Transaminases, also known as aminotransferases, are enzymes found within cells throughout the body. These enzymes serve as biological catalysts, accelerating chemical reactions fundamental to human metabolism. Their primary function involves the movement of amino groups between molecules, a crucial step for the body to process food into usable energy and build new proteins. Although normally contained within cells, their presence in the bloodstream is often measured to check for signs of cellular damage or injury. The level of these enzymes in circulation provides a window into the health of certain organs.
The Core Function of Transaminases
The fundamental biochemical process catalyzed by transaminases is called transamination, which is central to amino acid metabolism. This reaction involves transferring an amino group from an amino acid to a corresponding alpha-keto acid. This transfer results in the formation of a different amino acid and a new alpha-keto acid.
Transamination is a reversible process that allows the body to synthesize non-essential amino acids as needed. It also manages the processing of excess amino acids. By converting amino acids into alpha-keto acids, the body can redirect the carbon skeletons into energy-producing pathways, such as the citric acid cycle, or use them for glucose production through gluconeogenesis. Vitamin B6, in the form of pyridoxal-5′-phosphate, acts as a coenzyme for all these transamination reactions to occur efficiently.
Specific Types and Their Distribution in the Body
The two transaminases most frequently measured in a clinical setting are Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST). ALT is predominantly concentrated in the liver, making it a highly specific indicator of liver cell damage.
While AST is also found in the liver, it is distributed more widely across other organs. Significant concentrations of AST exist in the heart, skeletal muscle, kidneys, and red blood cells. Therefore, an elevation in AST levels alone may indicate damage to non-liver tissues, whereas a high ALT level is more reliably traced back to the liver.
Clinical Measurement and Interpretation
Transaminases are typically measured through a blood test, often included as part of a comprehensive liver function panel. The result is expressed as a concentration in the blood, measured in units per liter. Healthcare professionals compare these measured values against established reference ranges, which can vary slightly depending on the laboratory and patient factors like age and sex.
Elevations in these enzyme levels signify that cells have been injured, releasing their contents into the circulation. The magnitude of the elevation can offer clues about the severity and nature of the damage. For instance, extremely high levels, sometimes exceeding ten times the upper limit of normal, are often seen in cases of acute viral hepatitis or severe drug-induced liver injury, such as from an acetaminophen overdose.
The relationship between the two enzymes, expressed as the AST:ALT ratio, is a valuable tool for interpretation. A ratio where AST is significantly higher than ALT, often two-to-one or greater, is highly suggestive of alcohol-related liver disease. Conversely, in conditions like Non-Alcoholic Fatty Liver Disease (NAFLD) or acute viral hepatitis, the ALT level is typically higher than the AST level. This ratio helps to narrow the potential causes of the cellular injury, although it does not provide a definitive diagnosis on its own.
Common Causes of Elevated Levels
One of the most frequent causes of elevated transaminase levels is Non-Alcoholic Fatty Liver Disease (NAFLD), a condition strongly associated with obesity, diabetes, and high cholesterol. Fat accumulation within the liver cells leads to inflammation and subsequent leakage of enzymes into the bloodstream. This often presents with ALT being higher than AST, particularly in the early stages.
Alcohol consumption is another major contributor, where chronic heavy use causes direct damage to the liver cells, often resulting in the characteristic AST:ALT ratio of two-to-one or higher. Acute or chronic viral hepatitis, caused by viruses like Hepatitis B or C, also triggers significant liver inflammation and cell death, leading to high transaminase readings. Certain medications, including commonly prescribed drugs like statins and even over-the-counter pain relievers, can cause temporary or sustained enzyme elevations as a side effect.
Beyond liver injury, the broader distribution of AST means non-hepatic causes must be considered. Conditions involving severe muscle damage, such as intense exercise, trauma, or certain muscle disorders, can elevate AST levels. Less common causes include heart attacks, where AST is released from damaged heart muscle, and other systemic illnesses like celiac disease or thyroid disorders. Because elevated transaminases merely indicate cellular damage and not the specific cause, any abnormal result requires a thorough medical investigation by a healthcare professional to determine the underlying issue.