Rapeseed napin and cruciferin are readily digested by poultry

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The seed storage proteins of rapeseed (Brassica napus) consist of approximately 60% cruciferin (known as 11S globulin, rich in lysine and methionine), 20% napin (2S albumin, rich in glutamine, proline and cysteine) and minor proteins such as thionins, trypsin inhibitor and lipid transfer protein (Berot et al., 2005; Bos et al., 2007). Cruciferin (molecular weight, MW 300–360 kDa) consists of six subunits that are arranged as two trimers, held together by hydrogen bonds and salt bridges (Wanasundara and McIntosh, 2013). The cruciferin subunit of this hexameric assembly (~50 kDa) contains an acidic or α‐chain (29–33 kDa) and a basic or β‐chain (20–23 kDa), that are linked by single disulphide bond (Schatzki et al., 2014). Napin (MW ~13–18 kDa) is a dimer of a large or heavy polypeptide (10–12 kDa) and a small or light (3–6 kDa) polypeptide that are connected by four disulphide bonds (Rask et al., 1998; Wanasundara and McIntosh, 2013; Schatzki et al., 2014).
Rapeseed co‐products are of considerable interest as a protein source in animal feeds due to a high content of protein with a greater content of sulphur‐rich amino acids (cysteine, methionine) compared to a standard soya bean meal (Wickramasuriya et al., 2015). During rapeseed oil production, whole seeds are de‐fatted by hexane extraction producing a rapeseed meal (RSM), or by cold pressing producing a rapeseed cake (RSC) (Untersmayr and Jensen‐Jarolim, 2008). The crude protein content of the co‐products may range from 329 to 437 g/kg dry matter (DM) (Seneviratne et al., 2011a,b; Maison and Stein, 2014). However, protein content and individual amino acid levels will vary depending on rapeseed variety and oil extraction method used (Kasprzak et al., 2016). Several studies have shown that rapeseed protein is less digestible (by an absolute decrease of 14–16%) than soya bean protein or casein protein in standard diets (Savoie et al., 1988; Adedokun et al., 2008). This difference in nutritional value of protein is not only attributed to variation in chemical composition between the co‐products, but also to the compact structure and relatively high content of disulphide bonds in rapeseed protein. When in vitro models were used, napin was reported to be extremely resistant to pepsin digestion and denaturation caused by heat and low pH (Murtagh et al., 2003; Abeysekara and Wanasundara, 2009; Wanasundara, 2011).
To the best of our knowledge, there is no in vivo study focusing on digestibility of napin and cruciferin in the gastrointestinal tract of non‐ruminants when examining rapeseed proteins. The aim of the current study was to identify proteins in de‐fatted rapeseed co‐products, and the corresponding ileal digesta from broilers fed rapeseed diets.
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