Protein degradation kinetics of untreated and treated soybean meal using SDS-PAGE

This study was completed to determine dry matter (DM) and crude protein (CP) degradation characteristics of untreated, autoclaved (127°C for 10 min), ethanol (700 ml/l for 1 h) and xylose (20 g/kg DM) treated soybean meal (SBM) using nylon bags and sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) techniques. Nylon bags of untreated and treated SBM were suspended in the rumen of three Holstein steers from 0 to 48 h, and data was fitted to a non-linear degradation model to calculate effective rumen degradation (ERD). Autoclave, ethanol and xylose treatments decreased (P<0.05) ERD of CP. The ERD of untreated, autoclaved, alcohol and xylose treated SBM CP at rumen outflow rate of 0.05/h were 0.683, 0.480, 0.466 and 0.299, respectively. From slab gel analysis and densitometric scanning, SBM proteins were composed of two major components; β-conglycinin and glycinin, accounting for approximately 30 and 40% of buffer soluble meal protein, respectively. The approximate molecular weights of the β-conglycinin ά, α and β subunits were 90.5, 71.5 and 55.2 kDa and the glycinin acidic and basic subunits were 37.6 and 19.8 kDa, respectively. Electrophoretic patterns of untreated, autoclaved, ethanol and xylose treated SBM protein residues revealed that β-conglycinin α and ά subunits were degraded completely within 2, 12, 12 and 6 h of incubation in the rumen, whereas the β subunit of β-conglycinin were degraded after 8, 12, 24 and 12 h of incubation, respectively. Acidic subunits of glycinin were degraded after 12 and 48 h of incubation, but basic subunits of glycinin were not degraded completely until 48 h of incubation in untreated and treated SBM, respectively. The in vitro digestibility (mimic of post-ruminal digestion) of undegradable CP from untreated and treated SBM at 0, 8, 12 and 24 h of incubation increased as incubation time increased. Autoclaving and ethanol treatment increased, but xylose treatment decreased (P<0.05), in vitro digestibility of CP. Processing of SBM by autoclave and with ethanol had greater potential to increase rumen undegradable protein, with positive effects on in vitro CP digestibility, than xylose treatment. SDS-PAGE indicated that the basic subunit of glycinin make an appreciable contribution to metabolizable protein when untreated SBM (whereas β-conglycinin and glycinin when autoclaved, xylose or ethanol treated SBM) is fed to ruminants.