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The incorporation of [³H]tyrosine ([³H]tyr) into different hypothalamic peptide fractions isolated from normal and castrated rats on a Sephadex G-25 column has been studied in vitro. Luteinizing hormone releasing factor (LH-RF) activity was determined in the different fractions by measuring their ability to elicit release of radioimmunoassayable luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in ovariectomized rats treated with oestrogen and progesterone.

For further purification, the fraction with LH-RF activity was applied to a CM-Sephadex G-25 column eluted with a gradient of ammonium acetate. The large radioactive peptides emerged from the Sephadex G-25 column in fraction S-1, while the small peptides with LH-RF activity were eluted in fraction S-2. Gonadectomy significantly increased the incorporation of [³H]tyr into the peptides of fractions S-2. Only in the purified fraction with LH-RF activity was the radioactivity incorporated higher in gonadectomized than in normal rats. The enhanced incorporation in fraction CM-3 observed after castration implies an increase in the hypothalamic synthesis of peptides with LH-RF activity. The addition of testosterone (2 µg/ml) to the incubation medium of hypothalamus from gonadectomized rats, corrected these modifications.

Gonadectomy decreased the incorporation of tyrosine into the large peptides, and incubation with testosterone corrected this change.

The modifications in the incorporation of [³H]tyr into the large and small peptides produced by castration appear to indicate that gonadectomy, as well as stimulating the production of LH-RF, enhances the synthesis of other hypothalamic peptides while inhibiting the synthesis of proteins. On the other hand an increase in the breakdown of large peptides into small peptides cannot be excluded.