Distribution of 3H-acetyl human growth hormone in subcellular fractions of rat liver

Abstract

3H-Acetyl human growth hormone ( 3H-HGH) (∼60 μg) was injected intravenously into hypophysectomized rats. Radioactivity appeared in all the subcellular fractions of liver as early as 2 minutes after the injection. Until 10 minutes postinjection virtually all of the tissue radioactivity was precipitable with trichloroacetic acid, whereas at 20 and 40 minutes after injection all of the radioactivity was soluble in trichloroacetic acid. Liver uptake (cpm/mg protein) peaked at about 20 minutes postinjection. The appearance of radioactivity in the crude nuclear fraction closely followed that of the whole homogenate. Radioactivity in the microsomal and mitochlondrial fractions was highest at the early time periods (up to 10 minutes) and then declined progressively. The radioactivity in the final supernatant or cytoplasmic fraction increased slowly, peaked at 20 minutes, and remained the highest. Pretreatment with unlabeled or 14C-labeled HGH significantly reduced the mitochondrial and microsomal uptake of 3H-HGH. In contrast with these results, when 3H-HGH was added in vitro just before homogenization and fractionation of liver, most of the radioactivity (77%) appeared in the cytoplasmic fraction. These results are interpreted to mean that the appearance of 3H-HGH in the particulate fractions at these early time periods may have functional significance, and that important initial binding sites of growth hormone in the liver cell may be mitochondria and microsomes.