Protein is the most abundant source of N in most ruminant diets. Feed protein is usually broken down rapidly by the microbial proteases secreted by ruminal bacteria, protozoa and fungi present in the rumen, giving rise to peptides and amino acids, which are then taken up by the microbial cells (Fig. 15.1) (Nolan, 1975; Wallace, 1996; Wallace et al., 1997; Cotta and Russell, 1997). A number of factors, which affect the precise rate and extent of breakdown, ultimately determine the nutritive value of the protein (Wallace et al. 1997). Various chemical and physical characteristics of proteins, such as solubility, the degree of secondary and tertiary structure and the presence of disulphide bonds (Henderickx, 1976; Mahadevan et al., 1980; Wallace, 1983) are important determinants of the susceptibility of proteins to digestion by ruminal microorganisms. Because some proteins are fermented faster than others in the rumen (Wallace, 1994), the rate and extent of protein breakdown to generate peptides and amino acids which are then either incorporated into microbial protein or broken down to ammonia by the ruminal microorganisms is thought to affect ammonia incorporation patterns significantly in the rumen (Hristov and Broderick, 1994). Hristov and Broderick (1994) found that soluble proteins such as casein favoured greater amino acid and lower NH3 incorporation in to microbial protein.
A number of different types of proteolytic enzymes are produced and secreted by many strains and species of ruminal ciliate protozoa, bacteria and fungi which have been found to be proteolytic (Wallace, 1994; Wallace et al., 1997). The type of diet is also known to have a major influence on proteolytic activity and the microbial species responsible for that activity (Nugent and Mangan, 1981; Siddons and Paradine, 1981; Hazlewood et al., 1983). Fresh herbage was reported to promote higher proteolytic activity than dry rations, the higher soluble-protein content of the herbage enriching proteolytic bacteria (Nugent and Mangan, 1981; Hazlewood et al., 1983; Nugent et al., 1983). Cereal diets were also reported to encourage higher proteolytic activities than dry-forage diets, probably because these diets are suitable for the proliferation of amylolytic luminal species which are known to be proteolytic rather than cellulolytic (Siddons and Paradine, 1981). It is noteworthy that the pattern of proteolytic enzymes has been quite variable in animals offered the same or similar diets and housed together (Wallace et al., 1997; Falconer and Wallace, 1998).
Recently, a new suggestion has been put forward that proteolysis is carried out not only by ruminal microorganisms, but also by endogenous plant proteases, which may play a significant role in the breakdown of fresh herbage protein in the rumen (Zhu et al., 1999). Plant proteinases play a significant role in protein breakdown in the silo, and it is logical that they may also have a role in the rumen of grazing ruminants.
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