Reversible denaturation of cyclosporin synthetase by urea

The reversible denaturation of the multifunctional potypeptide, cyclosporin synthetase, by urea was analyzed. It is possible to discriminate between at least two stages of enzyme denaturation. While at low urea concentration (up to 0.8 M) cyclosporin A formation is inhibited, synthesis of the diketopiperazine cyclo-(D-alanyl-N-methylleucyl), a molecule representing a partial sequence of cyclosporin A is still detectable. At higher concentrations of urea the enzyme preparation is totally inactive. This inactivation is a consequence of conformational change(s) of cyclosporin synthetase as shown by fluorescence emission spectra of native and denatured enzyme. These data imply a consecutive folding/defolding mechanism for the different domains forming the multifunctional polypeptide.