Monoclonal antibodies specific for the free alpha subunit of glycoprotein hormones and their use in the development of a sensitive immunofluorometric assay.

Glycoprotein hormone free alpha subunit has been used as a marker for some pituitary tumors and to study the reactivity of glycoprotein hormone-producing cells under different circumstances. We describe a highly sensitive and specific immunofluorometric assay for the measurement of serum free alpha subunit levels. The assay is based on a monoclonal antibody, specific for free alpha subunit, bound to microtiter plates. As tracer antibody we employed an europium-labelled free/complexed alpha subunit specific monoclonal antibody. Using overnight incubation and 50-microliters samples, the least detectable dose was of the order of 4 ng/l. Cross-reactivity with LH, TSH, FSH and hCG was 6.5, 1.2, 4.3 and 1.1%, respectively. Normal adult males showed values ranging from 120 to 790 ng/l, not different from normal adult premenopausal females (88 to 604 ng/l). In post-menopausal females, serum concentrations were significantly higher, ranging from 341 to 4071 ng/l. In 56 patients with untreated pituitary tumors (18 "non-secreting", 25 GH-producing and 13 prolactin-producing tumors), 10 showed high values, 3 of them from the first group, 3 from the second and 4 from the third. We conclude that this highly sensitive assay can be a valuable tool for the diagnosis and follow-up of selected patients with pituitary tumors and in other circumstances in which the glycoprotein hormone-producing cells of the pituitary require evaluation.