Description of an extraction from a Dublin clay loam soil of a colloidal organic matter complex that is urease active and, by X-ray analysis, free of clays. Urease activity in the clay-free precipitates, as in the soil, was not destroyed by the activity of an added proteolytic enzyme, pronase. This is attributed to the circumstance that native soil urease resides in organic colloidal particles with pores large enough for water, urea, ammonia, and carbon dioxide to pass freely, but nevertheless small enough to exclude pronase.
Burns, R. G., El-Sayed, M. H., & McLaren, A. D. (1972). Extraction of an urease-active organo-complex from soil. Soil Biology and Biochemistry, 4(1), 107–108. https://doi.org/10.1016/0038-0717(72)90048-X