Human serum albumin (HSA), the most abundant protein in plasma, has a very unique function, catalyzing the conversion of prostaglandin J(2) (PGJ(2)), a dehydration product of PGD(2), to yield Delta(12)-PGJ(2). These PGD(2) metabolites are actively transported into cells and accumulated in the nuclei, where they act as potent inducers of cell growth inhibition and cell differentiation, and exhibit their own unique spectrum of biological effects. The facts that (i) arachidonic acid metabolites bind to human serum albumin (HSA) and the metabolism of these molecules is altered as a result of binding, (ii) HSA catalyzes the transformation of PGJ(2) into Delta(12)-PGJ(2), and (iii) Delta(12)-PGJ(2) is stable in serum suggest that HSA may bind and stabilize Delta(12)-PGJ(2) in a specific manner. A molecular interaction analysis using surface plasmon resonance (Biacore) indeed suggested the presence of a specific Delta(12)-PGJ(2)-binding site in HSA. To investigate the molecular details of the binding of this PGD(2) metabolite to albumin, we analyzed the cocrystal structure of the HSA-Delta(12)-PGJ(2)-myristate complex by X-ray crystallography and found that two Delta(12)-PGJ(2) molecules bind to a primary site in subdomain IB of the protein. The electron density results suggested that one of the two Delta(12)-PGJ(2) molecules that specifically bind to the site covalently interacted with a histidine residue (His146). Using nano-LC-MS/MS analysis of the HSA-Delta(12)-PGJ(2) Complex, the formation of an unusual Delta(12)-PGJ(2)-histidine adduct at His146 was confirmed. Thus, our crystallographic and mass spectrometric analyses of the HSA-Delta(12)-PGJ(2) complex provided intriguing new insights into the molecular details of how this electrophilic ligand interacts with its primary producer and transporter.