Summary: Twenty-nine strains of 14 species of rumen bacteria were screened for their ability to hydrolyse Ala. Ala, Gly Arg-4-methoxy-2-naphthylamide (GlyArg-MNA) and Leu-MNA. Several species, notably , were active against Ala, and a smaller number, including , broke down Ala. had an exceptionally high leucine arylamidase activity. However, only hydrolysed GlyArg-MNA. Further investigation revealed that only and hydrolysed Ala to Ala and Ala, with little Ala being produced, in a manner similar to rumen fluid. The activity of against synthetic peptidase substrates, including GlyArg-MNA, LysAla-MNA, ArgArg-MNA, GlyPro-MNA, LeuVal-MNA, and Ala--nitroanilide, was similar to that of rumen fluid, whereas the activity of was quite different. Since the main mechanism by which peptides are broken down in the rumen is similar to dipeptidyl aminopeptidase type I, for which GlyArg-MNA is a diagnostic substrate, it was concluded that was the most important single species in peptide breakdown in the rumen.


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