Isoenzyme – Definition, Function and Diagnostics
Isoenzymes are multiple forms of the same enzyme that catalyze the same chemical reaction but differ in structure, tissue distribution, and biochemical properties.
Things worth knowing about "Isoenzyme"
Isoenzymes are multiple forms of the same enzyme that catalyze the same chemical reaction but differ in structure, tissue distribution, and biochemical properties.
What Is an Isoenzyme?
An isoenzyme (also called an isozyme) is one of several structurally distinct forms of an enzyme that all catalyze the same biochemical reaction. Enzymes are protein molecules that speed up chemical reactions in the body. When multiple variants of the same enzyme exist – with slightly different amino acid sequences or subunit compositions – they are referred to as isoenzymes. They typically arise from different genes or through alternative splicing of messenger RNA (mRNA).
Characteristics and Distribution
Isoenzymes can occur in different organs, tissues, or cell types in varying amounts. Despite their functional similarity, they differ in:
- their amino acid sequence (primary protein structure)
- their kinetic properties (e.g., substrate binding affinity)
- their electrophoretic mobility (behavior in an electric field)
- their tissue distribution (organ-specific occurrence)
- their regulation by hormones or other molecules
Clinical Significance
Isoenzymes play an important role in medical diagnostics. Because certain isoenzymes are found predominantly in specific organs, their concentration in the blood can provide information about damaged tissues. Common diagnostic applications include:
Myocardial Infarction Diagnosis
Creatine kinase (CK) exists in three isoforms: CK-MM (skeletal muscle), CK-MB (cardiac muscle), and CK-BB (brain). An elevated CK-MB level in the blood is a key indicator of a heart attack, as this isoenzyme is predominantly found in heart muscle tissue.
Liver Disease
Lactate dehydrogenase (LDH) exists in five isoforms (LDH-1 through LDH-5). LDH-1 and LDH-2 predominate in the heart muscle, while LDH-4 and LDH-5 are mainly found in the liver and skeletal muscle. Determining LDH isoenzyme patterns helps distinguish liver damage from cardiac damage.
Bone Metabolism
Alkaline phosphatase (ALP) exists in several isoforms, including bone-specific, liver-specific, and intestinal variants. Elevated levels of the bone-specific isoenzyme may indicate bone disorders such as Paget's disease or bone metastases.
Diagnostic Methods
Various laboratory techniques are used to distinguish and measure isoenzymes:
- Electrophoresis: Separation of isoenzymes based on their electrical charge and size in an electric field.
- Immunological methods: Use of antibodies that specifically bind to a particular isoenzyme (e.g., immunoassays for CK-MB).
- Chromatography: Separation of isoenzymes based on their chemical properties.
- Kinetic analyses: Measurement of enzyme activity under varying conditions.
Relevance in Pharmacology
Isoenzymes are also of great importance in pharmacology. The cytochrome P450 enzymes (CYP) are a family of isoenzymes found mainly in the liver that are involved in the metabolism of drugs. Different isoforms such as CYP3A4, CYP2D6, and CYP2C9 break down different medications. Genetic variations in these isoenzymes (polymorphisms) explain why individuals respond differently to drugs – a central topic in pharmacogenetics.
Summary
Isoenzymes are structurally distinct but functionally related enzyme variants with major significance in diagnostics and pharmacology. Their tissue-specific distribution makes them valuable biomarkers for diseases of the heart, liver, bones, and other organs.
References
- Berg JM, Tymoczko JL, Stryer L: Biochemistry. W.H. Freeman and Company, 9th edition, 2019.
- Tietz NW (ed.): Fundamentals of Clinical Chemistry and Molecular Diagnostics. Saunders/Elsevier, 7th edition, 2015.
- World Health Organization (WHO): Use of Anticoagulants in Diagnostic Laboratory Investigations. WHO/DIL/LAB/99.1 Rev. 2, 2002. Available at: https://www.who.int
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