The presence of endogenous inhibitors of NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase (PGDH) has been indicated by increasing total activity after the initial purification step of PGDH in human placenta. Based on this observation, we tried to characterize and analyze endogenous inhibitors of PGDH in human placenta in this study. The inhibitors were extracted from the supernatant by precipitation at pH 5.2 and partially purified by acetone precipitation and by thin layer chromatography. The inhibitors were stable to heating at 100 degrees C for 10 min, and to trypsin digestion. The pattern of inhibition was competitive with regard to PGE2 and uncompetitive with regard to NAD at pH 8.0. The Ki value for PGE2 was 18.9 microM. Analysis by gas chromatography and mass spectrometry indicated that the inhibitors consisted of fatty acids which were palmitic, stearic, oleic and linoleic acids. Myristic, palmitic and stearic acids were confirmed to exert an inhibitory action on PGDH and showed a competitive inhibition pattern. Stearic acid was less potent in inhibition than other fatty acids. These findings suggest that intracellular fatty acids may play a unique role in the control of PGDH activity.