The measurement of the serum levels of numerous enzymes has been shown to be of diagnostic significance as Diagnostic Enzymes. This is because the presence of these enzymes in the serum indicates that tissue or cellular damage has occurred resulting in the release of intracellular components into the blood.
Enzymes are biological catalysts responsible for supporting almost all of the chemical reactions that maintain animal homeostasis. Because of their role in maintaining life processes, the assay and pharmacological regulation of enzymes have become key elements in clinical diagnosis and therapeutics.
Hence, when a physician indicates that he/she is going to assay for liver enzymes, the purpose is to ascertain the potential for liver cell damage.
Commonly assayed enzymes are the aminotransferases:
- Alanine Transaminase (ALT): sometimes still referred to as Serum Glutamate-Pyruvate aminoTransferase, SGPT) and
- Aspartate Aminotransferase (AST): It is also referred to as Serum Glutamate-Oxaloacetate aminoTransferase, SGOT);
- Lactate dehydrogenase (LDH)
- Creatine Kinase (CK): It is also called creatine phosphokinase, CPK);
- Gamma-Glutamyl Transpeptidase (GGT)
Other enzymes are assayed under a variety of different clinical situations but they will not be covered here.
The typical liver enzymes measured are AST and ALT. ALT is particularly diagnostic of liver involvement as this enzyme is found predominantly in hepatocytes.
The measurement of LDH is especially diagnostic for myocardial infarction because this enzyme exists in five closely related, but slightly different forms (isozymes). The 5 types and their normal distribution and levels in non-disease/injury are listed below.
- LDH 1 – Found in heart and red-blood cells and is 17% – 27% of the normal serum total.
- LDH 2 – Found in heart and red-blood cells and is 27% – 37% of the normal serum total.
- LDH 3 – Found in a variety of organs and is 18% – 25% of the normal serum total.
- LDH 4 – Found in a variety of organs and is 3% – 8% of the normal serum total.
- LDH 5 – Found in liver and skeletal muscle and is 0% – 5% of the normal serum total.
Following a myocardial infarction, the serum levels of LDH rise within 24-48 hours reaching a peak by 2–3 days and return to normal in 5-10 days. Especially diagnostic is a comparison of the LDH-1/LDH-2 ratio. Normally, this ratio is less than 1.
A reversal of this ratio is referred to as a “flipped LDH”. Following an acute myocardial infarction, the flipped LDH ratio will appear in 12–24 hours and is definitely present by 48 hours in over 80% of patients. Also important is the fact that persons suffering chest pain due to angina only will not likely have altered LDH levels.
- CPK is found primarily in heart and skeletal muscle as well as the brain. Therefore, measurement of serum CPK levels is a good diagnostic for injury to these tissues. The levels of CPK will rise within 6 hours of injury and peak by around 18 hours. If the injury is not persistent the level of CK returns to normal within 2–3 days. Like LDH, there are tissue-specific isozymes of CPK and their designations are described below.
- CPK3 (CPK-MM) is the predominant isozyme in muscle and is 100% of the normal serum total.
- CPK2 (CPK-MB) accounts for about 35% of the CPK activity in cardiac muscle, but less than 5% in skeletal muscle and is 0% of the normal serum total.
- CPK1 (CPK-BB) is the characteristic isozyme in the brain and is in significant amounts in smooth muscle and is 0% of the normal serum total.
Since most of the released CPK after a myocardial infarction is CPK-MB, an increased ratio of CPK-MB to total CPK may help in the diagnosis of an acute infarction, but an increase of total CPK in itself may not.
CPK-MB levels rise 3–6 hours after a myocardial infarct and peak 12–24 hours later if no further damage occurs and returns to normal 12–48 hours after the infarct.
Some Enzymes and Related Diseases (Diagnostic Enzymes):
- Acid Phosphatase – Some Prostatic diseases
- Alanine aminotransferase (ALT) -Liver, Heart diseases
- Aldolase – Some muscle diseases
- Alkaline Phosphatase – Liver and Bone diseases
- Amylase – Pancreatic diseases
- Angiotensin-Converting Enzyme (ACE) – Active Sarcoidosis
- Aspartate aminotransferase (AST) – Heart, Liver diseases
- Cholinesterase (pseudocholinesterase) – Acute organophosphorus poisoning
- Creatine Kinase (CK or CPK) – Heart, Muscle diseases
- Gamma-Glutamyltransferase (GGT) – Liver disease, alcohol rehabilitation
- Lactate Dehydrogenase (LDH)– Heart, Liver, Brain diseases
- Lipase -Pancreatitis
- Lysozyme – Some acute leukemias
So there is more importance of enzymes in clinical diagnosis.