Characteristics of the rumen proteolysis of fraction I (18S) leaf protein from lucerne (Medicago sativa L)

<jats:p>1. The rate of proteolysis of fraction 1 (18S) leaf protein in the rumen of sheep and cattle was affected by diet and the rate on fresh lucerne (<jats:italic>Medicago sativa</jats:italic> L) was three to nine times the rate on a hay + concentrate diet.</jats:p><jats:p>2. Simultaneous rumen fermentations in vivo and in an artificial rumen showed that the rates of proteolysis of fraction I in vitro was approximately 80% of the rates in sheep.</jats:p><jats:p>3. Using <jats:sup>14</jats:sup>C uniformly-labelled fraction I protein at low concentrations, proteolysis exhibited 1st-order kinetics. Over a wide range of protein concentrations the velocity <jats:italic>v</jats:italic>. substrate concentration curve showed Michaelis–Menten characteristics typical of an enzyme-catalysed reaction. With rumen fluid from a hay + concentrate-fed sheep the maximum velocity was 2.6 mg protein nitrogen/1 per min and the Michaelis constant was 75 mg nitrogen/1.</jats:p><jats:p>4. Rapid adsorption of <jats:sup>14</jats:sup>C-labelled fraction I protein onto bacterial cells preceded proteolysis.</jats:p><jats:p>5. Sucrose-density-gradient analysis showed initial incorporation of <jats:sup>14</jats:sup>C from protein into rumen bacteria followed by partial transfer to rumen protozoa.</jats:p><jats:p>6. No peptides were detected during proteoiysis showing that the rate-limiting step occurred during the initial stages of proteolysis. Only small amounts of free amino acids were released except for leucine, isoleucine, valine and ornithine, which showed significantly increased levels.</jats:p><jats:p>7. Volatile fatty acids were the main <jats:sup>14</jats:sup>C-labelied end products and were rapidly produced in descending concentrations: acetate > propionate > 3-methyl + 2-methyl butyrate > butyrate > isobutyrate > valerate.</jats:p>