The influence of beta-93 sulfhydryl groups, organic phosphate and heme concentration on methemoglobin reduction.

Native and modified methemoglobin (beta-93-SH groups blocked) were reduced by NADH-dependent methemoglobin reductase in the absence and the presence of organic phosphate (inositol hexaphosphate). These experiments were performed with dilute as well as concentrated methemoglobin solutions (physiological heme concentration). It is shown that: (a) in dilute solutions the blockage of beta-93-SH groups lowers the rate of methemoglobin reduction in the absence of organic phosphate but the rates of native and modified methemoglobin reduction are similar in the presence of organic phosphate; (b) at physiological heme concentration the blockage of beta-93-SH groups does not affect the rate of reduction but the organic phosphate stimulates the reduction of both native and modified methemoglobins in a similar fashion, as it does in dilute solutions. It is concluded that, although in dilute solutions the blockage of beta-93-SH groups alters the reduction rate, at physiological heme concentration the presence of free beta-93-SH groups does not have any significant effect on methemoglobin reduction. On the contrary, the organic phosphates do accelerate the rate of reduction at all ranges of heme concentration.