Methaemalbumin formation in sickle cell disease: effect on oxidative protein modification and HO-1 induction.

Normally, cell free haemoglobin is bound by haptoglobin and efficiently cleared. However, the chronic haemolysis in sickle cell disease (SCD) overwhelms haptoglobin binding capacity and protein turnover, resulting in elevated cell free haemoglobin. Cell free haemoglobin acts as both a scavenger of vasoactive nitric oxide and a pro-oxidant. In addition, methaemoglobin (metHb) releases the haem moiety, which can bind to albumin to form methaemalbumin (metHSA). This study used electron paramagnetic resonance to detect metHSA in SCD plasma and demonstrated that haptoglobin prevents haem transfer from metHb to HSA. MetHSA may either provide a second line of defence against haemoglobin/haem-mediated oxidation or contribute to the pro-oxidant environment of SCD plasma. We demonstrated that HSA inhibited oxidative protein modification induced by metHb. Additionally, we showed that while metHb induced haem oxygenase 1 (HO-1), an indicator of oxidative stress, HSA attenuated metHb induction of this enzyme, thereby limiting the potential benefits of HO-1. Furthermore, HO-1 induction by metHSA was less than HO-1 induction by equimolar metHb not bound to albumin. Our findings confirm the presence of metHSA in SCD and suggest that haem transfer from metHb to HSA reduces the oxidative effects of free haemoglobin/haem on endothelium with both beneficial (reduced protein oxidation) and potentially harmful (reduced HO-1 induction) outcomes.