Anti-hypertensive substances in fermented soybean, natto

Natto is a traditional Japanese fermented food made by fermenting boiled soy beans with Bacillus natto. Its contents of inhibitors against the angiotensin converting enzyme (ACE, EC3.4.15.1) were investigated. Relatively strong inhibitory activity (IC50: 0.4 mg/ml, 11.8 inhibition units/g natto) was detected in natto extracts and the inhibitory activity observed in the viscous fraction was more potent than in the bean extract. Two groups of inhibitors in the viscous material, high and low molecular weight inhibitors, were resolved by dialysis test. The inhibitor of high molecular weight was a protein with low IC50 value (0.12 mg/ml). The two types of low molecular weight inhibitors were detected in ethanol extracts (IC50: 0.53 mg/ml and 0.95 mg/ml) and they were found to be stable over a wide range of pH and temperature up to 100 degrees C. They were different in the mode of ACE inhibition. One is competitive, and the other noncompetitive against the hydrolysis of Bz-Gly-His-Leu by ACE.