3-13C-methionine-labelled E. coli alkaline phosphatase.

3-13C-methionine has been biosynthetically incorporated into E. coli alkaline phosphatase using strain CW3747 which is auxotrophic for Met. 13C NMR of the dimeric native enzyme labelled at the eight methionine residues of the primary structure shows a dispersion of resonance signals permitting resolution of at least five methionine environments, none of which coincide with the chemical shift position of free methionine. At acid pH, 13C signal intensity is shifted to a chemical shift consistent with solvent exposure. However, three discrete resonances are observed, suggesting a retention of defined structure. The labelled protein thus can serve as a probe of conformational alterations of the enzyme.