The fermentation of soybean meal by rumen microbes in vitro reveals different kinetic features for the inactivation and the degradation of trypsin inhibitor protein

Soybean meal is a widely used protein supplement in animal feeds. When fed to monogastrics, thermal pretreatment (roasting) is needed to destroy antinutritional components. The major antinutritive compound of soybeans is trypsin inhibitor, a protein of 21 kDa molecular mass. Ruminants can tolerate this compound due to the microbial fermentation in the rumen rendering the inhibitor ineffective. This process was monitored under controlled conditions in an in vitro fermentation system. Raw soybean meal was added at different levels to a basal roughage and incubated with bovine rumen fluid. Samples were taken at regular time intervals up to 24 h of incubation. The samples were assayed enzymatically for trypsin inhibitor activity, and parallel to this the proteolytic degradation of trypsin inhibitor was documented qualitatively and quantitatively through polyacrylamide gel electrophoresis and densitometric methods. It was shown that trypsin inhibitor is both inactivated and degraded in the rumen, but with different kinetic characteristics. Its concentration in the soluble fraction showed a more pronounced initial increase than the apparent activity, and the time of complete degradation lagged 1–2 h behind the complete loss of activity. Protein concentration and activity were related to each other by calculating specific trypsin inhibitor activity. As this parameter showed a rapid, presumably exponential decay with the onset of fermentation, inactivation must be due to a mechanism preceding and not directly related to proteolysis.