B ring regulation of colchicine binding kinetics and fluorescence.
Several properties of the colchicine-tubulin interaction such as association rate, reversibility, and the promotion of drug fluorescence have been related to the B ring of colchicine. The B ring itself retards the binding rate, and substitution at C-7 leads to further binding rate decreases that appear to be related to both substituent bulk and the presence of a N-acyl group. Thus, the decreasing order of binding rates is 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone greater than deacetamidocolchicine greater than deacetylcolchicine greater than or equal to colcemid greater than colchicine greater than N-benzoyldeacetylcolchicine, etc. The apparent irreversibility of the binding seems more closely related to the presence of an N-acyl group rather than the bulk of the substituent at C-7. Substitution at C-7 also affects the tropolone fluorophore. Thus, amines (deacetylcholchicine, colcemid, or N-methylcolcemid) fluoresce poorly in the presence of tubulin, whereas substitution of the amino group with an acyl group enhances fluorescence. The presence of an N-acyl group at C-7 is essential for enhanced fluorescence. We conclude that, in addition to A- and the C-ring portion of the molecule, the B ring of colchicine is a third determinant recognized by the binding site on tubulin.
Bhattacharyya
B
B
Howard
R
R
Maity
S N
SN
Brossi
A
A
Sharma
P N
PN
Wolff
J
J
D000818:Animals; D003078:Colchicine / Q000031:analogs & derivatives* / Q000378:metabolism; D007700:Kinetics; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D014404:Tubulin / Q000378:metabolism*
D014404:Tubulin; D003078:Colchicine