Prostaglandin H synthase (PGHS) as apo- and holoenzyme and the enzyme inactivated during the conversion of arachidonic acid into prostaglandin H2 has been modified by diethyl pyrocarbonate (DEPC). DEPC (40 mol/l mol protein) rapidly, but quantitatively differently interacted with the three forms of the enzyme (pH 6.0, 25 degrees C). The exhausted reaction with DEPC corresponded to modification of seven histidine residues in apo-PGHS and four residues in holo-PGHS. All of the 18 histidine residues were available for modification in the enzyme inactivated during the catalysis. The modification of apo-PGHS was accompanied by a concerted loss of the combined cyclooxygenase plus peroxidase and peroxidase activities. The velocities of the tryptic cleavage of the three forms of the enzyme into the 33 and 38 kDa polypeptides were essentially different, but the modification of each enzyme form did not affect the velocity of its cleavage. Two of the three histidine residues essential for the interaction with the heme within the 38 kDa fragment might be His-309 and His-388. Based on the comparison of availability for the reaction with DEPC of all the 18 histidine residues in the enzyme molecule inactivated by the interaction with arachidonic acid and on the abnormally high velocity of the tryptic cleavage of this form of PGHS, a hypothesis has been put forward about the fast and dramatic changes in the protein structure in the course of catalysis.