Stereospecificity of Hydrogen Transfer Reactions of the Pseudomonas aeruginosa Pyridine Nucleotide Transhydrogenase

The stereochemistry and mechanisms of hydrogen-transfer reactions catalyzed by the Pseudomonas aeruginosa pyridine nucleotide transhydrogenase have been investigated. In the presence of specific tritium-labeled pyridine nucleotides, the Pseudomonas transhydrogenase is able to catalyse a direct hydrogen transfer between the reduced and the oxidized pyridine nucleotides. The reduction of DPN+ involved the 4B atom of the TPNH; the 2'-AMP-activated reduction of TPN+ results in the removal of the 4B atom of the pyridine ring of DPNH. The reduced products of the respective reactions all have tritium in the 4B position. The mechanism of transfer involves no exchange of hydrogen with the medium, and this phenomenon is not altered by changes of temperature. The Pseudomonas transhydrogenase catalyzes the reduction of the thionicotinamide nucleotide analogue of DPN with stereospecificity exhibited equivalent to that of the reaction with the natural pyridine nucleotides. Indirect evidence indicates that FAD participates in the hydrogen transfer reaction. Exchange of the hydrogen with the medium can be induced in a 1 m urea medium. Available spectral evidence suggests the possibility that conformational changes of the transhydrogenase occurs in 1 m urea.