Protection of cyclooxygenase activity during heme-induced destabilization.

The initial step in the conversion of arachidonic acid to prostaglandins and related derivatives is catalyzed by the microsomal enzyme, cyclooxygenase. The ovine enzyme (from sheep vesicular glands) requires heme which is tightly bound as a prosthetic group (l-3). The bovine enzyme also requires heme (4) from which it is readily dissociated (5). In addition to free hematin, a variety of hemeproteins, including hemoglobin and myoglobin, can provide the heme necessary for activity (2, 4-6). Attempts to purify and study the cyclooxygenase from ovine vesicular glands have been hampered by enzyme instability (1,79). Recently, while attempting to prepare holoenzyme, Ogino et al. (5) noted that heme caused a rapid destabilization of bovine vesicular gland cyclooxygenase (50% inactivation in a few minutes). However, several agents such as epinephrine, hydroquinone, tryptophan, and phenol provided protection. The mechanism of the destabilizing process was not resolved since removal of O2 or addition of scavengers for peroxide, superoxide, singlet oxygen, and hydroxy radical did not appear to protect the cyclooxygenase (5). Almost without exception, the compounds which stabilized cyclo-