Enzyme reaction rates at limited water activities.

A well-mixed powder consisting of dry urea and urease exposed to air containing discrete amounts of water vapor showed a release of carbon-14 dioxide above 60-percent relative humidity. The relative activity of urease followed the water-vapor adsorption isotherm of urease. The minimum amount of water required for the reaction observed was 1.

Enzyme action: comparison on soluble and insoluble substrate.

The action of trypsin on gelatin solution is compared with its action on swollen gel microspheres in suspension. Both the solution gelatin and gel spheres, which are readily permeable to the enzyme, follow Michaelis-Menten kinetics. The apparent rate constants for dissociation of the enzyme-substrate complexes to hydrolysis products are essentially the same for both solution gelatin and spheres, an indication that gel structure in this system has a negligible influence on reaction rate once the enzyme forms a complex with the substrate.