Previous studies have demonstrated that mutation in prostaglandin endoperoxide synthase-1 of Cys313 or Cys540 to Ser residues reduces cyclooxygenase and peroxidase activities by 80-90%. In the present work, we investigated the effect of these Cys-to-Ser mutations on the sensitivity of the enzyme to inhibition by cyclooxygenase inhibitors, the ability of the enzyme to form homodimers, the extent of glycosylation of the enzyme, and the sensitivity of the enzyme to maleimide enzyme inhibitors. No significant differences were observed between native and mutant enzymes in any of these parameters. The results suggest that the loss of activity observed in the mutant enzymes is not due to major differences in protein folding or aggregation. Most surprising was the finding that the sensitivity of prostaglandin H synthase-1 to maleimide-containing inhibitors was not affected by mutating any of the Cys to Ser. This indicates that the inhibition of cyclooxygenase activity affected by N-ethylmaleimide and N-carboxyheptylmaleimide is not due to modification of a cysteine residue.