Using different experimental conditions and applying various substrates, a wide range of metabolic rates could be observed in incubations with human erythrocytes. 2. Lower rates of substrate utilization resulted in a decrease of the ATP and, more pronounced, of the 2,3-bisphosphoglycerate concentration while carbon utilization rates beyond 14 mumol C/ml packed cells in 120 min yielded constant levels of ATP and 2,3-bisphosphoglycerate indicating that steady state had been achieved. 3. It can be concluded, that a carbon utilization rate of 14 mumol C/ml cells in 120 min is able to cover the energy requirement of the red cells under steady state conditions which could be calculated to amount to 3.6 mumol ATP/ml cells in 120 min at 37 degrees C. 4. The contribution of the 2,3-bisphosphoglycerate bypass to carbon metabolism could be calculated to range between 15--25% at metabolic rates below 14 mumol C/ml cells in 120 min (presteady state). When steady state has been achieved, the share of this bypass increases up to 61% with increasing metabolic rates.
