1-Acetyl-7-deacetylforskolin: a potential non-specific inactive analog of forskolin for estimation of its specific high-affinity binding and adenylyl cyclase stimulation in vitro.

Abstract Labeled and unlabeled 1-acetyl-7-deacetylforskolin and forskolin were synthesized by acetylation of 7-deacetylforskolin with labeled and unlabeled acetyl chloride. The binding of 1-acetyl[1-acetyl- 11 C]-7-deacetylforskolin ([ 11 C]1-acetyl-7-deacetylforskolin) and [7-acetyl- 11 C]forskolin ([ 11 C]forskolin) to rat brain membranes was studied using filtration assay. The [ 11 C]forskolin binding was decreased with an increasing load of unlabeled forskolin, whereas [ 11 C]l-acetyl-7-deacetyl-forskolin binding was always very low, the level of which agreed with that of the non-specific binding in forskolin. However, binding of [7-acetyl- 11 C]1,9-dideoxyforskolin, which has been used as a non-specific inactive analog of forskolin, had a higher binding ratio than that of the non-specific binding of forskolin. The binding of [ 11 C]forskolin was not affected by an increased load of cold 1-acetyl-7-deacetylforskolin.Forskolin activated adenylyl cyclase (AC) in cultured human endothelial cells, whereas 1-acetyl-7-deacetylforskolin did not. These data show that the 1-acetyl-7-deacetylforskolin lacks specific binding affinity and the ability to stimulate AC, while it has similar physical properties with forskolin. The compound 1-acetyl-7-deacetylforskolin would be a suitable “non-specific inactive analog” of forskolin with which to estimate its specific high-affinity binding capacity and to validate forskolin-specific AC stimulation in vitro.