[Effect of surface-active agents on peroxidase activity. IV. Effect of lipids and fatty acids from milk]

The effect of surfactants, lipids and fatty acid salts isolated from cow milk on the activity of heme-containing horseradish peroxidase in solution was studied. As the surfactant concentration increases, the rate of the enzymic reaction successively decreases, increases, and again decreases, down to zero in the case of the fatty acid salts. The initial deceleration of the reaction rate results from the enzyme inhibition. The subsequent increase is caused by an improved accessibility for the substrate and the enhanced activity of the catalytic site of the enzyme due to its immobilization in the surfactant aggregates. A shielding of the protein by these aggregates can explain the secondary deceleration of the enzymic reaction rate. The general character of the dependence is similar and does not depend on the surfactant structure for a series of fatty acid salts and phospholipids; however, it is quite different in the case of cholesterol and sphingomyelin.