IDENTIFICATION OF SULFUR-RICH PROTEINS WHICH RESIST RUMEN DEGRADATIONAND 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 e lectrophoresis, The S-rich proteins rice prolamin (10 kDa), maize zein (10 kDa) and the 3.2 kDa pumpkin (Cucurbita maxima L.) trypsin inhibi tor-1 (CMTI-1) were highly resistant to rumen fluid degradation, relat ive to control proteins of known degradation rate (casein, bovine seru m albumin (BSA) and pea (Pisum sativum) albumin-1 (PA1)), Comparison o f PA1 and a recombinant N-terminal epitope-tagged PA1 indicated that a ddition of the epitope caused a slight increase in resistance to rumen degradation, The proteins were also incubated with a mixture of tryps in (EC 3.4.21.4) and chymotrypsin (EC 3.4.21.1), PA1, BSA and casein w ere 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 e xpression as foreign proteins in pasture plants to increase throughput and uptake of essential amino acids in sheep.