Rumen protein breakdown

Many food constituents are potentially toxic to the animal consuming them. Microbial contaminants are an obvious example the digestive enzymes kill many bacteria, but some organisms may damage the gut, which allows them or the toxins they produce to invade the animal s tissues. Foreign proteins, especially those with endocrine activity, could harm the animal if absorbed, but the gut provides an effective barrier to prevent their absorption before they are hydrolysed. The same is true of nucleic acids (whose breakdown is a matter of concern, as some animal foods may now be derived from genetically modified plants). Some of the toxic constituents of pasture plants are broken down in the rumen of cattle, sheep and goats (see p. 494). As mentioned above, the animal body avoids excessive intake of the mineral elements calcium and iron by selective absorption. 

The ammonia in rumen liquor is the key intermediate in microbial degradation and synthesis of protein. If the diet is deficient in protein, or if the protein resists degradation, then the concentration of rumen ammonia will be low (about 50 mg/1) and the growth of rumen organisms will be slow in consequence, the breakdown of carbohydrates will be retarded. On the other hand, if protein degradation proceeds more rapidly than synthesis, then ammonia will accumulate in rumen liquor and the optimum concentration will be exceeded. When this happens, ammonia is absorbed into the blood, carried to the liver and converted to urea (see Fig. 8.8). Some of this urea may be returned to the rumen via the saliva and also directly through the rumen wall, but the greater part is excreted in the urine and thus wasted. 

Rumen breakdown of food protein is not always desirable, because microbial protein may be inferior to it in both quantity and quaiity. Thus, food proteins are sometimes protected from attack by rumen microorganisms. in suckled ruminants, there is a device known as the oesophageal groove, which channeis miik directly to the abomasum. 

Degradability will be affected by such factors as the surface area available for microbial attack and the protective action of other constituents as well as the physical and chemical nature of the protein. Claims have been made that the solubility of a protein is correlated with ease of breakdown, but these do not survive critical examination. Thus, casein, which is readily degraded in the rumen, is not readily soluble whereas albumin, which is resistant to breakdown, is readily soluble. It has been suggested that a major factor affecting degradability is the amino acid sequence within the protein molecule. If this is so, then the nature of the microbially produced rumen peptidases is of considerable importance and it seems doubtful whether any simple laboratory test for degradability is possible. 

The extent to which a protein is broken down in the rumen depends not only on its innate degradability but also upon the length of time for which it is exposed to breakdown and therefore upon its rate of passage through the rumen. It may be defined as follows ... 

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