Aldolase

Aldolase is an enzyme involved in synthesizing glucose and breaking it down into products of energy. Elevated serum aldolase levels may indicate damage to different organ systems.

Reference ranges for aldolase are as follows ^[1] :

• Adult: 3-8.2 Sibley-Lehninger units/dL or 22-59 mU/L at 37°C (SI units)
• Child: Approximately two times the adult values
• Newborn: Approximately four times the adult values

An aldolase blood test is used to help identify injury to organs such as muscle, liver, kidney, or heart. There are three subclasses of aldolase; A, B, and C. Aldolase A is expressed in muscle, erythrocytes, and the brain; aldolase B is expressed in the liver, kidneys, and enterocytes; aldolase C is expressed in the brain.

Aldolase levels are measured in units per liter and range from 1.0-16. There are slight variations in reference ranges between men and women based on height, weight, muscle mass and methods used in laboratories.

Elevated aldolase levels may be seen in conditions such as damage to the skeletal muscle (trauma), dermatomyositis, polymyositis, infectious mononucleosis, muscular dystrophy, myocardial infarction, nonspherocytic hemolytic anemia, viral / auto-immune hepatitis or hepatic cancer, colon cancer,  pancreatic cancer, prostate cancer, and osteosarcoma. ^[2]

Interpreting results requires understanding of the subclasses of aldolase. When damage occurs to aldolase containing cells, aldolase is released into the bloodstream at high levels. For certain diseases, monitoring aldolase can help direct treatment therapy. For example, aldolase A is contained in muscle tissue and used to monitor the course of muscular dystrophy.

Measurement of aldolase A can also help differentiate between muscle versus neurological myopathy. If aldolase A is elevated, the primary cause of myopathy may be related to an inflammatory state of the muscle. If aldolase A is not elevated, the pathology may be related to a secondary cause. An example is a patient with a history of multiple sclerosis who presents with muscle weakness and has normal aldolase A levels.

Aldolase B levels are elevated in liver disease or heart disease (myocardial infarction). This particular marker is not widely used because other blood tests are more specific for liver and heart-related diseases (LFT’s and Cardiac enzymes).

Serum aldolase

Serum aldolase collection details are as follows:

• Specimen: Blood

• Container: Red-top tube (plain, no gel tube).

• Collection method: Routine venipuncture, 3.0 mL of blood (minimum of 0.2 mL for processing)

Fasting for at least 8 hours and avoiding strenuous activity are important in order to obtain accurate results; not fasting or strenuous activity may lead to muscle breakdown and falsely elevate aldolase levels, making it uninterruptible. Certain medications may also falsely elevate aldolase levels (statin induced myopathy or steroid induced myopathy). Consideration must be taken to cease medications if accurate test results are to be obtained.

The test measures total enzyme activity and is not specific for Aldolase A, B or C.

The specimen is collected in a red-top (plain, no gel) tube. A minimum of 0.2 mL must be present in the sample. Hemolyzed specimens and serum gel tubes unacceptable samples. They must be frozen and forwarded promptly to processing centers where they go under special processing through centrifuge, removing serum aliquot.

Aldolase is indirectly measured from a product using a substance that will react with the aldolase enzyme within the collected sample. The measurement of the end product determines the level of aldolase contained in the patient’s serum.

This test is ordered separate from all other blood panels.

Description

Aldolase is a cytoplasmic enzyme involved in glucose and fructose metabolism. It specifically catalyzes the reversible reaction of converting fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehydes 3-phosphate.

The enzyme works on six reversible reactions in gluconeogensis and glycolysis. In gluconeogenesis, aldolase catalyzes the reduction of phosphoenolpyruvate to fructose 1,6-bisphosphate. In glycolysis, aldolase catalyzes fructose 1,6-bisphosphate to phosphoenolpyruvate through an oxidative reaction. ^{[3, 4]} Aldolase also reversibly cleaves fructose 1-phosphate to glyceraldehyde and dihydroxyacetone phosphate. ^[5] Subgroup B works in both gluconeogenesis and glycolysis whereas subgroups A and C focus primary on glycolysis.

Aldolase A is primary contained in the muscle and erythrocytes. The liver, enterocytes and kidney contain aldolase B and the brain contains both aldolase A and C.

Indications/Applications

Cancer cells are known to upregulate glycolysis to adapt to the increased demand for the production of energy. In a recent study where aldolase A expression was investigated, it was found that the mRNA expression level of aldolase A was higher in human colon cancer tissues. High aldolse A expression was also related to colon cancer progression and metastasis. ^[6]

Hereditary fructose intolerance (HFI) is a genetic disease characterized as a defect in aldolase B enzyme, causing improper processing of fructose-1-phosphate (F1P), leading to a toxic buildup of F1P in bodily tissue. ^[7] In addition to toxicity in tissue, F1P traps phosphate in an unusable form that cannot generate a phosphate pool. This results in a depletion of phosphate and lower ATP stores. The lack of available phosphate causes cessation of glycogenolysis in the liver and results in hypoglycemia. ^[5]

The disease is characterized as autosomal recessive with mutations occurring on A149P allele (approximated 53% of the time). ^{[7, 8]}

The accumulation also affects gluconeogenesis, further reducing available glucose. The loss of ATP leads to inhibition of protein synthesis, hepatic, and renal dysfunction. Testing for aldolase B may help guide for diagnosis. By avoiding foods containing sucrose, sorbitol and fructose, patients can live symptom free. ^[5]

Considerations

Aldolase levels vary with age. Newborns start with higher levels and trend down towards adulthood. Normal values can vary depending on how the lab conducts testing. Inappropriate fasting time (less than 8 hours) and medications may give false positive results.

Because of the introduction of liver function tests and cardiac enzyme panel, aldolase is no longer routinely ordered to assess liver or heart function. Aldolase A levels may be useful to determine primary pathology between muscular versus neurological manifestations of myopathy.