Aim: To demonstrate the role of the 8 kDa peptide in regulation of testosterone production by rat Leydig cells. Methods: A peptide similar to 8 kDa peptide purified from immature rat Leydig cells was isolated and purified from rat lung cytosol. Immunological and structural similarity between the peptides purified from lung and Leydig cells was established by Western blot and tryptic map comparison respectively. Results: Addition of the 8 kDa peptide 10, 50, 100, and 150 g decreased the production of testosterone in Leydig cells dose-dependently. But the addition of the peptide 150 g along with hCG had no effect on hCG-stimulated increase in testosterone production. Conclusion: In vitro addition of the peptide purified from lung cytosol to adult rat Leydig cells resulted in a concentration-dependent decrease in basal testosterone production although it had no effect on hCG-stimulated testosterone production.

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