NADP⁺ reduction by human lymphocytes

The hexose monophosphate shunt (HMPS) is known to be responsible for the reduction of NADP⁺ by lymphocytes. We tried to find other enzymatic systems that might provide the lymphocytes with NADPH. By measuring the absorbance at 340 nm we noted that the addition of NADP⁺ to a preparation of disrupted lymphocytes resulted in the formation of NADPH at a rate of 4 nmol/10⁶ cells per min. This phenomenon could not be changed by negative feedback inhibition of HMPS, and could not be attributed to the low concentration of glucose, glucose-6-phosphate (G-6-P) and isocitrate found in the cell preparation (NADP⁺-dependent isocitrate dehydrogenase in addition to HMPS NADP⁺ reducing enzymes was found to be present in lymphocytes). Because of the activity of a NADP⁺-dependent lactate dehydrogenase, pyruvate oxidized the NADPH as it was being formed. Here we demonstrate the presence of an unknown NADP⁺ reducer in lymphocytes which seems to play an additional role to HMPS in NADP⁺ reduction by lymphocytes. NADP⁺-dependent lactate dehydrogenase may play a role in regulating the NADP⁺/NADPH ratio.