Revised Manuscript Received December 7, 1989 abstract: Albumin catalyzes the transformation of prostaglandin D2 to 9-deoxy-A9, A12 (£)-prostaglandin D2 and to isomeric prostaglandin D2 compounds including A12 (£)-prostaglandin D2. Both of these compounds are a,/3-unsaturated ketones, which should render them susceptible to nucleophilic addition. We therefore examined the ability of the compounds to form conjugates with thiols glutathione and cysteine. During incubation with excess glutathione, both 9-deoxy-A9, A12 (£)-prostaglandin D2 and A12 (£)-prostaglandin D2 formed a conjugate. Conjugation of 9-deoxy-A9, A12 (£)-prostaglandin D2 occurred very rapidly; approximately 70% was conjugated within 2 min. In contrast, conjugation of A12 (£)-prostaglandin D2 with glutathione proceeded at a much slower rate; only 38% was conjugated at 60 min. The formation of both conjugates was enhanced by glutathione 5-transferase. Conjugation of bothcompounds with cysteine was found to occur more rapidly than with glutathione. This effect was more pronounced with A12 (£)-prostaglandin D2 in which 60% conjugated with cysteine within 2 min. These differences are likely attributed to greater steric hindrance for conjugation across the A12 double bond compared to that across the A9 bond. Analysis by fast atom bombardment mass spectrometry confirmed the formation of the glutathione conjugate of 9-deoxy-A9, A12 (£)-prostaglandin D2. Following prolonged incubation of 9-deoxy-A9, A12 (£)-prostaglandin D2 with excess glutathione in the presence of glutathione 5-transferase, a small quantity of a bis conjugate of this compound was also detected by mass spectrometry.'H NMR analysis determined that in the mono glutathione conjugate of 9-deoxy-A9, A12 (£)-prostaglandin D2 the glutathione was conjugated at C-9. To assess the formation of conjugates in vivo, tritiated 9-deoxy-A9, A12 (£)-prostaglandin D2 was injected intravenously in a rat, and urine and bile were collected for 4 h. Of the recovered radioactivity, 90% was excreted into the bile. Essentially all of the radioactivity in the bile was present in the form of a polar conjugate. HPLC analysis of the bile revealed a major radioactive peak with an elution volume characteristic for that of the glutathione conjugate of 9-deoxy-A9, AI2 (£)-prostaglandin D2, suggesting that conjugation of this prostaglandin with glutathione may also occur in vivo. In addition, in view of the fact that glutathione is ubiquitous and present in mostcells, it may be important to consider the possibility that conjugation of these prostaglandins with glutathione may play a role in modulatingtheir biological actions.