Screening for production of proteinase inhibitors by Antarctic Streptomycetes

Three out of 17 Streptomycetes strains - Streptomyces sp. 35 LBG09, Streptomyces sp. 36 LBG09, and Streptomyces sp. 39 LBG09, were selected based on the high production of proteinase inhibitors with trypsin serine proteinase activity. The strains were isolated from soil samples taken from the area around the Bulgarian station on Livingston Island, Antarctica. Biosynthesis of proteinase inhibitors by the promising strains started at different stages of their development but was generally not associated with the growth of the producers. Peak levels were reached in the stationary phase (96-120 h) of their cultivation. Inducing effects on strain development, biomass accumulation, and proteinase inhibitor biosynthesis were based on the composition of the nutrient medium: the polypeptones contained in Taguchi medium and glucose as a carbon source. The most productive out of the three strains was Streptomyces sp. 36 LBG09. Its maximum inhibitory activity was reached at 96 h in culturing media modified by three different carbon sources. The active proteinase inhibitor biosynthesis proceeded at pH values between 6.8 and 8.6 and the dynamics of production depended on the type of carbon source. Peak levels of extracellular protein and dry biomass were reached at 120 h in the stationary growth phase. The residual sugars were minimal at the end of the process when using soluble starch as a carbon source, and maximal when glucose was used.