Methemoglobin formation after administration of hemoglobin conjugated to carboxylate dextran in guinea pigs. Attempts to prevent the oxidation of hemoglobin.

In 1990, McGown demonstrated in vitro a limitation of extracellular methemoglobin (metHb) formation by releasing and recycling of ascorbic acid by red blood cells. In order to investigate the autoxidation of free or modified hemoglobin in plasma and the possibility of reproducing McGown's phenomenon in vivo, we performed a 50% blood mass exchange in guinea-pigs with a 70 +/- 5 g/l dex-BTC-Hb solution (metHb < 5%). Methemoglobin was determined according to Evelyn-Malloy's method. We observed a clear but limited oxidation of plasmatic hemoglobin (MetHb approximately 30-40% at t = 12 hrs up to t = 24 hrs). A similar blood mass exchange was performed with the same hemoglobin solution which was previously totally oxidized into metHb. 40% of this methemoglobin was found to be reduced after 12 hrs. These results demonstrated a marked reducing activity by residual blood as shown by others. The addition of potentially protective compounds such as ascorbic acid (non enzymatic intraerythrocytar reduction pathway), methylene blue or riboflavin (enzymatic intraerythrocytar pathway), allowed a significant drop in the methemoglobin level. On the contrary, we didn't observe any reducing effect with reduced glutathione.