Contribution of different catalytic types of peptidases to soil proteolytic activity

Abstract

Soil organic nitrogen is largely composed of proteinaceous material, hence, the extracellular peptidases that are widely produced by microorganisms play a critical role in the recycling of soil organic nitrogen. But why do microbes produce such a variety of functionally different peptidases? In theory, this could be an adaptation to substrate heterogeneity, but it may also be an adaptation to variable soil conditions. Here we characterized the contribution of different catalytic types, or classes, of peptidases present in soil with the intent to determine if their relative contributions would vary as a function of soil properties. We screened specific peptidase inhibitors and optimized their concentrations to work effectively in soil. Total potential proteolytic activity was partitioned among several peptidase classes by adding class-specific inhibitors to the peptidase assay. Using Pepstatin A, EDTA (ethylenediaminetetraacetic acid), PMSF (phenylmethylsulfonyl fluoride), and E64 (epoxysuccinyl-L-leucylamido (4-guanidino) butane), we were able to discriminate among aspartic, metallo-, serine, and cysteine peptidases, respectively. We found that diverse peptidases were active and contributed to the proteolytic activity in soil. Extracellular peptidase profiles varied among different soils and were associated with soil chemical and microbial properties. Metallopeptidases contributed 30–50% of the soil proteolytic activity in all soils. A higher relative contribution of metallopeptidase activity was found in less acidic soils, reflecting its neutral pH optimum. Serine peptidases were only detected in soil from Douglas-fir (Pseudotsuga menziesii) stands (10–20% of total proteolytic activity) but not in soils under red alder (Alnus rubra). The relative activity of aspartic peptidase correlated positively with the fungal:bacterial ratios of the soils. Our results lend support to the view that microbial communities might modify their peptidase profile to optimize protein utilization in response to soil and other environmental factors.