Stereochemical aspects of the conversion of cyclopeptine into dehydrocyclopeptine by cyclopeptine dehydrogenase from Penicillium cyclopium

Cyclopeptine dehydrogenase, an enzyme from Penicillium cyclopium, catalyses the reversible transformation of the benzodiazepine alkaloids cyclopeptine and dehydrocyclopeptine. By the dehydrogenation of cyclopeptine two hydrogen atoms are removed from the positions 3 and 10. It was demonstrated that, from the two optical isomers of cyclopeptine, only the naturally occurring 3S-compound was used as substrate by cyclopeptine dehydrogenase. To test the stereospecificity of the enzyme with respect to the second hydrogen which is eliminated from C-10 a mixture of cyclopeptine-3S-[10R-³H₁] and cyclopeptine-3R-10S-³H₁] was prepared. The 3S-isomer was transformed by the enzyme into radioactively labelled dehydrocyclopeptine. This demonstrated that cyclopeptine dehydrogenase removes the 10-proS hydrogen atom from the cyclopeptine molecule. Because the formed dehydrocyclopeptine has the trans-configuration it is probable that a synperiplanar elimination takes place. The hydride ion removed from cyclopeptine is transferred to the 4-proR-position of NAD⁺. Cyclopeptine dehydrogenase thus belongs to the A-specific dehydrogenases.