| CONCENTRATIONS OF REDOX COENZYMES IN THE CELL
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| G6PDH and 6-phosphogluconate dehydrogenase maintain a cytoplasmic ratio of NADPH/NADP+ ≈100. Interestingly, because NAD+ is required for glycolysis, the ratio of NADH/NAD+ in the cytoplasm is nearly the inverse, less than 0.01. Although the total concentrations (oxidized plus reduced forms) of NAD(H) and NADP(H) in the RBC are similar (∼25 μmol/L), the cell maintains these two redox systems with similar redox potentials at such different set-points in the same cell by isolating their metabolism through the specificity of cytoplasmic dehydrogenases. The glycolytic enzymes (G3PDH and LDH) use only NAD(H), while pentose phosphate pathway enzymes use only NADP(H). There are no enzymes in the RBC that catalyze the reduction of NAD+ by NADPH, so that high levels of both NAD+ and NADPH can exist simultaneously in the same compartment.
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