Enzymes are catalysts that enhance reactions without directly participating in them. Individual enzymes, each of which has its own substrate and product specificity, exist for nearly all of the metabolic reactions that maintain body functions.
Enzymes are normally intracellular molecules. Because certain metabolic reactions occur in many tissues, the involved enzymes exist in many cell types. Enzymes with more restricted metabolic functions are found in only one of several specialized cell types. The presence of enzymes in circulating blood indicates cellular changes that have permitted their escape into extracellular fluid. The continuous synthesis and destruction of the cells of the enzymes' origins, for example, allow small amounts of enzymes to appear in the blood. Cellular disruption caused by damage by disease, toxins, or trauma, as well as increased cell wall permeability, also elevates serum enzyme levels. Additional causes of elevated enzyme levels are an increase in the number or activity of enzyme-containing cells and decreases in normal excretory or degradation mechanisms.
Decreased serum enzyme levels rarely have diagnostic significance because so few enzymes are present in substantial quantity. Enzyme levels may decline if the number of synthesizing cells declines, if generalized or specific restriction in protein synthesis occurs (enzymes are proteins), or if excretion or degradation increases.
Very few enzymes are studied routinely. Although highly specialized enzyme analysis is applied to the study of many genetically determined diseases, most diagnostic enzyme studies involve only those enzymes with changing values in serum, providing inferential or confirmatory evidence of various pathological processes. A major goal of enzyme analysis is to localize disease processes to specific organs, preferably to specific functional subdivisions or even to specific cellular activities. Enzymes unique to a single cell type or found in only a few sites are particularly useful in this regard. The source of elevations of those enzymes with widespread distribution also can be determined by partitioning total activity into isoenzyme fractions. Isoenzymes are different forms of a single enzyme with immunologic, physical, or chemical characteristics distinctive for their tissue of origin.
Efforts to standardize the study of enzymes (enzymology) have led to new terminology for naming and measuring enzymes. The Commission on Enzymes of the International Union of Biochemistry (IUB) has classified enzymes according to their biochemical functions, assigning to each a numerical designation that embodies class, subclass, and specification number. The IUB has also assigned descriptive names according to the specific reaction catalyzed and, in many cases, a practical name useful for common reference. One result of this standardization is that enzymes that have been studied for years have been renamed according to the new terminology. For example, the liver enzyme that was formerly called glutamic-oxaloacetic transaminase (GOT) is now named aspartate aminotransferase (AST).
Another attempt to standardize enzymology is the introduction of international units (IU) for reporting enzyme activity. One IU of an enzyme is the amount that catalyzes transformation of 1 µmol of substrate per minute under defined conditions. The actual amounts vary among enzymes, and the IU is not a single universally applicable value that can be used to compare enzymes of different characteristics.26
In this section, enzymes associated with organs and tissues such as the liver, pancreas, bone, heart, and muscle are discussed. Enzymes specific to red and white blood cells are included in Chapter 1 - Hematology and Tests of Hematopoietic Function.