REDUCTION OF METHEMOGLOBIN VIA ELECTRON-TRANSFER ACROSS THE BILAYER-MEMBRANE OF HB VESICLES

The hemoglobin vesicle (HbV) has a cellular structure which encapsulat es concentrated Hb in the inner aqueous phase of a phospholipid bilaye r vesicle. Hb is gradually autoxidized to methemoglobin (metHb), which can not bind oxygen during oxygen transport under physiological condi tions. In order to reduce metHb in HbV, we evaluated the reduction of metHb by electron transfer across the bilayer membrane of HbV from a r eductant added to the outer aqueous phase. Water-soluble methylene blu e (MB) and hydrophobic ubiquinone 10 (UQ) were selected as electron me diators. Under a nitrogen atmosphere, the addition of the reduced form nicotinamide-adenine dinucleotide (NADH) to the outer aqueous phase o f UQ-incorporated HbV showed only a slow reduction rate for metHb. On the other hand, when MB and NADH were added under a nitrogen atmospher e to HbV containing 40% metHb, a rapid decrease in the metHb percentag e was observed. The entire reaction was controlled by a reaction with NADH and MB in the outer aqueous phase. Under aerobic conditions, the decrease in the efficiency of the metHb reduction and rapid oxidation after reaching the minimal metHb percentage were observed. This was co nfirmed to be due to the influence of hydrogen peroxide; the decrease was prevented by the co-encapsulation of catalase.