Alkaline Phosphates (ALP) : Level, Quantification principle, Isoenzyme & Clinical Symptoms

 Alkaline phosphates enzyme plays an important role in the growth and development of bones and teeth. It is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules.

  It is produced by osteoblasts of the bone and is associated with calcification process. It is localized in cell membranes and hence an ecto-enzyme.

  Alkaline phosphatase hydrolase enzyme catalyzes hydrolytic removal of phosphate group from mono-phosphoric esters including glycerophosphates, phenyl-phosphates, nucleotides, and proteins at alkaline pH (9 and 10) as its name indicates.
It is activated by magnesium (Mg++) and manganese(Mn++).

  Alkaline phosphatase (ALP) is an enzyme present in the canalicular and sinusoidal membranes of the liver and also active in many other tissues, particularly in bone, kidney, intestine and placenta. It is useful diagnostic test in bone and liver disorders.

Principle of Quantitative Analysis of ALP

  The principle for quantitative analysis of ALP in lab is Alkaline phosphatase in serum acts on the substrate paranitrophenyl phosphate buffered at pH 10 at 37°C to liberate paranitrophenol which gives yellow colour complex and this is measured at 405 nm.

Isoenzymes of Alkaline Phosphates

  It exists in six forms of isoenzymes. The isoenzymes are due to variation in carbohydrate content (sialic acid content), so they can also be called as isoforms. Affinity electrophoresis using polyacrylamide gel is used in separation and identification of fraction of ALP isoenzymes to differentiate liver or bone disorders.

The six isoenzymes of ALP are:

1. Alpha-1ALP:
  •  Alpha-1 ALP is 10% of total ALP.
  •  It moves in alpha 1 position in electrophoresis.
  • It is synthesized by epithelial cells of biliary canaliculi and increased in obstructive jaundice, biliary cirrhosis and to some extent in metastatic carcinoma of liver.

2. Alpha-2, heat labile ALP:

  • It is 25% of total ALP. 
  • It is alpha 2 heat labile, stable at 56°C but loses its activity when kept at 65°C for 30 min.
  • It is synthesized by hepatic cells and increased in hepatitis.

3. Alpha-2, heat stable ALP:

  • It is only 1% of total ALP. It is also alpha 2 but heat stable isoenzyme.
  • It is not destroyed at 65°C but inhibited by phenylalanine.
  • It is of placental origin and found in blood in normal pregnancy.
  • It increases in 2nd and 3rd trimester of normal pregnancy and its decrease indicates placental insufficiency and foetal death.

4. Pre-Beta ALP:

  • It constitutes 50% of total ALP.
  • It is pre-beta ALP and is most heat labile, loses its activity at 56°C within 10 min.
  • It originates from the bone and is increased in rickets and Paget's disease.

5. Gamma ALP:

  • It is 10% of total ALP.
  • It is gamma ALP and is inhibited by phenylalanine.
  • It is synthesized by intestinal cells and is increased in ulcerative colitis and after gastrectomy surgery.

6. Leukocyte ALP:

  • It constitutes 4% of total ALP.
  • It originates from leukocytes.
  • It is increased in lymphomas and significantly decreased in chronic myeloid leukaemia.

Abnormal ALP Isoenzymes

Regan isoenzyme (carcino-placental isoenzyme):
  • It is named after the first patient in whom it was detected.
  • It is an abnormal ALP isoenzyme closely resembling placental ALP isoenzyme.
  • It is seen in about 15% cases of carcinoma of lung, liver and gut. Chronic heavy smoking also increases regan isoenzyme in blood.

Nagao isoenzyme:

  • It is an abnormal ALP isoenzyme present in carcinomas and metastasis.

Normal Levels of ALP

  • Normal level of ALP in Adult is 40-125 IU/L.
  • Normal level of ALP in Children is 42-362 IU/L.
  • In children, ALP levels are more because of the increased osteoblastic activity in children.

Clinical Significance of Alkaline Phosphates

  • Moderate (2-3 times) increase in ALP level is seen in hepatic diseases such as infective hepatitis, alcoholic hepatitis or hepatocellular carcinoma.
  • Very high levels of ALP (10-12 times of upper limit) may be noticed in extrahepatic obstruction (obstructive jaundice) caused by gallstones or by pressure on bile duct by carcinoma of head of pancreas.
  • Intrahepatic cholestasis may be due to virus (infective hepatitis) or by drugs (chlorpromazine).
  • ALP is produced by epithelial cells of biliary canaliculi and obstruction of bile with consequent irritation of epithelial cells leads to secretion of ALP into serum.
  • Drastically high levels of ALP (10-25 times of upper limit) are seen in bone diseases where osteoblastic activity is enhanced such as Paget's disease (osteitis deformans), rickets, osteomalacia, osteoblastoma, metastatic carcinoma of bone and hyperparathyroidism.

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