Presence of sterol-binding sites in the cytosol of French-bean (Phaseolus vulgaris) roots.
Vitamin D3 (cholecalciferol) and stigmasterol have been shown to stimulate Ca2+ uptake and to induce calmodulin synthesis in cultured French-bean (Phaseolus vulgaris) roots. In addition, the appearance of calmodulin in the cultures in response to vitamin D3 could be prevented by RNA-synthesis inhibitors. To investigate the possibility that the sterols affect root DNA transcription through a receptor-mediated mechanism, the existence of sterol-binding sites in P. vulgaris roots was investigated. Specific binding of [3H]vitamin D3 could be demonstrated with intact tissue and the cytosolic fraction obtained therefrom. Equilibrium in the binding reaction with cytosol was attained after 4 h of incubation at 0 degrees C. The [3H]vitamin D3 was reversibly bound, since it could be displaced by an excess of unlabelled sterol. An equilibrium binding constant (KD) of (3.48 +/- 0.09) x 10(-9) M and a maximum binding-site concentration (nmax) of 32 +/- 2.54 (3) pmol/mg of protein could be calculated by Scatchard [(1949) Ann. N.Y. Acad. Sci. 51, 660-672] analysis. In addition to vitamin D3, stigmasterol and sitosterol were effectively able to compete with [3H]vitamin D3 for binding to root cytosol. Cortisol, oestradiol and progesterone displaced bound labelled vitamin D3 to a lesser extent, whereas 5 beta-dihydrotestosterone, lanosterol and diosgenin were ineffective. The affinity and specificity of the root sterol-binding sites are in agreement with the characteristics of tissue responses to the sterols (Ca2+ uptake and calmodulin synthesis).
Vega
M A
MA
Departamento de Biología, Universidad Nacional del Sur, Bahia Blanca, Argentina.
Boland
R L
RL
D001665:Binding Sites; D002762:Cholecalciferol / Q000494:pharmacology; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D007887:Fabaceae / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism; D010946:Plants, Medicinal; D013261:Sterols / Q000494:pharmacology*; D013329:Structure-Activity Relationship
D010940:Plant Proteins; D013261:Sterols; D002762:Cholecalciferol