Spectrophotometric confirmation for oxygen ligation to the reduced haem d ₁ of Pseudomonas nautica nitrite reductase

Abstract

Cytochrome cd ₁ was discovered in the Gram-negative bacterium Pseudomonas (Ps.) aeruginosa by Horio and coworkers [1]. It was tagged a cytochrome oxidase on the base of its capacity to reduce oxygen. Only a few years later was its main, if not only, physiological function as a nitrite reductase made clear [2]. Numerous articles have since been published on the spectrophotometry of cytochromes cd ₁ isolated from various bacteria incuding Ps. aeruginosa (see [3] for an overview) or Ps. nautica [4]. It was earlier believed that the nature of the electron donor was responsible for the considerable discrepancies observed between visible spectra of the same cytochrome cd ₁, in the regions of reduced haem d ₁ absorption peaks [5]. However, Shimada and Orii [6] demonstrated that an oxygenated-haem d ₁ reaction intermediate was the main reason for these discrepancies. Optical spectra of Ps. nautica cytochrome cd ₁ recorded under anaerobic conditions still exhibited differences and an investigation was decided to assess and compare respective effects of pH, buffer nature and level of oxygen contamination.