Alkaline Phosphatase

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Abstract
The major clinical advances in the knowledge of alkaline phosphatases are reviewed. Alkaline phosphatase is not a single enzyme. Methods are now available for the differentiation of tissue alkaline phosphatases from one another and for the identification of the origin of circulating enzymes. Alkaline phosphatase in serum of patients with skeletal disorders is of bony origin. Alkaline phosphatase in serum of patients with hepatobiliary disorders is a different enzyme, probably of hepatobiliary origin. Normal sera contain enzymes from both sources. The intestine contributes 25% of serum alkaline phosphatase in some subjects. The presence of intestinal alkaline phosphatase in serum depends on the blood group, the secretor status and the state of nutrition of the individual. This enzyme probably enters the circulation by way of the thoracic duct. Serum alkaline phosphatase elevation in pregnancy is due to the presence of placental enzyme in the maternal circulation. This material is identifiable by its remarkable resistance to denaturation and by its inactivation with a specific antibody. The serum alkaline phosphatase of the fetus shares neither of these characteristics. Most of the available evidence is against the hypothesis that alkaline phosphatase is excreted by way of the biliary tract. The entire concept of the "excretion" of this group of substances is regarded as incorrect. The function of the various alkaline phosphatases remains unknown.