Effect of aging on the quaternary structure of recombinant refolded nitrilase. Gel filtration analysis on Superose 6B of freshly refolded enzyme (A), and enzyme stored for 30 and 40 days at 4°C (B and C, respectively). Fractions were collected from the last separation, and analyzed by electron microscopy using material eluted between 26 and 27 min (D), 30 and 31 min (E), 33 and 34 min (F), and 37-38 min (G). The homogeneous round-shaped particles observed in the latter fractions (G) were analyzed in an analytical ultracentrifuge using sedimentation velocity (H) and sedimentation equilibrium (I) experiments as detailed in Methods. Fitted data with residual plots showing goodness of fit are shown together with calculated continuous size distribution c(s) of sedimenting species.