Angiotensin I-converting enzyme inhibitory peptides in douchi, a Chinese traditional fermented soybean product

Douchi, a soybean product originating in China, produces angiotensin I-converting enzyme (ACE) inhibitors with the potential to lower blood pressure. The ACE inhibitory activities of douchi qu pure-cultured by Aspergillus Egyptiacus for 48 h, and 72 h were compared with douchi secondary-fermented for 15 d. The results showed that ACE inhibitory activities were improved following the fermentation. ACE inhibitory activities of 48 h-primary-fermented douchi qu did not change dramatically after preincubation with ACE, but increased greatly after preincubation with gastrointestinal proteases. The results suggest they were pro-drug-type or a mixture of pro-drug-type and inhibitor-type inhibitors. The ACE inhibitors in 48 h-fermented douchi qu were fractionated into four major peaks by gel filtration chromatography on Sephadex G-25. Peak 2, which had the highest activity, had only one peptide, composed of phenylalanine, isoleucine and glycine with a ratio of 1:2:5.