ALPHA FETO PROTEIN (AFP)

Clinical Significance:

Alpha feto protein (AFP) is an oncofetal protein found at a high concentration in fetal and maternal blood and also in patients with certain neoplastic and non-neoplastic disorders. The AFP derived from the fetus can be detected in the maternal serum in concentration about 100-fold less than in amniotic fluid. The association between high concentrations of AFP with open neural tube is confirmed by many researchers. AFP is a single chain sialylated glycoprotein composed of approximately 580 amino acid residues and 3-4% carbohydrates. The molecular weight of AFP is 67,000 Daltons and is a negatively charged protein with an isoelectric pH 4.57 to 5.08. This variation in charge is due to sialic acid content does not affect its antigenic property. AFP expression is regulated by transcriptional mechanisms involving a relatively large promoter and three distant enhancers. The AFP is synthesized during the G1 and S phases of cell cycle, thus it has been hypothesized that it affects cell growth. AFP binds to estrogen thus plays a role in sexual differentiation of the brain by protecting the fetus from the effects of circulating estrogen. In addition to binding estrogen, AFP, like albumin, is able to bind other steroids as well as endogenous and exogenous substances such as fatty acids, bilirubin and various pharmaceutical agents, suggesting that AFP may play a general transportation role. Moreover, because cellular internalization of the protein has been reported, AFP could also interact with cytoplasmic chaperone proteins that normally escort nuclear receptors or transcription cofactors through the cytoplasm toward organelle interfaces. AFP has also been proposed to be a protein that protects the embryo against the maternal immune system.