Aldo-keto reductase 1B1 and 1B3 (AKR1B1 and AKR1B3) are the pri-mary human and mouse prostaglandin F2a(PGF2a) synthases, respectively, which catalyze the NADPH-dependent reduction of PGH2, a common intermediate of various prostanoids, to form PGF2a. In this study, we found that AKR1B1 and AKR1B3, but not AKR1B7 and AKR1C3, also catalyzed the isomerization of PGH2 to PGD2 in the absence of NADPH or NADP + .