Pituitary growth hormone (GH) filtered on Sephadex G-200 is heterogeneous: “large” growth hormone (LGH) elutes with the void volume, small growth hormone (SGH) migrates identically with purified GH, and at least two intermediate sized forms can be identified. Male rat anterior pituitaries were incubated with 14C and 3H-labeled amino acids or nucleosides, extracted, and the 12, 000 × g supernate was filtered over Sephadex G-200. LGH, SGH, and intermediate forms were identified in pulse labeled pituitary glands. LGH radioactivity decreased from 45$ to 15% during the first 35 min of the chase period and was stablethereafter. The percent of immunoprecipi table radioactivity migrating as SGH increased during the chase period until immunoprecipitable (IP) GH radioactivity began to appear in the incubation medium. SGH radioactivity then decreased in complementary fashion. During continuous exposure to radiolabeled amino acids IP-LGH decreased from an initial value of 50-80$ to 32-36$ by 40 min and was then stable. Percent pituitary SGH was complementary, and IP-GH radioactivity progressively accumulated in incubation medium. The ratio of IP 3H/14C in SGH from pituitaries exposed simultaneously to 3H-serine and 14C-leucine changed with time according to a pattern predicted using the amino acid sequence of human GH. The calculation assumes that the firstGH molecules to appear in the SGH peak have only their carboxy terminal portions synthesized inthe presence of isotope, and that progressively more extensively labeled molecules accumulate over time. The ratio eventually stabilizes and is characteristic of the serine-leucine content of the entire GH molecule. 3H-uridine was incorporated almost exclusively into IP-LGH. Ribonuclease treatment of chromatographically isolated LGH, labeled with 3H-uridine and 14C-leucine, released IP-GH which was retarded by Sephadex and labeled with 14C alone. We conclude that LGH is the earliest detectable biosynthetic form of pituitary GH. It is associated with RNA and may represent the GH polysome. Transfer of IP isotope from LGH to SGH and incubation medium identifies the sequential relationship of these pituitary GH forms during synthesis and secretion.
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