The reaction was started by the addition of purified brain hexokinase I (0.24 μg) (gift from J. Wilson) to 1.0 ml of reaction mixture containing 0.05 M Tris-HCl (pH 8.5), 7.4 mM MgCl2, 6.6 mM ATP, 0.65 mM NADP, 11.1 mM monothioglycerol, and 1 unit of glucose 6-phosphate dehydrogenase.