Serum enzymes in the diagnosis of disease in man and animals

Abstract

The clinical pathologist is concerned with aiding the diagnosis of disease by investigating the disease process in the patient. These investigations seek to identify the pathogen (e.g., toxin, virus, bacterium or parasite) or to detect diagnostic pathological changes by haematology, cytology and biochemistry. Clinical enzymology is a branch of clinical biochemistry which has grown rapidly in the last 50 years. This rapid growth has been associated with the development of methods of enzyme assay. The molecular concentrations of enzymes in serum are very much lower than those of most other metabolites of clinical importance. Only special radioimmunoassay techniques are sufficiently sensitive for the direct determination of serum enzyme protein concentration. Fortunately, enzymes can be determined indirectly by measuring their activity, i.e. their ability to accelerate the particular chemical reactions which they catalyse. Thus the activity of an enzyme may be determined by incubation with its substrate and measurement of loss of substrate, formation of product or change in a coenzyme. Activity varies with temperature, pH, substrate concentrations and the presence of activators or inhibitors, so that enzyme assay conditions must be carefully controlled. It is desirable to saturate the enzyme with substrate to ensure zero-order kinetics during the assay. There is considerable variation in the stability and assay requirements of different enzymes. Enzymes which are very unstable or difficult to assay are of limited value in diagnostic investigations. Practical instructions for many enzyme assays are provided by Bergmeyer ( 1983). Early investigators used a variety of units of enzyme activity. In 1961 the Commission on Enzymes of the International Union of Biochemistry (IUB) recommended a standard international unit (IU) as that amount of enzyme which will catalyse the transformation of 1 micromole of substrate per minute under standard conditions. It is convenient to perform serum enzyme assays at 30°C and to express the results in IU per litre. The newer SI System recommends the “katal” (kat) as the unit of enzyme activity. This is the amount of activity which transforms 1 mole of substrate per second. Thus 1 p kat is equivalent to 60 IU. The enzyme names and numbers recommended