Dual Roles of Cyanopeptides as Substrates and Inhibitors of Enzymes in Surface Waters.

Harmful cyanobacterial blooms produce diverse bioactive metabolites, cyanopeptides, whose environmental persistence remains uncertain. First, we investigated enzymatic reactions with cyanopeptides as substrates in aquatic systems. The aquatic enzyme pool from the riverine biofilm showed peptidase and oxidase activities. Incubation of cyanopeptides with the natural aquatic enzyme pool revealed novel transformation products (TPs) of two microginins (nostoginin BN741 and microginin FR6) and TPs of anabaenopeptins that also formed when incubated with intact biofilm. Next, we probed underlying mechanisms with peptidases as functional analogs of the aquatic enzyme pool. Carboxypeptidase Y produced identical hydrolysis TPs of the two microginins. A multicopper oxidase rapidly transformed tyrosine-containing microcystins, anabaenopeptins, and microginins, in the presence of the mediator 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS). Tyrosine-free congeners remained stable. Lastly, we investigated cyanopeptides as enzyme inhibitors. Cyanopeptolins inhibited trypsin (IC50 = 53.75 nM), chymotrypsin (IC50 = 2.2 μM), and analogous activities in the aquatic enzyme pool. Anabaenopeptins and microginins showed only mild inhibition of carboxypeptidase Y. As different cyanopeptides are co-produced by cyanobacteria, biotransformation is expected to differ in mixtures compared to single-compound assessments due to the observed inhibition taking place at the same time. Similarly, their release during bloom events may influence the transformation of other organic compounds in surface waters.