Stability and properties of alkaline phosphate immobilized by a rendzina soil

Abstract

Different factors (agitation time, type of buffer, enzyme solution/soil ratio, enzyme solution concentration) affecting alkaline phosphatase immobilisation by a rendzina soil were studied. Appreciable quantities (>48%) of commercial active phosphatase were immobilised by the soil colloidal particles in a process well adjusted to a Langmuir isotherm (r = 0·999 at P<0·001), giving an index of bonding energy of 0·115 ml h μmol p-nitrophenol−1 and an enzymic sorption maximum of 79·9 μmolp-nitrophenol g soil−1 h−1. Some properties and the stability of the immobilised enzyme (kinetics, effect of soil, pH-activity curves, inhibition by inorganic phosphate, persistence, thermal stability, and exposure to proteinases) were also investigated. The Km value for the immobilised enzyme was greater than that for the soluble enzyme, the pH optimum was similar to that of the indigenous soil enzyme but higher than that of the enzyme in solution, and there were no appreciable differences in the inhibition by inorganic phosphate of the three types of enzyme (immobilised, indigenous and soluble phosphatase). The enzyme studied was stable when added to soil, and appeared to have a thermal stability and a protection against pronase and trypsin larger than that of the soluble enzyme but lower than that of indigenous soil alkaline phosphatase.