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Conundrums of Protein and Peptide Metabolism in the Rumen
N retention in ruminants is inefficient due to rumen microbial activity. Protein in forages is particularly vulnerable, because a high proportion of forage carbohydrate comprises cellulose or hemicellulose, which are slowly degraded, in contrast to the soluble protein, which degrades rapidly. The release of energy-yielding sugars, which enables ruminant microbes to trap the forage protein N, therefore does not match the availability of amino acids, which are degraded excessively, leading to high N emissions by the animal. Protein supplements are almost as vulnerable. The problem with supplements is that they tend to be variable in their degradation rate; thus, diet formulation relies upon knowledge of their rate of degradation in the rumen. How to assess protein degradation and the chemistry and microbiology of the process have long been interests of Glen Broderick. It was therefore natural that Glen might want to spend time at the Rowett Research Institute, where Bob Orskov and I shared this interest. Malt whiskies and old Scottish castles also had their part to play! Bob was a committed advocate of the nylon-bag method, a technique that Glen abhorred. I sat on the fence on that one. It might seem like a trivial thing to do, to develop a method to assess the rate and extent of protein degradation. Many have tried to develop in vitro incubations that enable the determination of those characteristics, soon finding that something simple, like the release of ammonia, was vastly in error. Protein supplements are not comprised entirely of protein, indeed starchy carbohydrate even exceeds protein in some supplements. Thus, an enzyme system is fraught with difficulty. The physical form of the supplement is an issue - to grind or not to grind? Furthermore, when ruminal digesta is used as an inoculum for protein degradation assays hydrolyses in vitro, microbes mop up the released amino acids and ammonia as they ferment the carbohydrate. Glen and his colleagues devised an in vitro system that overcame these problems, and at the same time revealed some fundamental properties of ruminal protein metabolism. Peptides as an intermediate of the proteolysis cascade were a contentious topic when Glen first arrived on Scottish shores. He and I had great fun in establishing the sequence characteristics that dictate microbial degradation rates of peptides – particularly the importance of the N-terminal amino acids and whether the peptide was basic or acidic.
Keywords:
Proteolysis; peptide metabolism; rumen