An enzyme immunoassay for the quantitative determination of human growth hormone (HGH) concentration in serum.
Human Growth Hormone (hGH, somatotropin) is a polypeptide secreted by the anterior pituitary. It is 191 amino acids in length, has a molecular mass of approximately 22,000 daltons, and its metabolic effects are primary anabolic.1 hGH promotes protein conservation and is engaged in a wide range of mechanisms for protein synthesis. It also enhances
glucose transport and facilitates glycogen storage. Its cascade of growth-promoting action is mediated by another family of peptide hormones, the somatomedins.2 hGH measurement is primarily of interest in the diagnosis and treatment of various forms of abnormal growth hormone secretion. Disorders caused by hyposecretion include dwarfism and unattained growth potential. Hypersecretion is associated with gigantism and acromegaly.3 Caution must be exercised in the clinical interpretation of growth hormone levels. These vary throughout the day, making it difficult to define a normal range or to judge an individuals status based on a single determination. Many factors are known to influence the rate of growth hormone secretion, including periods of sleep and wakefulness, exercise, stress, hypoglycemia, estrogens, corticosteroids and L-dopa. Because of its similarity to prolactin and placental lactogen, earlier GH immunoassays were often plagued with falsely high values in pregnant and lactating women.2,3,6 Growth hormone-deficient individuals have fasting and resting levels similar to those found in normal individuals. Various challenge tests have therefore been devised to differentiate them. For example, with the onset of deep sleep or after 15 to 20 minutes of vigorous exercise, growth hormone levels normally rise.4 Other tests of growth hormone responsiveness are based on the administration of
L-dopa, arginine and insulin. Propanolol or estrogen are sometimes given in conjunction with the primary stimulus to accentuate the response.4,5 A small number of dwarfism cases have been documented in which both the basal level of HGH and the responses to challenge testing were normal. Such cases may involve tissue insensitivity to either growth hormone or the somatomedins, or immunoreactive but biologically inactive growth hormone.4 The Human Growth Hormone Enzyme Immunoassay provides a rapid, sensitive, and
reliable test for GH measurement. There is no cross-reactivity with hCG, TSH, LH, FSH and prolactin
The HGH ELISA is based on the principle of a solid phase enzyme-linked immunosorbent assay (ELISA). The assay system utilizes a sheep anti-HGH antibody for solid phase (microtiter wells) immobilization and a mouse monoclonal anti-HGH antibody in the antibody-enzyme (horseradish peroxidase) conjugate solution. The test sample is allowed to react simultaneously with the antibodies, resulting in HGH molecules being sandwiched between the solid phase and enzyme-linked antibodies. After a 45 minute incubation at room temperature, the wells are washed with water to remove unbound labeled antibodies. A solution of TMB is added and incubated for 20 minutes, resulting in the development of a blue color. The color development is stopped with the addition of 1N HCl, and the color is changed to yellow and measured spectrophotometrically at 450 nm. The concentration of HGH is directly proportional to the color intensity of the test sample.