In vitro metabolism of the epoxide substructure of cryptophycins by cytosolic glutathione S-transferase: Species differences and stereoselectivity

Abstract

The enzyme kinetics of the glutathione (GSH) conjugation of cryptophycin 52 (C52, R-stereoisomer) and cryptophycin 53 (C53, S-stereoisomer) by cytosolic glutathione S-transferases (cGSTs) from human, rat, mouse, dog and monkey liver were studied. V_max, K_m, and CL_int values for glutathione conjugation of C52 (R-stereoisomer) were 0.10 ± 0.01 nmol min⁻¹ mg⁻¹, 3.24 ± 0.23 µM, and (3.15 ± 0.09) × 10⁻² ml min⁻¹ mg⁻¹, respectively, in human cytosol. Due to limited solubility relative to the K_m, only CL_int values were determined in rat ((7.76 ± 0.10) × 10⁻² ml min⁻¹ mg⁻¹) and mouse ((7.61 ± 0.50) × 10⁻² ml min⁻¹ mg⁻¹) cytosol. Enzyme kinetic parameters could not be determined for C53 (S-stereoisomer). Microsomal GSH conjugation in human, rat, and mouse was attributed to cytosolic contamination. No GSH conjugation was seen in any biological matrix from dog or monkey. There was little GSH conjugation of C53 by cytosol or microsomes from any species. The metabolism of C52 and C53 by epoxide hydrolase was also investigated. No diol product was observed in any biological matrix from any species. Thus, cGSTs are primarily responsible for C52 metabolism.

Key words::

• Cryptophycin
• cytosol
• gluthathione S-transferase (GST)

Acknowledgements

The authors would like to thank Dr John-Michael Sauer, Dr James Grace, Dr Jenny Chien, Ms Barbara Ring and Mr Ken Ruterbories for insightful discussion and technical assistance.