A Comparative Study of the Nutritional and Physiological Significance of Pure Soybean Trypsin Inhibitors and of Ethanol-extracted Soybean Meals in Chicks and Rats

Abstract

Several experiments were conducted with the pure soybean trypsin and α-chymotrypsin inhibitor (AA) and the crystalline Kunitz soybean trypsin inhibitor (CSBTI) to study their specific effects on growth rate of chicks and rats, on pancreatic hypertrophy and on intestinal proteolytic activity. For comparative purposes raw soybean meal (RSBM), heated soybean meal (HSBM) and the residues after ethanolic extraction of RSBM (EERSBM) were used. The addition of the inhibitors to HSBM diets of chicks and rats resulted in a slight depression of growth rate but caused a significant hypertrophy of the pancreas. However, the RSBM+HSBM diet (1:2) which possessed the same trypsin-inhibiting activity and approximately 40% of the chymotrypsin-inhibiting activity as the HSBM+AA diet, caused a considerable growth depression. The proteolytic activity in the small intestine was decreased by addition of inhibitor AA but not by CSBTI. This probably arises from the small activity of CSBTI towards chymotrypsin (8% of that of AA) and from its inactivation during peptic digestion. The comparative stability of AA was also indicated by the various ethanolic extractions of RSBM during which AA maintained its full potency, whereas CSBTI was almost completely inactivated. Addition of methionine to the different meals at two protein levels (25% and 15%) resulted in a decrease in growth impairment caused by the low protein RSBM diets. These experiments clearly indicate the minor role of soybean trypsin and chymotrypsin inhibitors in growth depression caused by RSBM diets and suggest the presence of other responsible factor(s) in RSBM.