| Abstrakt: |
Fluoride activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] is significantly enhanced (2 to 5 times) by a protein factor isolated from rat brain. The fluoride-dependent adenylate cyclase stimulator (FCS) is nondialyzable, trypsin-labile, and stable at 90 degrees C for 10 min. FCS stimulates adenylate cyclase activity only in the presence of NaF (2-25 mM) and this effect is independent of added GTP, 5'-guanylylimidodiphosphate, or calcium. FCS has been purified roughly 3000-fold from a 12,000 X g supernatant fraction of rat brain homogenate. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and sucrose density gradient sedimentation suggest that FCS is a monomer with an apparent Mr of 59,000. Isoelectric focusing indicates FCS has a pI of 8.9. FCS from rat brain stimulates fluoride-activated adenylate cyclase from a variety of cell types, and FCS can also be isolated from rat liver. The effects of FCS are not reversed by washing membranes when the membranes and FCS are preincubated with NaF. The Km of adenylate cyclase for ATP and the fluoride concentration causing half-maximal activation are unchanged by FCS; however, FCS increases the Vmax by 2.5-fold. FCS may act to increase the catalytic efficiency of fluoride-activated complexes of the GTP-binding unit with adenylate cyclase or to enhance the formation of additional active complexes. |
