Alkaline Phosphatase (ALP) Overview

Alkaline Phosphatase (ALP) Overview

• ALP is a hydrolase enzyme.
• It removes phosphate groups from molecules like nucleotides, proteins, and alkaloids in a process called dephosphorylation.

Location

• ALP exists in several tissues including the liver, small intestine, bones, kidney, and placenta.
• Highest concentrations are in cells of the bone and liver.

ALP in the Liver

• In the liver, ALP can be found on the edges of cells that form bile ducts: small tubes that drain bile to the bowels, aiding in fat digestion.

ALP in Bone

• ALP in bone is produced by osteoblasts, which are special cells involved in bone formation.

When to Order an ALP Test

• An ALP test can be part of routine lab testing, often included in a liver function test.
• It may also be ordered with other tests when someone has liver or bone disorder symptoms.
• Liver disorder symptoms can include weakness, fatigue, loss of appetite, nausea, vomiting, abdominal pain or swelling, jaundice, dark urine, light-colored stool, and itching (pruritus).

Factors Affecting ALP Levels

• Conditions affecting bone growth or increasing bone cell activity can change ALP blood levels.
• Children and adolescents have higher ALP levels than adults due to bone growth.
• ALP is often very high during growth spurts in both boys and girls.

Clinical Significance

• ALP is used to detect liver disease, bone disorders, biliary obstruction, and cancer.

Elevated ALP Enzyme Conditions

• Biliary obstruction (blocked bile duct): ALP is high in the edges of cells that form bile ducts in this case.
• Osteoblastic bone tumors: Detects cancers that have spread to bones.
• Osteomalacia: Bone softening in adults, typically from prolonged vitamin D deficiency.
• Hepatitis: Damaged liver cells release more ALP into the blood.
• Leukemia: Cancer of WBCs, starts in the bone marrow.
• Lymphoma: Cancer that starts in the cells of the lymph system.
• Paget's Disease: Diagnoses Paget's disease, which causes malformed bones.
• Sarcoidosis: Immune system overreaction leads to clusters of inflamed tissue (granulomas) in organs.
• Hyperthyroidism: Too much thyroid hormone (T3 and T4) causes increased ALP, with excess T3 inducing apoptosis of hepatocytes and liver dysfunction.
• Hyperparathyroidism: An excess of parathyroid hormone (PTH) from the parathyroid gland can elevates ALP due to high bone resorption.
• Myocardial infarction: Increased ALP levels are related to promoting vascular calcification in the case of a myocardial infarction, as the ALP enzyme inhibits vascular calcification.
• Pregnancy: Increased ALP from placental release and fetal bone growth.

Lowered ALP Enzyme Conditions

• Low ALP levels may occur after blood transfusions or heart bypass surgery.
• Zinc and magnesium deficiencies, as seen in hypothyroidism, can lower ALP levels.
• Hypophosphatasia is a rare genetic bone metabolism disorder that causes low ALP levels.
• Malnutrition, protein deficiency, and Wilson's disease are possible causes.
• Some drugs, like oral contraceptives, may decrease ALP levels, while anti-epileptics may increase them.
• The normal ALP range for adults is 32-92 IU/L.

Principle of ALP Test

• The colorimetric determination of ALP activity involves a reaction where phenylphosphate converts to phenol and phosphate.
• Free phenol from hydrolysis then reacts to form a red-colored complex, measured at 510 nm, directly proportional to ALP activity.
• Sodium arsenate is also used within this test.