For example, albumin binds bilirubin, salicylic acid, fatty acids, calcium and magnesium ions, cortisol, and some drugs. This charac- teristic is also exhibited with certain dyes, providing a method for the quantitation of albumin.
Globulins The globulin group of proteins consists of fractions. Each fraction consists of a number of different proteins with different functions. The following subsections describe selected examples of the globulins.
α 1 - A n t i t r y p s i n . α 1 - A n t i t r y p s i n i s a n a c u t e - p h a s e r e a c t a n t . I t s m a i n f u n c t i o n i s t o n e u t r a l i trypsin-like enzymes (i.e., elastase) that can cause hydrolytic damage to structural protein. α1-Antitrypsin is a major component (approximately 90%) of the fraction of serum proteins that migrates electrophoretically immediately following albumin. z e
α 1 - F e t o p r o t e i n . α 1 - F e t o p r o t e i n ( A F P ) i s s y n t h e s i z e d i n i t i a l l y b y t h e f e t a l y o l k s a c a then by the parenchymal cells of the liver. In 1956, it was first discovered in fetal serum to have an electrophoretic mobility between that of albumin and α1-globulin. It peaks in the fetus at about 13 weeks' gestation (3 mg/mL) and recedes at 34 weeks' gestation. At birth, it recedes rapidly to adult concentrations, which are normally very low. n d
The physiologic function of AFP is not well established. It has been proposed that the protein protects the fetus from immunolytic attack by its mother, modulates cell growth, transports compounds such as steroids, and is required for the functional development of the female reproductive system. AFP is detectable in the maternal blood up to month 7 or 8 of pregnancy because it is transmitted across the placenta. Therefore, measurement of the level of AFP in maternal serum is a screening test for any fetal conditions in which there is increased passage of fetal proteins into the amniotic fluid.
The normal time for screening is between 15 and 20 weeks' gestational age. The maternal AFP increases gradually during this period; therefore, interpretation requires accurate dating of the pregnancy. Maternal AFP levels are also affected by maternal weight, which reflects blood volume (inverse