Rates of proteolysis in the rumen of the soluble proteins casein, fraction I (18S) leaf protein, bovine serum albumin and bovine submaxillary mucoprotein.

The rate of proteolysis in the rumen was dependent on the soluble protein used. With a sheep on a hay + concentrate diet the rates (approximately Vmax) for casein, Fraction I leaf protein and bovine serum albumin were 16.5, 3.4 and 0.9 mg protein nitrogen/l per min respectively. Change of diet from hay + concentrate to fresh lucerne (Medicago sativa) increased the proteolytic rates for all three proteins. Bovine submaxillary mucoprotein degraded extremely slowly in the rumen at approximately 0.5-0.6 mg N/l per min and its sialic acid component was degraded at a similar rate to that of the protein chain. Uniformly 14C-labelled Fraction I leaf protein was used to demonstrate that in the presence of a second protein, competition for enzymic sites occurred. In Fraction I and bovine serum albumin mixtures, reduced rates for the individual proteins of the mixture were observed compared with the proteins treated separately. Treatment of bovine serum albumin with dithiothreitol (0.2 g/l) to cleave disulphide bridges increased the rates of proteolysis by as much as 8.5-fold.