Structur
e-activity r
elationship studi
es hav
e establish
ed that th
e A and C rings of colchicin
ecompris
e th
e minimum structural f
eatur
e n
ec
essary for high affinity drug-tubulin binding. Thus, colchicin
eacts as a bifunctional ligand by making two points of attachm
ent to th
e prot
ein. Furth
ermor
e, analogu
esb
elonging to th
e iso s
eri
es of colchicin
e ar
e virtually inactiv
e in binding to tubulin and inhibiting microtubul
eass
embly. In th
e pr
es
ent study, w
e found that th
e substitution of a hydrophobic dansyl group on th
eB-ring sid
e chain (C7 position) of isocolchicin
e r
ev
ers
es th
e structural alt
erations at th
e C ring and th
en
ewly synth
esiz
ed -NH-dansyl isocolchicin
e r
estor
es th
e lost biological activity of th
e compound. It inhibitsmicrotubul
e ass
embly
effici
ently with an IC
50 valu
e of 10
![](/imag<font color=)
es/
entiti
es/mgr.gif">M and comp
et
es with [
3H]colchicin
e forbinding to tubulin. Mor
eov
er, although -NH-dansyl colchicin
e binding to tubulin involv
es two st
eps, th
e-NH-dansyl isocolchicin
e-tubulin int
eraction has b
een found to occur via a on
e-st
ep proc
ess. Also, th
eaffinity constant of th
e -NH-dansyl isocolchicin
e-tubulin int
eraction is roughly only 3 tim
es low
er thanthat of th
e -NH-dansyl colchicin
e-tubulin int
eraction. Th
es
e r
esults sugg
est that th
e enhanc
ed microtubul
einhibitory ability of -NH-dansyl isocolchicin
e is th
er
efor
e r
elat
ed to th
e affinity of th
e drug-tubulinint
eraction and not to any conformational chang
es upon binding tubulin. W
e also obs
erv
ed that th
ecomp
etition of -NH-dansyl isocolchicin
e with [
3H]colchicin
e for binding to tubulin was d
ep
end
ent on th
etubulin conc
entration. In conclusion, this pap
er for th
e first tim
e indicat
es that a biologically activ
ebifuntional colchicin
e analogu
e can b
e d
esign
ed wh
er
e th
e drug binds tubulin through its A and B rings,whil
e th
e C ring r
emains inactiv
e.