Abstract
Studies were conducted to detect and identify proteinase inhibitors in rice bran. Water extracts of rice bran exhibited significant inhibitory capacities against trypsin (EC 3.4.21.4) and pepsin (EC 3.4.23.1), but little and practically no inhibitory capacities against α-chymotrypsin (EC 3.4.21.1) and elastase (EC 3.4.21.11) respectively. The trypsin inhibitor appeared to be a thermolabile proteinaceous substance which was relatively abundant in the germ portion. The pepsin inhibitor distributed relatively evenly over the whole bran fraction, and was a highly thermostable substance containing phosphorus in its molecule.
Kinetic analyses of pepsin inhibition by the water extracts of rice bran revealed that the inhibitor combines with the substrate protein so rendering the protein poorly available for the peptic action. Dispersibility of the substrate protein, measured in the media of varied pH, was found to be significantly lowered by the added bran extracts especially under the pepsin assay conditions. Authentic phytate showed essentially the same effect on the protein dispersibility as observed with the bran extracts. Pepsin inhibitory capacities of cereals and soybean paralleled with their phytate contents. These results and other considerations lead to the conclusion that phytate is a factor which accounts for most of the pepsin inhibition by the bran extracts, and also suggest the possibility that the inhibition by phytate may be of some nutritional significance in animals that consume relatively large amounts of phytate.
