, 200 and 300 mM on the proteolytic activity of bovine rumen fluid was assayed in vitro with azocasein as substrate in 1 h incubations at 39°C. NaCl was used to control the osmolarity and ionic strength changes in the medium. An increase of 100 mM in the acetic acid concentration produced a 6% decrease in the activity whereas increments of 200 and 300 mM of the same compound increased the activity by 16% and 47%, respectively. Propionic acid did not alter the activity at any of the concentrations tested. With increments of 100, 200, and 300 mM of butyric acid, proteolysis increased by 10%, 11%, and 15%, respectively. Lactic acid at 200 and 300 mM produced increases in activity by 27% and 69%, respectively, which were greater than those of any other substances assayed at the same concentrations. Glycerol reduced the proteolytic activity by about 20% with all the concentration increases tested. Proteolysis was affected by NaCl only when its concentration was increased by 300 mM which raised the activity by 16%. The effects found could not be attributed only to osmolarity and/or ionic strength changes in the medium. An alternative explanation based on alterations caused by the additions in the hydrophobicity/hydrophilicity of the medium is suggested.