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This protein reaches a maximum serum concentration at the age of 2-4 years and then decreases to about onethird of that concentration at about age 45 years. Later in life, a moderate increase is seen. This change with age is more pronounced in males than in females. There is a distinct difference in the reference values for males and females, adult females having higher values than males.

α2-Macroglobulin inhibits proteases such as trypsin, pepsin, and plasmin. It also contributes more than one fourth of the thrombin inhibition normally present in the blood.

Transferrin (Siderophilin). Transferrin, a glycoprotein, is synthesized primarily by the liver. Two molecules of ferric iron can bind to each molecule of transferrin. Normally, only about 33% of the iron-binding sites on transferrin are occupied. Transferrin is the major component of the β-globulin fraction and appears as a distinct band on high-resolution serum protein electrophoresis.

The major functions of transferrin are the transport of iron and the prevention of loss of iron through the kidney. Its binding of iron prevents iron deposition in the tissue during temporary increases in absorbed iron or free iron. Transferrin transports iron to its storage sites, where it is incorporated into apoferritin, another protein, to form ferritin. Transferrin also carries iron to cells, such as bone marrow, that synthesize hemoglobin and other iron-containing compounds.

Hemopexin. The parenchymal cells of the liver synthesize hemopexin, which migrates electrophoretically in the β-globulin region. Hemopexin can be determined by radial immunodiffusion. The function of hemopexin is to remove circulating heme. When free heme (ferroprotoporphyrin IX) is formed during the breakdown of hemoglobin, myoglobin, or catalase, it binds to hemopexin in a 1:1 ratio. The heme-hemopexin complex is carried to the liver, where the complex is destroyed. Hemopexin also removes ferriheme and porphyrins.

The level of hemopexin is very low at birth but reaches adult values within the first year of life. Pregnant mothers have increased plasma hemopexin levels.

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