Characterization of proteases from Rhizopus species after growth on soybean protein

Abstract

Culture filtrates of different fungi of the genus Rhizopus forming tempe (i.e. traditional Indonesian food) were grown on a soybean protein-raffinose-phytate medium and investigated for protease activity using soyprotein as substrate. All isolates belonging to the species R. oryzae, R. stolonifer, R. oligosporus, and R. microsporus var. chinensis, formed the well-known Rhizopus-pepsin (aspartic proteinase, 35 kD, isoelectric points: 5.9, 5.0, <4) and an additional protease mainly active under alkaline conditions. The new protease (33 kD, isoelectric points: variable and isolate specific) was purified approximately 300-fold and shown to be a serine protease (inhibitor studies). During fungal culture (12-135 h) the aspartic proteinase is expressed first followed by the serine protease. Both proteases are insensitive to the soybean Kunitz and Bowman-Birk inhibitors. The best rate of soyprotein degradation is achieved by the coordinate action of both proteases at pH 6.5. The examined Rhizopus isolates differ in the time course and intensity of the expression of the alkaline protease.