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The Influence of Clay Minerals on the Enzymatic Hydrolysis of Organic Phosphorus Compounds¹

ABSTRACT

Organic phosphorus compounds contain a large portion of the total phosphorus in many soils. The release of phosphate from the organic form is thought to be enzymatic in nature. Since clays compose an active and important part of the soil, a study was made of their effect on the enzymatic hydrolysis of several organic phosphorus compounds.

The effect of clays on the enzymatic hydrolysis of fructose-diphosphate, phytin, glycerophosphate, and lecithin was studied. The enzymes were prepared from bread yeast, wheat bran, bread yeast, and kidney cortex, respectively. The various clays studied included kaolinite, Wyoming bentonite (montmorillonite), Swygert (illite), and Cisne (a clay similar to Putnam). The activity of the enzyme was determined with no clay present, and then in the presence of various amounts and kinds of clays. The enzymatic reactions were performed paying special attention to the control of pH, temperatures, inhibitors, activators, and concentrations. The difference between the activity of the enzyme on the organic phosphorus compound in the presence of a suitable buffer and the activity in the presence of clays was considered a measure of the influence of the clay upon the enzyme activity. All of the clays used in this study exhibited an inhibiting influence upon the enzymatic hydrolysis of the organic phosphorus compounds. The amount of inhibition varied with the kind of clay according to the following series: Wyoming bentonite (montmorillonite) > Cisne (Putnam-like clay) > Swygert (illite) > kaolinite.

The inhibition of phosphatase activity by clays is roughly proportional to the base exchange capacity of the clay. A constant, evaluated for each clay in an empirical equation which describes hydrolysis of the organic compound in the presence of varying quantities of clay, is approximately equal to the ratios of the base exchange capacities of the various clays. The study indicates that the inhibition is due to the effect of the clay on the enzyme and not to adsorption of the organic phosphorus compound by the clay.

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana, Ill. Published with the approval of the Director. Part of a thesis submitted to the Graduate College, University of Illinois, by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Received for publication September 20, 1951.

² Formerly First Assistant in Soil Physics, now Assistant Professor of Agronomy, University of Wyoming, Laramie, Wyo. and Professor of Soil Physics, respectively.