N. A. Daugherty Colorado State University Fort Collins, CO 80523
A middle aged man who has just finished playing tennis, experiences chest pain and is admitted to a hospital. How does the admitting physician decide whether the patient's chest oains are caused by a heart attack or by some other cause such as indigestion, or simple over-exertion? A few )ears ago, the doctor would have had to relv on a physical rxamination and an elertnr.;lrdineram IEKG I. The resultv were often less than definitive. Now there is available a simple chemical test which allows the ohvsician to confirm or rule out a heart attack (myocardiai iifarction or M.1.) with almost 100% confidence. This new chemical test involves the measurement of isoenzymes. Almost all reactions which occur in biological systems are enzyme catalyzed. Enzymes are protein molecules, usually containing more than 100 amino acid residues and having molecular weights in excess of 10,000. The portion of the enzyme molecule responsible for the catalytic behavior is called the active site. The active site comnrises onlv a small oart of .-~. the enzyme molecule. Any structu;al changes in the enzyme which do not affect the active site will have little influence on the catalytic properties of the molecule. Most enzymes are produced in many different types of body tissue. In many cases, the different types of tissue produce sliahtlv different forms of the enzyme. These multiple forms of gi;en enzyme are called i s o e n h e s . For example, skeletal muscle, heart muscle, and brain tissue all produce the enzyme creatine kinase (CK).' Each of these tissues produces a slightly different form of CK. Because each isoenzyme catalyzes the same reaction. the isoenzvmes must differ in wavs which do not influence the artive site. Altering the poly peptidr, chain at a Iocntion other than the active site u,ould result in a dit. ferent form of the enzyme hut would not alter its catalytic behavior. The enzyme CK is a dimer consisting of two polypeptide chains. The isoenzyme produced by skeletal muscle is made up of two identical chains and is designated as the MM isoenzyme. Brain CK consists of a different pair of identical chains and is designated as the BB isoenzyme, while the cardiac enzyme is made up of a chain of each type and is designated as MB. The enzymes associated with cellular metabolism are present in relatively high concentrations within the cell. In healthv tissue. the cell membrane confines the enzvmes within the ceil and the concentration of the enzymes in ~xtracellular fluid or plasma is low. When the cells are damaged by disease or injury, the cellular enzymes are "leaked into the extracellular fluid and plasma causing a rise in the levels found in the blood. As a result, disease often causes an increase in the concentrations of enzymes found in the blood. The enzyme amylase is secreted by the pancreas. When the pancreas hecomes inflamed (pancreatitis), the level of serum amylase increases dramtically. Since the pancreas is the only major source of amylase, an increase in serum amylase is a good diagnostic sign of pancreatitis. When several tissues or organs are sources of an enzyme, the measurement of the total en~
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