Identification and Characterization of a Novel Extracellular Ferric Reductase from Mycobacterium paratuberculosis

A novel extracellular mycobacterial enzyme was identified in the ruminant pathogen Mycobacterium paratuberculosis. The enzyme was capable of mobilizing iron from different sources such as ferric ammonium citrate, ferritin, and transferrin by reduction of the metal. The purified reductase had a calculated M(r) of 17,000, was sensitive to proteinase K treatment, and had an isoelectric point of pH 9. Analysis of the amino acid composition revealed glycine, serine, asparagine (or aspartic acid), and glutamine (or glutamic acid) as the most frequently occurring residues. Enzymatic activity was highest at 37°C and between pH 5 and 10. The calculated K(m) and V(max) for ferric ammonium citrate were 0.213 mM and 0.345 mM min(−1) mg(−1), respectively. Using a specific antireductase antibody in immunoelectron microscopy, we were able to detect the enzyme associated with intracellular mycobacteria in naturally M. paratuberculosis-infected bovine tissue. We propose that the reductase of M. paratuberculosis represents an alternative strategy of mycobacteria to mobilize ferric iron and discuss its potential role in bacterial evasion of intracellular defense mechanisms.