CHARACTERIZATION OF THE INTERACTION OF THE MARINE CYANOBACTERIAL NATURAL PRODUCT CURACIN-A WITH THE COLCHICINE SITE OF TUBULIN AND INITIAL STRUCTURE-ACTIVITY STUDIES WITH ANALOGS

Curacin A, the major lipid constituent of a strain of the marine cyano bacterium Lyngbya majuscula obtained off the coast of Curacao, is a po tent antimitotic agent that we have previously shown to inhibit microt ubule assembly and colchicine binding to tubulin. In the present study , we report that curacin A probably binds in the colchicine site becau se it competitively inhibits the binding of [H-3]colchicine to tubulin with an apparent K-i value bf 0.6 mu M and stimulates tubulin-depende nt GTP hydrolysis, as do most other colchicine-site agents. The bindin g of curacin A to tubulin resembled the binding reactions of combretas tatin A-4 and podophyllotoxin in contrast to that of colchicine in tha t it occurred as extensively on ice as at higher temperatures. However , once bound, the dissociation rate of curacin A from tubulin is very slow, more closely resembling that observed with colchicinoids (thioco lchicine was the drug examined) than the faster dissociation that occu rs with combretastatin A-4 and podophyllotoxin. Because the molecular structure of curacin A is so different from that of previously describ ed colchicine-site drugs (e.g., there is no aromatic moiety, and there are only two conjugated double bonds in its linear hydrocarbon chain) , we have been examining the activities of natural isomers and synthet ic derivatives. So far, only modest enhancement or reduction of activi ty has been observed with a variety of structural changes.