POTENTIAL ACTIVITIES OF FRESHWATER EXO- AND ENDO-ACTING EXTRACELLULAR PEPTIDASES IN EAST TENNESSEE AND THE POCONO MOUNTAINS

Proteins represent a particularly bioavailable subset of organic carbon and nitrogen in aquatic environments but must be broken down into smaller, more easily metabolized pieces by microbially derived extracellular enzymes. Activities of extracellular peptidases (protein-degrading enzymes) have frequently been assayed in freshwater systems, but such studies have been limited to substrates for a single enzyme (leucyl aminopeptidase) out of more than 300 biochemically-recognized peptidases. We have analyzed the kinetic measurements of extracellular hydrolysis of substrates for five distinct classes of peptidases in twenty-eight freshwater bodies in the Delaware Water Gap National Recreation Area in the Pocono Mountains (Pennsylvania, USA) and near Knoxville, Tennessee (USA), between 2013 and 2016. The assays putatively test for four aminopeptidases (arginyl aminopeptidase, glyclyl aminopeptidase, leucyl aminopeptidase and pyroglutamyl aminopeptidase), which cleave N-terminal amino acids from proteins, and trypsin, an endopeptidase, which cleaves proteins mid-chain. Exopeptidases and the endopeptidase were active in all water bodies studied, indicating that a diverse set of enzymes were present in the water bodies samples. However, ratios of peptidase activities were variable among sites: aminopeptidases dominated at some sites, trypsin at others. At a given site the ratios remained fairly consistent over time, suggesting that they are driven by ecological factors. The variability of the endopeptidase: aminopeptidase ratio introduces about an order of magnitude uncertainty in total peptidolytic capacity of an environment, based on measurements of leucyl aminopeptidase activity alone. In studies in which enzyme activities span many orders of magnitude, this may be negligible, but in studies in which the range of activities is narrower, using leucyl aminopeptidase as a proxy for total aminopeptidases may introduce substantial error.