Transaminases are specialized proteins that act as catalysts, speeding up essential chemical reactions within the body’s cells. They are involved in metabolic processes, primarily the breakdown and synthesis of amino acids. These enzymes are normally contained within the cells of various organs, and their presence in the bloodstream is typically minimal. When a routine blood test reveals elevated transaminase levels, it signals that cellular damage has occurred, causing these enzymes to spill into the circulation. This finding is a sensitive indicator that a tissue, such as the liver, has been injured or inflamed.
The Biochemical Role of Transaminases
Transaminases are formally known as aminotransferases. Their purpose is to facilitate transamination, a biochemical reaction involving the transfer of an amino group (NH2) from an amino acid to a keto acid molecule. This process is crucial for managing the body’s nitrogen balance and recycling the building blocks of protein.
Through transamination, the body synthesizes non-essential amino acids and manages the breakdown of excess amino acids. The resulting keto acids can then enter other metabolic pathways, such as the citric acid cycle, to be converted into energy or glucose. These enzymes link protein metabolism directly to energy production, which is essential for growth, tissue repair, and maintaining metabolic function.
Specific Types and Tissue Concentration
Two transaminases are routinely measured in blood tests as markers of organ health: Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST).
Alanine Aminotransferase (ALT)
ALT, also referred to as SGPT, is found in the highest concentration within the cytosol of liver cells (hepatocytes). Because of this specific location, an elevation in ALT levels is generally considered a sensitive indicator of liver injury.
Aspartate Aminotransferase (AST)
AST, historically known as SGOT, is present in the liver but also in significant amounts in the heart, skeletal muscle, kidneys, and brain. This broader tissue distribution means that while elevated AST can point to liver damage, it is a less specific indicator than ALT. The different location profiles of these two enzymes allow them to be used together to help pinpoint the source of a health issue.
Interpreting Enzyme Levels in Health Screening
Measuring transaminase levels in the blood is standard practice because it provides indirect evidence of cellular damage. In a healthy state, transaminases are primarily confined within the cell membranes of organs like the liver, and only very small amounts are detectable in the serum. When cells are damaged or die due to inflammation, disease, or injury, their membranes become compromised and leak their contents into the bloodstream.
The magnitude of the enzyme elevation often relates to the severity of the cellular damage. Very high levels, sometimes exceeding ten times the normal upper limit, typically suggest acute and severe injury, such as from acute viral hepatitis or toxin exposure.
Clinicians also examine the relationship between the two enzymes by calculating the AST:ALT ratio, sometimes called the De Ritis ratio, to gain a diagnostic clue. A ratio where ALT is higher than AST (a ratio less than 1) is frequently seen in many forms of chronic liver disease, including non-alcoholic fatty liver disease (NAFLD). Conversely, an AST:ALT ratio of 2:1 or greater is highly suggestive of alcoholic liver disease, a condition where the enzyme mechanism favors the release of AST over ALT. If AST is elevated but ALT remains near normal, it can suggest the source of the enzymes is outside the liver, often indicating an issue with muscle tissue.
Common Factors Leading to Elevated Levels
The causes of elevated transaminase levels can be broadly categorized into those originating in the liver (hepatic) and those originating in other organs (non-hepatic).
Hepatic Causes
The most common cause of mildly elevated levels is Non-Alcoholic Fatty Liver Disease (NAFLD), often linked to obesity and metabolic syndrome. This condition, characterized by fat accumulation in the liver, can lead to inflammation, causing the enzymes to leak.
Other significant hepatic causes include chronic viral infections (Hepatitis B and C), which cause ongoing inflammation and destruction of liver cells. Excessive alcohol consumption leads to alcoholic liver disease, ranging from simple fatty liver to severe alcoholic hepatitis and cirrhosis. In cases of advanced liver scarring (cirrhosis), transaminase levels may be only slightly elevated or near normal, as there are fewer healthy cells left to be damaged.
Less common causes include hereditary hemochromatosis (iron overload) and Wilson disease (copper overload), which cause progressive liver damage.
Non-Hepatic Causes
Non-hepatic factors are important to consider, especially when the AST level is disproportionately high compared to ALT. Strenuous physical activity, such as intense weightlifting or long-distance running, can cause temporary elevation of AST due to muscle fiber breakdown. Certain muscle disorders, such as polymyositis or dermatomyositis, also cause muscle cell damage that releases AST into the bloodstream.
The use of certain medications and supplements is a frequent cause of transaminase elevation, known as drug-induced liver injury. Common examples include:
- High doses of over-the-counter pain relievers, like acetaminophen, which can be toxic to the liver.
- Prescription drugs, such as statins used to manage cholesterol.
- Some anti-seizure medications.