Modulation of Chloroplast Phosphofructokinase by NADPH

Phosphofructokinase has been partially purified from spinach (Spinacia oleracea) chloroplasts and studied from the standpoint of light/dark regulation. At concentrations reported to occur physiologically, NADPH effected a sharp inhibition of the enzyme by: (a) lowering its affinity (increasing the apparent K(m)) for both of its substrates, ATP and fructose 6-phosphate; and (b) lowering its V(max). Inhibition by NADPH was independent of pH and was observed both at pH 7.9 (pH of chloroplast stroma in the light) and pH 7.0 (stromal pH in the dark). The results are consistent with the conclusion that NADPH provides a mechanism for linking light to the modulation of phosphofructokinase activity and thereby to the regulation of glycolysis in chloroplasts.