There is the possibility of protecting peptides physically as there is for the protection of protein and amino acids (see below). Chemical protection, by N-terminal acylation, is also effective (Wallace, 1992b; Wallace et al., 1998; Witt et al., 1998) because peptides are broken down by rumen microbial aminopepti-dases. In terms of enzyme inhibition, the two steps of peptide hydrolysis can be considered separately. The breakdown of oligopeptides, catalysed mainly by dipeptidyl peptidases of Prevotella spp. (Wallace and McKain, 1991; McKain et al., 1992) can be inhibited by a variety of structural-analogue inhibitors, including benserazide, GlyPhe-diazomethyl-ketone, Ala2-chloromethylketone and diprotin A (H.R. Wang, N. McKain, N.D. Walker and R.J. Wallace, unpublished). Dipeptidase activity is strongly cation-dependent, and is sensitive to metal-ion chelators such as 1,10-phenanthroline (Wallace and McKain, 1996; Wallace et al., 1996). Both of these groups of peptidase inhibitors inhibit the rate of ammonia production from peptides and protein in rumen fluid, but whether they would prove to be suitable feed additives remains to be established.

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