Alanine aminotransferase (ALT, EC 2.6.1.2), also known as glutamate-pyruvate transaminase (GPT), is a pyridoxal-5′-phosphate (PLP)-dependent enzyme that catalyzes the reversible transfer of the amino group of L-alanine to α-ketoglutarate, to yield pyruvate and L-glutamate. Because ALT is abundantly expressed in hepatocytes and has a relatively low rate of leakage into the serum with membrane damage, ALT is an important biomarker of hepatocyte damage in clinical chemistry.
Figure 1. The Krebs Cycle showing the important role of alanine aminotransferase and aspartate aminotransferase in amino acid metabolism, catalyzing the conversion of L-alanine and α-ketoglutarate into L-glutamate and pyruvate[1].
Alfa Chemistry offers high-purity alanine aminotransferase (CAS 9000-86-6) for biochemical research, diagnostic assay development, and enzymology studies.
For a complete range of product categories and technical details, please refer to our product catalog or contact our team.
This article will delve into the biochemical mechanism of ALT, its isoforms, detection techniques, applications in disease research, and the potential role of advanced ALT reagents and kits.
What Is the Biochemical Mechanism of ALT?
ALT Catalytic Cycle and Cofactor Chemistry
ALT uses pyridoxal-5′-phosphate (PLP) as an essential prosthetic group. As is typical with a ping-pong mechanism:
a. PLP (in its aldehyde form) binds to a lysine residue in the enzyme's active site, forming a Schiff base (internal aldimine).
b. L-alanine binds, and the amino group is transferred to PLP, converting it to pyridoxamine phosphate (PMP) and releasing pyruvate (first half-reaction).
c. In the second step, α-ketoglutarate binds; the amino group is transferred from PMP to α-ketoglutarate, generating L-glutamate, thereby regenerating PLP.
d. The enzyme is returned to its baseline, internal aldimine state.
This two-step transamination is fully reversible under physiological conditions (i.e., the direction depends on substrate concentration) and plays a central role in amino acid and nitrogen metabolism. Of particular note, the addition of exogenous PLP to enzymatic assays is capable of "rescuing" the apoenzyme form of the enzyme (enzyme in the absence of the cofactor) and thus increases the apparent measured activity of the total (apo + holo) enzyme.
Figure 2. Ping-Pong mechanism of transaminase reactions (usually highly enantioselective)[2].
Tissue Distribution and Isoforms of ALT
Although ALT is most abundantly expressed in the liver, it is also present at lower levels in the kidney, skeletal muscle, heart, and other tissues. In humans, there are isoenzymes encoded by different genes (e.g., ALT1 and ALT2), which are differentially expressed in the cytosolic and mitochondrial pools.
In the setting of serum leakage, the cytosolic (cALT) form predominates. In contrast, in the baseline setting, the mitochondrial form (mALT) is present but contributes little to serum ALT but may be of importance in the setting of severe mitochondrial damage.
Figure 3. Relative tissue expression of ALT, AST, and GGT isoenzymes in humans[3].
ALT in Clinical Pathology
In clinical laboratories, ALT is typically measured as part of "liver function tests" or "liver panels" to assess hepatocellular injury. Elevated serum ALT activity is interpreted as evidence of hepatocellular membrane damage, necrosis, or increased permeability.
Serum ALT concentrations are typically expressed in IU/L or U/L (units per liter). Normal reference intervals vary by laboratory, but common values range from ~7 to 55 U/L in men and ~7 to 45 U/L in women. Mild elevations may result from nonspecific causes (e.g., strenuous exercise, a high-fat diet, medications), while large elevations typically reflect acute hepatitis, toxin exposure, or severe liver injury.
Although ALT alone is sensitive, it is not very specific—it is often necessary to combine it with other markers (AST, ALP, GGT, bilirubin) and imaging or biopsy to determine the etiology.
ALT in Research Applications
In preclinical and translational research, ALT measurements are widely used for:
- Drug toxicity screening (hepatotoxicity) in animal or cell models. Liver injury models (e.g., carbon tetrachloride, acetaminophen overdose) to quantify hepatocyte leakage.
- Metabolic disease research, such as non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).
- Systems biology and metabolomics experiments requiring quantification of transaminase flux and nitrogen balance.
- Enzyme kinetics, inhibitor screening, and mutagenesis studies to investigate the catalytic function or small molecule regulation of ALT.
In these settings, stringent ALT reagents (purified enzyme, highly stable substrates, and well-validated kits) are crucial for generating accurate and reproducible data.
Figure 4. Mendelian randomization (MR) was used to investigate whether circulating ALT, AST, ALP, and GGT might influence hepatocellular carcinoma (HCC) susceptibility. The three underlying assumptions of MR (left) and the primary design of this study (right) are shown[4].
What Are the Available Methods for Detecting ALT?
Kinetic Spectrophotometry
The most common methods couples production of pyruvate from ALT with a subsequent enzymatic or chemical reaction (results in a change of absorbance) (e.g., NADH consumption in a coupled lactate dehydrogenase reaction) to allow real-time monitoring of kinetics.
- Advantages: high sensitivity, wide dynamic range, and continuous readout.
- Disadvantages: sample matrix interference, need for coupled enzyme, dependence on precise calibration.
Endpoint Colorimetric Methods
Simpler kits convert pyruvate into a colored product in a terminal reaction after a fixed incubation time.
- Advantages: ease of use, compatible with microplate readers, low cost.
- Disadvantages: narrow dynamic linear range, low temporal resolution, and endpoint artifacts.
Fluorescence/Amplification Methods
These methods rely on the production of a fluorescent substance in a coupled reaction, allowing detection in low-concentration or diluted samples.
- Advantages: high analytical sensitivity and improved signal-to-noise.
- Limitations: quenching of fluorescence by sample components, need for calibration.
ELISA/Immunoassay Detection
In some studies, ALT protein concentration (rather than activity) is measured by immunoassay. This method is useful when the enzyme is inactivated due to mutation or inhibitor binding.
- Advantages: Direct protein quantification, capable of detecting inactive enzymes.
- Limitations: Does not reflect catalytic activity; may have antigen-antibody cross-reactivity; requires high-quality antibodies.
LC-MS/Mass Spectrometry
Highly multiplexed workflows using LC-MS can be used to quantify isotopically labeled substrate (substrate tracing) or ALT reaction products in a metabolomics setting.
- Advantages: molecular specificity, multiplexing capability, and absolute quantification.
- Limitations: complexity, expense, need for stable isotope standards, and instrument availability.
Figure 5. Electrochemical detection of ALT. ALT catalyzes the transamination of alanine and α-ketoglutarate to produce pyruvate and glutamate. Since neither glutamate nor pyruvate is electrochemically active, ALT is coupled to (1) pyruvate oxidase or (2) glutamate oxidase and relies on subsequent detection of H2O2 at the modified electrode.[5].
What Special Considerations Are There When Using ALT in Research?
Purified ALT enzyme is sensitive to freeze-thaw cycles, buffer composition, and the presence of cofactors. Adding cryoprotectants (e.g., glycerol, BSA) and maintaining PLP saturation can help maintain enzyme activity.
Biological matrices (serum, cell lysates) may contain endogenous inhibitors, high salt, hemoglobin (from hemolysis), or nonspecific proteases. Proper blank/control subtraction, matrix-matched standard curves, and validated buffer conditions are essential.
As mentioned above, incomplete PLP binding can lead to enzyme "latency"; adding exogenous PLP to the assay can rescue this latency. Failure to do so will underestimate the actual potential of ALT.
- Linear Kinetics and Enzyme Concentration
It is important to ensure that the ALT reaction is measured in the linear phase and under initial rate conditions (i.e., substrate is not depleted and product is not inhibited). It is crucial to use enzyme concentrations and incubation times within the validated linear range. Isoform Identification
In some advanced studies, it may be necessary to distinguish between cytosolic and mitochondrial ALT (e.g., by immunoisolation or proteomic labeling). Saturated, generic ALT measurements may obscure isoform-specific behavior.
Alfa Chemistry is at the forefront of the pharmaceutical industry, specializing in the development and supply of active pharmaceutical ingredients (APIs). We have carefully selected a number of specialty APIs of exceptional quality. Please browse the product list below to find the product you need.
References
- Tiller NB., et al. Exercise‑induced increases in "liver function tests" in a healthy adult male: Is there a knowledge gap in primary care? Journal of Family Medicine and Primary Care. 2023, 12(1), 177.
- Noshahri NG., et al. The Promising Role of Amine Transaminase Cascades in the Synthesis of Non-Canonical Amino Acids. Processes. 2024, 12, 2566.
- Vaillant A. Transaminase Elevations during Treatment of Chronic Hepatitis B Infection: Safety Considerations and Role in Achieving Functional Cure. Viruses. 2021, 13(5), 745.
- Qin S., et al. Liver function indicators and risk of hepatocellular carcinoma: a bidirectional mendelian randomization study. Front Genet. 2022, 14, 1260352.
- Qin S., et al. Towards better diagnostic tools for liver injury in low-income and middle-income countries. BMJ Global Health. 2019, 4(4), e001704.
Related Products & Services
Please kindly note that our services are for research use only.
