Studies onin VitroSynthesis and Secretion of Growth Hormone and Prolactin. II. Evidence Against the Existence of Precursor Molecules

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Abstract
In order to establish the possible existence of hormone precursors, the biosynthesis of rat growth hormone. (GH) and prolactin (PRL) was investigated in pituitary slices pulselabeled for a short time (5 min) with 14C-Lleucine and then incubated in chase medium up to 40 min. Proteins were analyzed by two different techniques of gel electrophoresis (the nondissociating gel system of Ornstein and Davis and a high resolution SDS gel system) and the rate of incorporation of 14C-L-leucine and the evolution of the specific radioactivity with chase in total protein as well as in GH and PRL and in other protein bands were followed. No kinetics of hormone labeling compatible with the existence of precursors was ever obtained when samples were adequately solubilized with detergents prior to electrophoresis on either system. Hormone bands appeared highly labeled at the end of the pulse and their specific radioactivity did not change significantly during chase incubation. Only when homogenates were incompletely solubilized by treatment with urea prior to purification did the labeling of hormones increase during chase incubation, even in the absence of net protein synthesis. Such a result is due to a greater resistance to solubilization of the cell organelles segregating the newly synthesized hormone molecules, the rough-surfaced microsomes, relative to that of secretion granules. We concluded that rat GH and PRL appear to be synthesized directly as finished molecules in rat pituitary cells. (Endocrinology94: 104, 1974)

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