ENHANCEMENT OF GROWTH-HORMONE BIOACTIVITY BY ZINC IN THE ELUTED STAINASSAY SYSTEM

The effects of ionic zinc (Zn2+) on human (h) GH bioactivity have been examined using a lactogenic bioassay. The potencies of pituitary-deri ved hGH (IRP 80/505), recombinant 22K hGH (IRP 88/624), Pituitary-deri ved human PRL (IRP 84/500), and a recombinant methionyl 20-kilodalton variant of hGH in the presence of selected concentrations of ZnCl2 wer e investigated with an eluted stain assay that uses Nb2 rat lymphoma c ells. This precise colorimetric bioassay is based upon the reduction o f a yellow tetrazolium salt, ,5-dimethyl-thiazol-2-yl]2,5-di-phenyl-te trazolium bromide, to its purple formazan by lactogen-activated Nb2 ce lls. Zinc (6-100 mum) enhanced the bioactivity of low doses (< 0.045 n M) of both pituitary-derived and recombinant 22K hGH, although the mag nitude of enhancement was considerably less than might have been antic ipated from previous binding studies (13). Higher concentrations of pi tuitary-derived hGH (> 0.045 nM) were inhibited by Zn2+. The bioactivi ty of recombinant methionyl 20K hGH was greatly enhanced by zinc (3-10 0 muM). In contrast to hGH, the bioactivity of hPRL was not potentiate d by Zn2+. These discriminatory effects of Zn2+ when stimulating via t he lactogenic receptor are in concordance with the results of previous radioligand binding studies (13). The striking enhancement of 20K hGH lactogenic bioactivity was observed at Zn2+ concentrations within the physiological range for normal human serum (5-20 mum).