When castrated male rats were injected intraperitoneally with [1,2,6,7-3H]-testosterone, liver nuclei specifically retained [3H]-androstenedione. Treatment of labelled nuclei with 0.4 M KC1 resulted in extraction of an androstenedione-protein complex with a pI of 5.1. An androstenedione-protein complex with identical pI was also found in liver cytosol; this complex had a sedimentation coefficient of 3.3 s and was eluted at 0.55 column vol. when chromatographed on a Sephadex G-100 column. The 3.3 s protein bound androstenedione with a limited number of binding sites. In reconstitution experiments where labelled liver cytosol was incubated with unlabelled nuclei from castrated male rats it was possible to demonstrate specific nuclear uptake of the androstenedione-protein complex from the cytosol; it could also be shown that this process was saturable and that the uptake increased with time. It is suggested that the androstenedione-binding protein in rat liver cytosol plays a role in the intranuclear transfer of androstenedione.
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