The kinetic mechanism of ketoreductase activity of bovine lung prostaglandin F synthase, expressed in E. coli, was investigated. Data on initial velocity and radioisotope exchange between [3H]prostaglandin D2 and 9 alpha,11 beta-prostaglandin F2 suggest that the enzyme obeys the ping-pong mechanism. Using a fluorescence technique we obtained a binding constant of 3 microM for NADPH. This is in close correlation with the kinetically determined intrinsic Michaelis constant for NADPH. Activation energy of the redox process was determined from the temperature dependence of maximal velocities for nitrobenzaldehyde and menadione and was found to be 119 and 96 kJ/mol, respectively.