Decomposition of protein compounds in an eutrophic lake: spatial and temporal distribution of exopeptidase and endopeptidase activities in various size fractions

Abstract

Activities of exopeptidases and endopeptidases in various size fractions and the possible regulation of these enzymes in response to the changes in substrate concentrations were studied. The endopeptidase activities were determined using a protein substrate labelled with a fluorochrome. Among the enzyme activities studied only leucine aminopeptidase and endopeptidase activities were recorded. The protein compounds did not seem to cleave by synergistic endo and exoenzyme mechanisms. The inhibitory effect of ethylenediamine tetraacetate (EDTA) and phenylmethylsulfonile fluoride (PMSF) suggested a high proportion of metallo-proteases and serine-proteases. The inhibitory profile at 1 and 5 m indicated that planktonic organisms probably produced the same type of proteases, whereas enzymes present in the 0–1.2 and 1.2–100 μm fractions were not similar. The mean percentages of aminopeptidase activity at 1, 5 and 14 m in the dissolved fraction were 12.5, 12.7 and 18.4%. This enzyme activity was low in the 0.2–1.2 μm fraction (12.1% at 1 m, 13.3% at 5 m and 19.1%, at 14 m) compared to that measured in the 1.2–100 μm fraction, whereas the average percentages of endopeptidase activities in this fraction were 50.9% at 1 m, 50.1% at 5 m and 53.5% at 14 m. The bacteria attached to particles had a higher specific activity than free-living bacteria. It was 11.7 times higher than the enzyme activity associated with larger free-living bacteria and 112.3 times higher than the activity of small free-living bacteria. In this study, the specific activity of the aminopeptidase (activity in the fraction 0.2–100 μm per number of bacteria) was correlated with Chl a at 1 m (r = 0.65, P <0.01), 5 m (r = 0.78, P <0.001) and 14 m (r = 0.96, P <0.001). The production of protein compounds by the phytoplankton could therefore regulate aminopeptidase activity.