Muscle Enzymes

Sarcoplasmic membrane muscle enzymes are released when damage of the muscle cell occurs or its membrane becomes defective. Serum muscle enzymes such as creatine kinase (CK), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), alanine amino-transferase (ALT), and aldolase are often elevated in DM and PM. Changes in the levels of serum enzymes depend on the form and severity of inflammatory muscle disease, and the applied therapy.

Creatine kinase (CK) catalyze the formation of adenosine triphosphate and the donation of a phosphate group to creatine, the combination of which is used as a high-energy storage molecule responsible for the energy transport in muscle fiber according to the so-called “creatine phosphate shuttle” [1]. The enzyme is a dimer consisting of two subu-nits, M (muscle) and B (brain), either with sulfhydryl groups present at the active sites of the enzyme. CK isoenzymes MM, MB, and BB are characteristic for skeletal muscle, myocardium, and brain respectively. In addition to differences in the activity ratios of enzymes and isoenzymes, the extent of CK increase also provides important diagnostic clues. CK is found in numerous organs such as skeletal muscles, myocardium, and brain in the ratio 10:2.5:1 respectively, but also in thyroid gland, kidney, and liver. In inflammatory muscle disease such as PM or DM, CK levels in sera could be elevated more than 50-fold higher than the normal upper limit [2]. CK-MM subtype is the most sensitive and the most specific enzyme marker for skeletal muscle damage. The CK-MB isoenzyme, which is usually a hallmark of acute myocardial injury, may also increase in DM/PM [3–5]. Recently, Hamilton and Sharpe [5] reported two cases of inflammatory muscle disease presenting as either PM or DM with raised serum levels of CK-MB isoenzyme and troponin T in the absence of acute myocardial damage. B units of CK can be abnormally produced by regenerating skeletal muscle when these patients are treated with corticosteroids [4, 6]. Inflammatory muscle diseases are an important cause for the increasement of cardiac injury markers such as CK-MB and troponins, which are raised in up to 75% of PM/DM patients [7]. CK is not a specific marker for myositis; however, in inflammatory muscle diseases, as well as after physical exercise or traumatic muscle damage, it is the most often elevated enzyme [1] (Table 30.1).