| Popis: |
Human chorionic gonadotropin is a glycoprotein hormone composed of α and β subunits, and shows a seat-belt structure three-dimensionally. The α subunit is identical to those of pituitary glycoprotein hormones, FSH, LH, and TSH. The biochemical characteristics of hCG, the amino acid sequences,carbohydrate structures, and biosynthetic pathways, were clarified in the 1970's. Recent genomic studies demonstrated that hCG is coded by multiple copies of genes that form a cluster at 19q13.33 on chromosome 19. A novel hCG molecule with a different immunoreactivity has been identified. It has the same amino acid sequences as regular hCG (R-hCG), but has a higher carbohydrate content than regular hCG, and has therefore been named hyperglycosylated hCG (H-hCG). Although R-hCG is secreted by the syncytiotrophoblast, H-hCG is produced in the cytotrophoblast, and promotes the invasion of the fertilized ovum into the endometrium during the first two weeks of gestation. H-hCG is also secreted in choriocarcinoma and is related to malignancies. By the 1980's, hCG was being measured immunologically using polyclonal antibodies. Sandwichimmunoassays with monoclonal antibodies and improved detection systems have now replaced the old immunoassay systems. The new systems have enabled the detection and discrimination of not only R-hCG and H-hCG but also various previously identified hCG-related peptides that result from degradation. However, false negative and false positive results, which are caused by increased levels of the hCG-related peptides and heterophilic antibodies, respectively, have been reported in some cases. To increase the accuracy of these analyses, specific protocols using both serum and urine as test samples should be applied to the measurement hCG levels, especially for diagnosis and treatment of choriocarcinoma. |
