The “acidophilic” granule (AG) fraction was isolated from homogenates of bovine anterior pituitary glands by centrifugation and shown by hormone and enzyme assays to be composed of growth hormone (GH) and prolactin (PL) as the only detectable proteins, and by electronmicroscopy to be a highly purified preparation of secretory granules. Biological assays for the GH and PL content of AG agreed well with results of radioimmunoassay. By density-gradient centrifugation, larger and more dense granules containing PL and GH were separated from the smaller and lighter population containing GH almost exclusively. The density-gradient fractions containing both PL and GH consisted of a population of granules with an average diameter of 450 m whereas the granules in the fraction containing primarily GH had an average diameter of 272 mμ. The total AG fraction was solubilized and fractionated on DEAE-cellulose; the identity of apparent GH and PL monomers and two PL polymers was established by gel filtration, amino acid analysis and radioimmunoassay, in comparison with purified NIH-PL and NIH-GH. The findings indicate that (a) the “acidophilic” granules are composed largely, if not entirely, of protein hormone, (b) PL and GH are each stored in a specific granule, presumably derived from specific cells, (c) GH is stored as readily solubilized packets of 25, 000 mol wt monomer, and PL both as a monomer of about 25, 000 mol wt and an aggregate form.
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