Proteolysis in ensiled forage results mainly

Proteolysis in ensiled forage results mainly from plant proteolytic enzymes (Ohshima and McDonald, 1978, McKersie, 1981, Heron et al., 1988). McKersie (1981) demonstrated the presence of at least three proteolytic enzymes in Lucerne (i.e., carboxypeptidase, aminopeptidase, s3i proteinase), and each differed in its pH and temperature optima and sensitivity to inhibitors. Metallo, aspartic and cysteine endopeptidases were in perennial ryegrass (PRG; Wetherall et al., 1995, Nsereko et al., 1998) and specific inhibitors of these classes of peptidase decreased proteolysis in ensiled PRG (Nsereko and Rooke, 1999, Nsereko and Rooke, 2000). However, despite applying specific inhibitors to ensilage, substantial proteolysis still occurred indicating that the targeted enzymes were not inhibited completely by spraying the inhibitor solution onto the surface of the forage, perhaps because most plant peptidases are located inside vacuoles and because other classes of peptidase may have been present. Pichard et al. (2006) reported that the main peptidases in extracts of six grass and legume species belonged to the serine class, whereas Nieri et al. (1998) described a metallopeptidase responsible for 90% of the proteolytic activity in senescent alfalfa leaves. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB, 1992), peptidases are divided into two classes, namely exopeptidases and endopeptidases, based on their actions on substrates and their active sites, respectively. Endopeptidases include four main peptidases, namely serine peptidase (E.C.3.4.21), cysteine peptidase (E.C.3.4.22), aspartic peptidase (E.C.3.4.23) and metallopeptidase (E.C.3.4.24). So far, the enzymes involved in proteolysis during alfalfa ensiling have not been systematically and well characterized. There have been few studies which have addressed contributions of different endo and exopeptidases to proteolysis, and to the formation of NPN compounds in ensiled alfalfa.
Materials and methods
Results
Discussion
Conclusions
Acknowledgements Financial supports from the National Natural Science Foundation of China (30800800), the Ph.D. Programs Foundation of Ministry of Education of China (200807301008) and the National Key Technology R&D Program in the 12th Five year Plan of China (2010BAD17B02) are gratefully acknowledged. The authors would like to express their highly appreciation to Mr. Malcolm Gibb (Formerly Environment and Grassland Research Institute, UK) for revision of the manuscript.
Introduction Carica papaya is widely cultivated in tropical and subtropical regions all around the world. Apart from the edible fruits, enzymes stored in its lactiferous cells can be produced and have found several applications (de Oliveira & Vitória, 2011). When these cells rupture, the coagulation of latex occurs. This represents an important defence mechanism of the plant against pathogens and other harmful attacks. In addition, the latex of C. papaya is a rich source of cysteine endopeptidases, including papain, glycyl endopeptidase, chymopapain and caricain, constituting more than 80% of total enzymes (Azarkan, El Moussaoui, Van Wuytswinkel, Dehon, & Looze, 2003). Papaya latex was used for preparing protein hydrolysates with bioactivities (Kittiphattanabawon et al., 2012, Ngo et al., 2011). Due to the abundance of glycine in gelatin molecules, glycyl endopeptidase, a major component which constitutes almost 30% of total protein in the latex of C. papaya, can serve as a potential protease, which preferably cleaves the peptide bonds in gelatin. However, undesirable off-odour of crude papaya latex leads to the offensive odour or flavour in the resulting gelatin hydrolysates, thereby causing consumer rejection.