Magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase (EC 1.14.13.81 Mg-protoporphyrin IX monomethyl ester (oxidative) cyclase) is an enzyme with system name magnesium-protoporphyrin-IX 13-monomethyl esterNADPH:oxygen oxidoreductase (hydroxylating) .