What tissues contain alkaline phosphatase (ALP)?

Tissues Containing Alkaline Phosphatase (ALP)

Alkaline phosphatase (ALP) is primarily found in the liver, bone, intestine, placenta, kidneys, and in lesser amounts in leukocytes. 1, 2

Major Sources of ALP

Liver

• Liver ALP exists in multiple forms:
  • Soluble liver ALP (Sol-liver)
  • Membrane-bound liver ALP (Mem-liver)
  • Lipoprotein-bound liver ALP 3
• Elevated liver ALP is commonly associated with:
  • Obstructive biliary diseases
  • Infiltrative liver diseases
  • Cholangiocarcinoma 4

Bone

• Bone ALP exists as:
  • Soluble bone ALP (Sol-bone)
  • Anchor-bearing bone ALP (Anch-bone) 3
• Bone ALP is a marker for:
  • Bone metastasis
  • Paget's disease
  • Other bone disorders 1

Intestine

• Intestinal ALP is thought to play a role in phosphate transport into intestinal epithelial cells 2
• Soluble intestinal ALP (Sol-intestinal) is clinically relevant in liver disease assessment 3

Placenta

• Placental ALP is encoded by a specific gene locus 2
• It's primarily present during pregnancy

Minor Sources of ALP

• Kidneys
• Leukocytes 4
• Germ cells (encoded by a specific gene locus) 2

Genetic and Structural Characteristics

• ALP is encoded by at least four different gene loci:

  1. Tissue-nonspecific (liver/bone/kidney) - located on chromosome 1
  2. Intestinal - located on chromosome 2
  3. Placental - located on chromosome 2
  4. Germ-cell ALP 3, 2

• The tissue-nonspecific gene undergoes post-translational modifications (e.g., differences in carbohydrate composition) to form bone and liver ALP 3

Clinical Significance

• Normal range for ALP in adults is typically 20-200 nmol/L (5-50 mg/L), though this varies by laboratory, age, and sex 1

• Elevated ALP levels are commonly associated with:

  1. Malignancy (57%) - particularly metastatic disease to liver or bone
  2. Bone disease (29%)
  3. Parenchymal liver disease (7%)
  4. Non-malignant infiltrative liver disease (2%) 5

• An isolated elevated ALP without obvious etiology should prompt investigation for:

  • Infiltrative intrahepatic malignancy
  • Bony metastasis
  • Primary bone disorders 5

Testing Considerations

• ALP isoenzyme testing can determine the source of elevated ALP (liver, bone, or other) 1
• A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children 3
• Reference ranges should account for age and sex of the patient 3

Understanding the tissue distribution of ALP is essential for proper interpretation of elevated levels and can guide diagnostic workup in clinical practice.