Effects of ascorbate on methemoglobin reduction in intact red cells

Abstract

An endpoint of 75% HbO 2/25% methemoglobin (MetHb) was approached in red cells incubated with a greater than physiologic concentration of ascorbate (10 m m). The presence of glucose (5 m m) with ascorbate shifted the endpoint to 90% HbO 2/10% MetHb while lactate (2 m m) plus pyruvate (0.1 m m) had no effect. These endpoints were approached regardless of the HbO 2 MetHb ratio at zero time. No hemoglobin degradation was observed. When red cells containing 100% MetHb at zero time were used, analysis of the initial rate of HbO 2 formation in the presence of various substrates showed synergistic interaction between ascorbate (10 m m) and glucose, additive activity with ascorbate and lactate, and less than additive activity with glucose and lactate. Incubation of red cells with a phsyiologic concentration of ascorbate (0.1 m m) resulted in no significant HbO 2 formation in the absence of other additions. When red cells were incubated with glucose and/or lactate plus pyruvate, an endpoint of about 99% HbO 2/1% MetHb was approached regardless of the HbO 2/MetHb ratio at zero time or the presence or absence of physiologic ascorbate. Physiologic ascorbate slightly but consistently increased the rate of HbO 2 formation in red cells incubated with glucose but not with lactate. HbO 2 formation was not increased by ascorbate in red cells which contained more than about 90% HbO 2 at zero time. The results indicate that excess ascorbate functions stoichiometrically driving cellular chemistry to a steady state between HbO 2 and MetHb formation whereas physiologic ascorbate functions catalytically allowing electron transport from glucose to MetHb via the hexose monophosphate shunt.