Tracing of Peroxidase Activity in Humic Lake Water

An enzyme assay was developed for studies on peroxidase activities in humic lake water. 3,4-Dimethoxybenzyl alcohol (veratryl alcohol, VeraOH) was used as tracer substrate, and peroxidase (EC 1.11.1.7) activity was measured by high-performance liquid chromatography. The chemical stability of VeraOH and its application as peroxidase substrate was tested under light and dark conditions, different hydrogen peroxide (H₂O₂) concentrations and humic matter contents. VeraOH was stable under low UV radiation at in situ conditions in lake water (<0.010...0.25 kJ m^–2 d^–1), laboratory conditions (<0.05...0.30 kJ m^–2 d^–1), and low (1...100 μM) H₂O₂ concentrations. However, peroxides oxidized VeraOH above 1...10 mM H₂O₂ concentration in sterile Millipore-Q and humic lake water. Dark incubations showed little VeraOH oxidation products. The developed peroxidase assay was tested in the growth medium of Phanerochaete chrysosporium and a bacteria isolate (P.M.D. 20.4.3.1) from mesohumic lake Pääjärvi. Peroxidase activities were also measured in natural microbial communities under standard laboratory and under in situ conditions in humic lake water. Incubation times of about 5 to 12 days were usually needed to record significant (P < 0.05) peroxidase activities, in lake waters. In situ peroxidase activities varied in pelagial surface water (0...0.5 m) on a seasonal scale between 74 nmol L^–1 h^–1 and 273 nmol L^–1 (mean: 176 nmol L^–1 h^–1) and within the water column between 110 nmol L^–1 h^–1 and 800 nmol L^–1 h^–1 (mean: 500 nmol L^–1 h^–1) in polyhumic lake Mekkojärvi.