Nitrite reductase from Achromobacter xylosoxidans forms stable trimers in dilute solution.

Nitrite reductase is a key metabolic enzyme in denitrifying anaerobic bacteria. In the past, nitrite reductase purified from a number of microorganisms has shown variation in subunit composition. In this study we show definitively using Sedimentation equilibrium i n the analytical ultracentrifuge that nitrite reductase from the species Achromobacter xylosoxiduns exists as stable trimers i n dilute solution, confirming the observations of Godden et al [ I ] . The molar masses (whole distribution weight averages. Mw) obtained at a loading concentration of 0.5mglml via the “M*” procedure [2] were ( 1 10000+5000) glmol and (105000+5000) glmol i n respectively phosphate and Tris HCI buffers, corresponding to trimers in both cases. From plots of point weight average molar masses [3] versus concentration i t is clear that the association is very strong with apparent dissociation constants < IpM.