Kinetic studies of proteolytic enzyme activity of arctic soils under varying toluene concentrations

The original focus of this study was an analysis of proteolytic enzyme activity of Alaskan arctic tundra soils, however initial results raised questions regarding the method (Watanabe and Hayano, 1995). Thus, the goals of the study changed to 1) an investigation of the method, and 2) a comparison of enzyme activities of two different soil layers from the arctic tundra. Methodological examination included the impact of toluene, used to prevent immobilization of the product, and blank correction on enzyme activity, and a search for a true 6-h linear rate of activity during a 48-h incubation. We measured native and potential, using casein as an artificial substrate, activities as net amino acid production in mineral and organic soil layer samples. Varying toluene concentration had no clear effect on activity; omitting toluene resulted in zero native activity and reduced potential for the organic samples, but not for the mineral. Comparison of activities with and without blank correction indicated, particularly for potential activity of samples with low native rates, that correction was required for accuracy. Native and potential activity of the organic samples, and native of the mineral were linear during the first 6 h of incubation; linearity was found during the 6–24 h incubation for potential activity of the mineral. Soil layer activity data indicated that native activity was higher in organic soils as compared with mineral. The organic layer potential activity was ten-fold greater than the native, suggesting substrate limitation; potential and native activities did not differ in the mineral layer, indicating substrate sufficiency. Casein addition changed the kinetic pattern for both layers from hyperbolic to sigmoidal for the mineral and linear for the organic, implying different enzyme pools or behavioral changes of existing pools. Native activity based on total soluble protein was higher for the mineral samples relative to the organic, reiterating substrate capacity differences and variations in enzyme/substrate interactions.

Research highlights► Higher protease activity per gram dry weight in organic vs mineral tundra soils. ► Higher activity in mineral vs organic tundra soils on total soluble protein basis. ► Organic soils substrate limited, mineral not. ► Linearity generally occurs during the first 6 h of incubation but not always. ► Toluene regardless of level reduced immobilization only in organic tundra soils.