Many cyanobacteria produce peptides that inhibit mammalian proteases. The hypothesis that inhibitors of mammalian proteases produced by cyanobacteria also interfere with digestive proteases of natural cladoceran grazers was tested by comparing the effects of cyanobacterial protease inhibitors on digestive proteases from Daphnia magna and on commercially available bovine proteases. The major digestive proteases of D. magna are trypsins and chymotrypsins, which differ from those of bovine origin in substrate specificity and susceptibility to synthetic inhibitors. An extract from Microcystis aeruginosa strain PCC 7806 inhibited both types of D. magna proteases. Subsequent fractionation of the extract by high-performance liquid chromatography indicated that several inhibitors are produced by M. aeruginosa that differ in their specificity for the trypsins and chymotrypsins of D. magna. Two fractions differed in their inhibitory effect on proteases of D. magna and bovine origin; therefore, assessment of the impact of cyanobacterial protease inhibitors on natural communities requires the use of digestive proteases from ecologically relevant grazers.

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This paper describes the characterization of proteases in Microcystis aeruginosa PCC7806 cells being inhibited by a metabolite produced by another Microcystis strain, Microcystis Ku1. With casein and oligopeptide substrates and specific inhibitors we detected activity similar to bacterial serine endoproteases. Substrate SDSpolyacrylamide gel electrophoresis revealed the presence of nine bands of proteases (ca. 35∼125 kDa). The cyanobacterial enzymes were insensitive to endogenous trypsin-inhibitory metabolites. Microcystis Ku1 produced a metabolite, tentatively characterized as microviridin, inhibiting both cyanobacterial proteases and trypsin at an estimated IC 50 of, respectively, 2.2 and 9.0 μg mL −1 . On activity gels, inhibitors specific to animal trypsin and elastase and the putative microviridin led to an inactivation of the proteases associated with the 88 and 110 kDa bands. We hypothesize that in Microcystis populations there is a "cross-talk" between the inhibitors and the proteases, and only the colonies of identical chemotypes can possibly aggregate to form blooms.

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