Enzyme kinetics in acoustically levitated droplets of supercooled water: A novel approach to cryoenzymology

The rate of the alkaline phosphatase-catalyzed hydrolysis of 4-methylumbelliferone phosphate was measured in acoustically levitated droplets of aqueous tris (50 mM) at pH 8.5 at 22 +/- 2 degrees C and in supercooled solution at -6 2 degrees C. At 22 degrees C, the rate of product formation was in excellent agreement with the rate observed in bulk solution in a cuvette, indicating that the acoustic levitation process does not alter the enzyme activity. The rate of the reaction decreased 6-fold in supercooled solution at -6 2 degrees C. The acoustic levitator apparatus is described in detail.