In practice, clinicians very often have to deal with an unexpected increase in alkaline phosphatase activity. Thanks to the use of multidisciplinary test kits for biochemical studies, it became possible to control the activity of this enzyme. By the level of activity of this enzyme, diseases of the hepatobiliary system and bone tissue can be detected.
Alkaline phosphatase is an enzyme that belongs to the class of hydrolases that cleaves a phosphate group from various biopolymers (proteins, nucleic acids, alkaloids). The highest enzyme activity is observed in an alkaline environment. The enzyme is in cells in a state bound to plasma membranes; therefore, it refers to membrane-bound enzymes. Alkaline phosphatase (ALP) consists of various isoenzymes that are located in the epithelium of the bile ducts, plasma membranes of hepatocytes and neurons, bones, intestines, placenta, kidneys.
Determine the activity of isoenzymes using electrophoresis. However, alkaline phosphatase isoenzymes do not have a clear organ specificity, and the electrophoretic technique is very complex. Therefore, to simplify research, a technique is used that is based on different sensitivity of isoenzymes to heat. Intestinal alkaline phosphatase belongs to the thermostable fraction, and isoenzymes from the liver and bone tissue belong to the thermolabile one. The most sensitive to heat is bone phosphatase.
Serum alkaline phosphatase is a mixture of bone and liver isoenzymes. The intake of the enzyme from different tissues into the blood depends on the physiological state and age of the body (pregnancy, childbirth, lactation intensity). The high activity of alkaline phosphatase in the blood of young people is explained by the intense functioning of osteoblasts in the bone tissue, which are caused by the processes of active growth of the body. During this period, the enzyme activity in the blood increases due to the bone isoenzyme. In the second half of pregnancy and during childbirth, an active increase and destruction of the placenta causes an increase in the concentration of placental isoenzyme.
An increase in the concentration of alkaline phosphatase in the blood is most often recorded in the pathology of the liver and bone tissue. Damage to the liver parenchyma causes a slight increase in the activity of the enzyme in the blood serum, because alkaline phosphatase is associated with cell membranes. In clinical hepatology, the hepatic isoenzyme is indicative of the diagnosis of cholestasis. This is due to increased synthesis of alkaline phosphatase by bile duct cells and impaired secretion of the enzyme in bile. Hyperfermentemia is especially high during the development of the pathological process and stasis of bile in the hepatic bile ducts. Then the enzymatic activity of alkaline phosphatase in blood serum increases tenfold. In case of damage to the intrahepatic bile ducts, the enzyme activity in the blood increases only 2-3 times. Norms of alkaline phosphatase for adults under 31 years old are 39.0–92.0 IU / L, after 31 years - 39.0–117.0 IU / L. Alkaline phosphatase is normal in children under 10 years old - 106.0-213.0 IU / l, up to 3 years old - 71.0-213.0 IU / l, and for newborns - 35.0-106.0 IU / l.
In bone pathology, when there is an increased activity of osteoclasts, during the development of rickets, osteodystorophia, hyperparathyroidism in the blood, the activity of bone alkaline phosphatase increases. In this case, the activity of total alkaline phosphatase in the blood increases by 3-10 times. For an early diagnosis of rickets and osteodystrophy, an increase in the activity of phosphatase in the synovial fluid is specific . An increase in alkaline phosphatase activity in the cerebrospinal fluid is a sign of damage to the membranes of neurons, where it is localized.