Hemoglobin vesicles containing methemoglobin and L-tyrosine to suppress methemoglobin formation in vitro and in vivo

Hemoglobin (Hb) vesicles have been developed as cellular-type Hb-based O-2 carriers in which a purified and concentrated Hb solution is encapsulated with a phospholipid bilayer membrane. Ferrous Hb molecules within an Hb vesicle were converted to ferric metHb by reacting with reactive oxygen species such as hydrogen peroxide (H2O2) generated in the living body or during the autoxidation of oxyHb in the Hb vesicle, and this leads to the loss of O-2 binding ability. The prevention of metHb formation by H2O2 in the Hb vesicle is required to prolong the in vivo O-2 carrying ability. We found that a mixed solution of metHb and L-tyrosine (L-Tyr) showed an effective H2O2 elimination ability by utilizing the reverse peroxidase activity of metHb with L-Tyr as an electron donor. The time taken for the conversion of half of oxyHb to metHb (T-50) was 420 min for the Hb vesicles containing 4 g/dL (620 mu M) metHb and 8.5 mM L-Tyr ((metHb/L-Tyr) Hb vesicles), whereas the time of conversion for the conventional Hb vesicles was 25 min by stepwise injection of H2O2 (310 mu M) in 10 min intervals. Furthermore, in the (metHb/L-Tyr) Hb vesicles, the metHb percentage did not reach 50% even after 48 h under a pO(2) of 40 Torr at 37 degrees C, whereas T-50 of the conventional Hb vesicles was 13 h under the same conditions. Moreover, the T-50 values of the conventional Hb vesicles and the (metHb/L-Tyr) Hb vesicles were 14 and 44 h, respectively, after injection into rats (20mL/kg), confirming the remarkable inhibitory effect of metHb formation in vivo in the (metHb/L-Tyr) Hb vesicles.