5(S)-HETE lactone is a cyclic ester formed by acid-catalyzed nucleophilic addition of the C-5 hydroxyl to the C-1 carboxyl of 5(S)-HETE. The ability of (±)5-HETE lactone to inhibit RBL-1 cell 5-LO (IC50 = 27 μM) may be entirely due to the 5(S) isomer, but the enantiomers have not been tested separately.{690}