Creatine kinase (CK, EC 2.7.3.2) catalyze creatine phosphate and ADP to produce creatine and ATP. Hexokinase catalyze creatine and glucose to produce glucose-6-phosphate. Glucose-6-phosphate dehydrogenase (G6P-DH,Glucose-6-phosphate dehydrogenase) catalyze glucose-6-phosphate and NADP+ to produce NADPH which have a maximum absorption peak at 340 nm. The CK activity can be calculated by measuring the OD values at 340 nm.