Association, substrates - soil organics

The enzymatic activity in soil is mainly of microbial origin, being derived from intracellular, cell-associated or free enzymes. Only enzymatic activity of ecto-enzymes and free enzymes is used for determination of the diversity of enzyme patterns in soil extracts. Enzymes are the direct mediators for biological catabolism of soil organic and mineral components. Thus, these catalysts provide a meaningful assessment of reaction rates for important soil processes. Enzyme activities can be measured as in situ substrate transformation rates or as potential rates if the focus is more qualitative. Enzyme activities are usually determined by a dye reaction followed by a spectrophotometric measurement. 

Proteinases in soil extracts have been assayed using casein and substituted amides and peptides as assay substrates. Mayaudon et al. found that the casein hydrolysing activities of soil extracts were directly related to soil organic matter contents and inversely related to clay contents. Specific activities doubled when associated coloured humic compounds were removed from the extracts. The partially-purified proteinases appeared to be of a serine protease type as judged from their responses to inhibitors. Optimal activity occurred at pH 8.5 and at 50 0. The estimated Km value (22mg casein ml was considerably higher than those (0.15 and 0.54mg casein ml reported by Nannipieri et for proteinases of crude extracts from two soils. 

Apart from its use in studies of nitrogen fixation, N-labeled substrates have been used to assess the association of contaminants with organic matter in soil, and in metabolism. 

J.P. Martin, A.A. Parsa and K. Haider, Influence of intimate association with humic polymers on biodegradation of [14C] labeled organic substrates in soil, Soil Biol. Biochem. 10 (1978) 483-486. 

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