22-KD GROWTH-HORMONE EXCLUSION ASSAY - A NEW APPROACH TO MEASUREMENT OF NON-22-KD GROWTH-HORMONE ISOFORMS IN HUMAN BLOOD

Human growth hormone (GH) exists in a variety of isoforms. In the pitu itary, the most abundant isoform is 22-kD GH (22 K GH), while other is oforms (non-22 K GH) are present in variable amounts. In human plasma, the GH heterogeneity contributes to the wide variability in GH levels measured by different immunoassays. The physiological role of the non -22 K GH isoforms is poorly understood, but they may represent a spect rum of agonists or antagonists of the GH receptor. It is possible that increased amounts of non-22K GH isoforms in the circulation contribut e to the growth failure observed in some short children and may be inv olved in the pathophysiology of acromegaly and other unrelated disease s. Currently, there is no method available to evaluate the ratio of no n-22 K GH isoforms to total GH in large sets of serum samples. In this report, a novel assay procedure is described in which monomeric and d imeric isoforms of 22 K GH are removed from serum and non-22 K GH isof orms are quantitated. The 22 K GH exclusion assay (22 K GHEA) was esta blished as a screening method to identify conditions in which the rati o of non-22 It GH isoforms to total GH in human blood is altered. A 22 K GH-specific monoclonal antibody (MCB) is used for binding to 22 K G H in serum. Magnetic beads coated with rat anti-mouse immunoglobulin G and a magnetic device are used to remove the 22 K GH-MCB complexes fr om serum. The non-22 K GH isoforms are measured by a polyclonal antibo dy-based immunoradiometric assay (GH-IRMA). The assay procedure was op timized systematically by statistical experimental designs. In serum s piked with monomeric or dimeric 22 K GH, the 22 K GH extraction was ef ficient at GH levels up to 100 mu g/l (range 96.3-100%). The intra- an d interassay precision for non-22 K GH levels of 3.9 mu g/l were 2.6% and 8.7%, respectively, while for levels of 0.6 mu g/l, which were ver y close to the detection limits of the assay, the coefficients were 17 .0% and 21.6%, respectively. The percentage of non-22K GH isoforms det ermined in serum samples from three different groups of subjects showe d clearly distinctive values. The 22 K GHEA is a new method for evalua tion of non-22 K GH isoforms in human blood under different physiologi cal and pathophysiological conditions.