The measurement of serum levels of muscle enzymes is a critical part of the evaluation of patients presenting with weakness or myalgias and is important in monitoring the course and response to therapy of certain muscular disorders. Creatine kinase (CK), lactate dehydrogenase (LD), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and aldolase are the serum enzymes that are measured in clinical practice.
This topic will review the biochemistry of muscle enzymes and their use in clinical practice. The biochemistry and use of serum enzymes in patients with renal failure are discussed separately. (See "Serum enzymes in patients with renal failure" and "Serum cardiac enzymes in patients with renal failure".)
PHYSIOLOGY OF MUSCLE ENZYMES
Creatine kinase — CK is the most widely used enzyme to diagnose and follow muscle disease. It is present in the highest concentrations in serum in response to muscle injury, is the most sensitive indicator of muscle injury, and is the best measure of the course of muscle injury .
CK is located on the inner mitochondrial membrane, on myofibrils, and in the muscle cytoplasm . It is involved in cellular energy storage and transfer via two major effects:
- It catalyzes the production of high energy ATP via transfer of a phosphate from creatine phosphate, which is the major storage reservoir of energy during muscle rest, to ADP (adenosine diphosphate).
- It participates in the transfer of high energy phosphate from its site of production in the mitochondria into the muscle cell cytoplasm where it is used during muscle contraction.