Dissociation constant of Tetrahymena tubulin-colchicine complex

Tetrahymena tubulin has a very low affinity for colchicine. The dissociation rate constant (k-1) and association rate constant(k+1) of Tetrahymena tubulin for colchicine were determined in comparison with those of porcine brain tubulin. The k+1 value of Tetrahymena tubulin was about one-twentieth that of porcine brain tubulin, and k-1 of Tetrahymena tubulin was about 680 times greater than that of brain tubulin. The dissociation constant (k-1/k+1) of Tetrahymena tubulin (2.7 X 10(-3) turned out to be ten thousand times greater than that of brain tubulin. This large Kd value explains well the high colchicine concentration requirement reported to block cell division of cilia regeneration of this protozoan.