It is helpful to remember that some enzymes are called upon to work in both directions at different times or in different tissues. For example, pyruvate is converted into lactate in muscle during anaerobic exercise, and lactate is converted into pyruvate in liver (this is called the Cori cycle). The same enzyme, lactate dehydrogenase, performs both jobs. The enzyme could have been named pyruvate reductase just as easily. Creatine phosphokinase (also called just creatine kinase) is an example of an enzyme that works in one direction at one time and the opposite direction at another time.