Creatine kinase.

Creatine kinase is present in significant concentrations in skeletal muscle and cardiac muscle and to a lesser extent in gastrointestinal tract and brain tissue. The enzyme has been purified from a variety of tissues and an examination of its kinetic and physical properties reveal that the enzyme consists of two subunits and can exist as three isoenzymes containing essential cysteine residues. These properties are important in understanding its stability, the assay conditions, and the techniques used to identify the different isoenzymes. The relationship between the properties and the determination of the enzyme in biological fluids will be a main thrust of the review. Creatine kinase activity in serum rises rapidly in conditions such as acute myocardial infarction and trauma to skeletal muscle. However, the interpretation of such increases is dependent upon a sound knowledge of the factors which influence both the total and isoenzyme activities. The nature of these factors will be discussed in detail.