Creatine kinase catalyzes the reversible transfer of a phosphate group from creatine phosphate to adenosine disphosphate [ADP] forming ATP.
creatine phosphate + ADP -> creatine + ATP
The ATP produced is a substrate for the coupled activities of glucokinase (GK) and glucose-6-phosphate dehydrogenase [G-6-PDH], resulting in the reduction of nicotinamide adenine dinucleotide phosphate [NADP+ to NADPH]
ATP + glucose -> glucose-6-phosphate + ADP
Glucose-6-phosphate + NADP+ -> 6-phosphogluconate + NADPH + H+
Conditions are such that the CK reaction is rate-limiting therefore, the resulting increase in absorbance at 340 nm is directly proportional to the CK level in the sample.
The International Federation of Clinical Chemistry has published recommended conditions for the measurement of CK. The Synermed® CK method is based upon these recommendations.