IDENTIFICATION OF SULFUR-RICH PROTEINS WHICH RESIST RUMEN DEGRADATION AND ARE HYDROLYZED RAPIDLY BY INTESTINAL PROTEASES

Several proteins with high proportions of S-containing essential amino acids were incubated in sheep rumen fluid in vitro and their rate of digestion was examined by sodium dodecyl sulphate-polyacrylamide-gel electrophoresis, The S-rich proteins rice prolamin (10 kDa), maize zein (10 kDa) and the 3.2 kDa pumpkin (Cucurbita maxima L.) trypsin inhibitor-1 (CMTI-1) were highly resistant to rumen fluid degradation, relative to control proteins of known degradation rate (casein, bovine serum albumin (BSA) and pea (Pisum sativum) albumin-1 (PA1)), Comparison of PA1 and a recombinant N-terminal epitope-tagged PA1 indicated that addition of the epitope caused a slight increase in resistance to rumen degradation, The proteins were also incubated with a mixture of trypsin (EC 3.4.21.4) and chymotrypsin (EC 3.4.21.1), PA1, BSA and casein were hydrolysed less rapidly than rice prolamin, maize zein and CMTI-1, Digestion by these intestinal proteases appeared to be complete, Thus, the prolamin, zein and CMTI-1 proteins are suitable candidates for expression as foreign proteins in pasture plants to increase throughput and uptake of essential amino acids in sheep.