Rat pituitary glands were incubated in vitro in the presence of pairs of amino acids labelled with either 3H or 14C in order to compare the relative ratios of leucine, lysine, alanine and serine incorporated into large growth hormone (LGH) with that into small growth hormone (SGH). After separating the two growth hormone moieties on a Sephadex G-200 column, label incorporation was assessed by immunoprecipitation. LGH isotope incorporation ratios differed from those of SGH isotope incorporation except when two different isotopic forms of the same amino acid were used, in which case relative incorporation into the two GH forms was identical. Labelled GH separated from LGH by 4M guanidene contained the same isotope ratios as did SGH. The data suggest that LGH is not a simple aggregate of SGH, but rather it is SGH, or a precursor of GH, associated in the pituitary gland with another simultaneously synthesized protein. The data also indicate that GH released into the medium contains the same amino acid ratio as does SGH (the storage form of GH in the pituitary).
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