ALKALINE PHOSPHATASE This is a widely distributed enzyme which releases inorganic phosphate from many organic phosphomonoesters and also pyrophosphates. The enzyme exhibits optimum activity at PH 9 – 10. The form present in the sera of normal adults originates mainly in the liver or biliary tract and bone (osteoblasts). The respective contribution of these two forms is markedly age-dependent with a high alkaline phosphate (bone) occurring during childhood and adolescence. Minor contribution also comes from intestine, placenta (during pregnancy) and rarely renal tissues.

Clinical significance Serum alkaline phosphatase is of particular interest in the investigation of two groups of conditions, hepatobiliary diseases and bone diseases associated with increased osteoblastic activity. The elevation tends to be more marked (more than three folds) in extrahepatic obstruction (e.g. by stone or by cancer of the head of pancreas) than intrahepatic. Liver diseases that principally affect parenchymal cells such as hepatitis, typically has normal or moderately elevated serum alkaline phosphatase activity, the degree of elevation is usually less than three folds and depends on the degree of biliary stasis.

Among bone diseases the highest level of serum alkaline phosphatase activity is encountered in paget’s disease. Moderate rises are observed in osteomalacia and rickets (with its activity tends to decrease following Vit. D therapy), primary and secondary hyperparathyroidism with skeletal involvement, very high enzyme activity is also present in patients with osteogenic bone cancer including metastases. An increase in alkaline phosphatase activity may be observed in women in the third trimester of pregnancy with additional enzyme being of placental origin. Serum alkaline phosphatase should, therefore, be interpreted with caution in pregnant women. Normal serum level of AP is 20-90 I.U/L (3-13) king. Armstrong units/d (K.A.U./dl) in adults.

Measurement of Serum Alkaline Phosphates Activity Principle The phenol released by enzymatic hydrolysis from phenyl phosphate substrate at a constant condition of time, temp. and PH is estimated colorimetrically. Phenyl phosphate ---ALP--------- phenol + phosphate The phenol liberated is measured in the presence of amino-4- antipyrine and potassium ferricyanide. The presence of sodium arsenate in the reagent stops the enzymatic reaction.