INACTIVATION OF PROTEINACEOUS PROTEASE INHIBITORS OF SOYBEANS BY ISOLATED FUNGI

Proteinaceous protease inhibitors, Kunitz Soybean Trypsin Inhibitor (K STI) and Bowman Birk Inhibitor (BBI), in legume seeds reduce the diges tibility of proteins in feed of monogastric animals. Enzymatic inactiv ation of these inhibitors will increase the nutritional value of the f eed. The aim of this study was to obtain microorganisms which produce proteases which specifically inactivate the inhibitors under condition s that occur in the stomach. Hereto, microorganisms were enriched at p H 3.0 and 37 degrees C on KSTI or BBI as sole nitrogen and sulphur sou rce. Nine fungal strains were isolated from the enrichment cultures. T he isolated strains were able to inactivate both inhibitors. KSTI was inactivated more effectively than BBI. KSTI inactivation was pH depend ent. At pH 5.5, only 30-45% of the KSTI was inactivated in 4 days, whe reas, 80-90% of the KSTI was inactivated in 2 days at pH 3.0. The isol ated strains varied in capacities to inactivate KSTI. In general, stra ins which inactivated KSTI to a higher extent had higher biomass yield s and higher general protease activities. The KSTI specificity of the most promising strain (K1) which had a relatively high KSTI inactivati on capacity and a relatively low general protease activity at pH 3.0, was studied in more detail. The KSTI specificity of the produced prote ases was affected by pH and by the type of substrates used [KSTI, bovi ne serum albumin fraction V (BSA), cysteine or without substrate]. KST I specificity of the proteases produced at pH 5.5 was low as compared to pH 3.0. The highest KSTI specificity, 1.7 mole KSTI per mole albumi n, was found when strain K1 was grown on cysteine at pH 3.0.