Recently we have shown that ferrica-hydroxyhaem bound tohaemoxygenase-1 canbe converted to ferrous verdohaem by approximately an equimolar amount ofO2in the absence of exogenous electrons [Sakamoto,H.,Omata,Y.,Palmer, G.,and Noguchi, M. (1999)J. Biol. – 18200]. Contrary to those results,other studies have claimed that the conversion requires both O2andanelectron.