Methaemoglobin as an Indicator of PMOR Activity

MetHb is often defined as oxidised Hb; that is the haem iron is oxidised from the ferrous (Fe) state to the ferric (Fe) state. In addition, in metHb the sixth coordination site of the haem iron is liganded to a water molecule; this shifts the absorbance maxima from that of Hb causing metHb to be chocolate brown in colour (acid form). At pH ~8 the water molecule is replaced by a bound hydroxyl group (alkaline form; Jaffé, 1981). This site is empty in deoxyHb and occupied by molecular oxygen in oxyHb (Jaffé, 1981). Due to this occupation of the sixth coordination site, metHb is unable to bind molecular oxygen. As the number of oxidised haem sites in the Hb molecule increases, unoxidised sites show an increase in oxygen affinity and a decrease in cooperativity, resulting in an inability to exchange molecular oxygen (Perrella, et al., 1993). The α-chain of Hb is more readily oxidised than the β-chain (Mansouri and Winterhalter, 1974), thus intermediate oxidised Hbs exist mainly as α2β2 rather than α2β2 (Tomoda, et al., 1978).