Frederick Team Synthesizes Cyclostreptin Analogue

Here’s one for you, if you like all of the nitty gritty science stuff.  I wish NCI or Ft Detrick had a site that would just tell us everything that has been published recently, don’t you?

ASAP Article, 10.1021/np800056m
Web Release Date: February 26,
2008 Copyright © 2008 American Chemical Society and American Society of Pharmacognosy
Interaction of a Cyclostreptin Analogue with the Microtubule Taxoid Site: The Covalent Reaction Rapidly Follows Binding^⊥

Ruoli Bai,^† Christopher D. Vanderwal,^‡ J. Fernando Díaz,^§ and Ernest Hamel^*†

Toxicology and Pharmacology Branch, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute at Frederick, National Institutes of Health, Frederick, Maryland 21702, Department of Chemistry, University of California at Irvine, Irvine, California 92697, and Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid 28040, Spain

Received October 1, 2007

Abstract:

The natural product cyclostreptin reacts covalently and stoichiometrically with microtubules, at either of two amino acid residues of β-tubulin, Thr-218 or Asn-226, but much less extensively and only at Thr-218 in unpolymerized tubulin. It was found that 8-acetylcyclostreptin (8AcCS) induces tubulin assembly in a manner almost identical with that of cyclostreptin. We therefore synthesized [¹⁴C-acetyl]8AcCS and studied the kinetics of its interaction with glutaraldehyde-stabilized microtubules and with unassembled tubulin. With the microtubules, we found that 8AcCS bound rapidly, with a minimal (unmeasurable with the radiolabeled analogue) lag prior to the occurrence of the covalent reaction. Apparent reaction rate constants for the overall reaction ranged from 6.2 × 10² M⁻¹ s⁻¹ at 0 °C to 5.6 × 10³ M⁻¹ s⁻¹ at 20 °C. The rate constants obtained at 0 and 10 °C indicate an activation energy for the reaction of about 27 kcal/mol, while those obtained at 10 and 20 °C indicate an activation energy of about 7.7 kcal/mol. With the unpolymerized tubulin, we did find a minimal covalent reaction occurred without apparent microtubule assembly, but a substantial reaction only occurred following assembly. In conclusion, the radiolabeled 8AcCS shows that an extensive covalent interaction of ligand with tubulin requires microtubule assembly and that the covalent reaction occurs rapidly after the initial binding interaction.

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