The deaminase activity of rumen bacteria does not vary widely with the protein content of the diet. In vitro deamination may be inhibited by toluene, and under these circumstances amino nitrogen rather than ammonia accumulates when protein is introduced into mixed suspensions of of rumen bacteria. Protein digestion in the rumen therefore appears to be a steady process of hydrolysis, through peptides of decreasing length to free amino acids which are largely destroyed by fermentative deamination with the production of carbon dioxide, ammonia, and short-chain fatty acids. Some peptides and amino acids may pass directly into bacterial cells, but the evidence suggests that many of the various species of rumen bacteria are able to synthesize their nitrogenous cell constituents using ammonia as a principle source of nitrogen, although some available sulphur and possibly certain carbon structures are necessary too. Some of the bacteria, however, need amino acids.
Ammonia is the principal soluble nitrogenous constituent of rumen fluid.
Its concentration is influenced by the quantity and solubility of the dietary protein, the quantity of urea that enters the rumen in saliva, the diffusion of urea through the rumen wall, and the rate at which ammonia is absorbed from the rumen. Rumen fluid can rapidly hydrolyse urea to ammonia and carbon dioxide. Increased quantities of sugars and starches in the diet decrease the concentrations of ammonia in the rumen, since the inclusion of rapidly fermenting carbohydrate substrates increases the speed with which organisms can incorporate ammonia nitrogen into their cell protoplasm. No mono- or diglycerides have been detected during the hydrolysis, so that if there are intermediary stages, they are very transient. Hydrolysis of triglycerides is due to rumen microorganisms, and the glycerol derived from the hydrolysis is fermented principally into propionic acid, although the transitory stages, succinic and lactic acids, have also been detected.