Hydrolysis of WR2721 by mouse liver cell fractions

A study of the dephosphorylation of WR2721 by broken cell preparations of mouse liver revealed the presence of at least two distinctive activities. One activity was inactivated by heat treatment and was present in the nuclear and microsomal fractions. It had an optimum pH at 9 and was inhibited by sodium vanadate, EDTA, and phenylalanine. Further subcellular fractionation demonstrated the localization of this activity in plasma membrane. A second WR2721 hydrolysis activity was detected in the cytosol fraction (postmicrosomal supernatant), which changed little with pH over the range of 5 to 10; sodium vanadate did not inhibit it. The cytosolic activity in response to heat treatment was complicated since there was an initial decrease followed by an increase in catalytic activity as a function of time at 55 degrees C. Enzyme kinetic analysis of the plasma membrane-associated activity in the microsomal fraction was performed, and Km and Vmax values of 12.5 and 69.9 nmol/min/mg protein, respectively, were obtained.