Soil maltase activity by a glucose oxidase–perioxidase system

The enzyme maltase (glucoinvertase; glucosidosucrase; maltase-glucoamylase; a-glucopyranosidase; glucosidoinvertase; a-D-glucosidase; a-glucoside hydrolase; a-1,4-glucosidase EC 3.2.1.20), is involved in the exo-hydrolysis of 1,4-a-glucosidic linkages and certain oligosaccharides into glucose which is an important energy source for soil microbes. This enzyme originates from different sources, which include plants, seaweeds, protozoa, fungi, bacteria, vertebrates, and invertebrates. The assay of soil maltase using maltose as substrate and the released glucose determined using a glucose oxidase–peroxidase system has not been explored or investigated to the best of our knowledge. A simple assay protocol using this system is proposed to evaluate and characterize maltase activity in soils. The protocol involves the release of glucose (determined using a glucose oxidase–peroxidase colorimetric approach) when 1 g soil is treated with toluene and incubated with 5 mM maltose in 67 mM sodium acetate buffer (pH 5.0) at 37 C for 1 h. The optimal activity using this procedure was at pH 5.0 and decreased at temperatures above 70 C. The calculated Km values ranged from 0.8 to 6.5 mM, and are comparable to those of enzymes purified from microorganisms. The Arrhenius equation plots for the activity in the four soils were linear between 20 and 70 C. The activation energy values ranged from 34.1 to 57.2 kJ mol, the temperature coefficients (Q10) ranged from 1.5 to 1.9 (avg. = 1.7), and the coefficients of variation (CV) of the proposed assay protocol for the soils used was \6%. While we recognize the availability of established assay protocols to determine soil a-glucosidase (referred in other literature as maltase) activity based on the p-nitrophenol (artificial product) released from p-nitrophenyl-a-D-glucopyranoside (artificial substrate), our interest was to assay its activity by determining the glucose (natural product) released from maltose (natural substrate).