The Effects of Sodium Perchlorate on Rabbit Muscle Enolase

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Abstract
Incubation of rabbit ββ enolase in NaClO4 (≤0.3 M) results in a loss of enzymatic activity and striking changes in the second‐derivative ultraviolet spectrum of enolase. HPLC gel filtration shows that dissociation of the dimeric enzyme is occurring. We have used molecular modelling, fluorescence and circular dichroic spectroscopy to examine the structural differences between the monomeric and dimeric forms of this protein. In the dimer, the tyrosine residues are in a non‐polar environment; upon dissociation, two of them that were at the dimer interface become exposed. This results in large changes in the second‐derivative spectrum. Both the tryptophan fluorescence emission spectrum and the aromatic region of the CD spectrum indicate that there are also changes in the environment of other aromatic residues. No perturbations in the peptide bond region of the CD spectrum are observed. We propose that the major structural effect of NaClO4, is to increase the flexibility of the loops connecting the helices and strands of the α/β, barrel of enolase. These loops, which contain about half of the aromatic residues, contain some of the residues of the active site and other residues involved in subunit contacts. Increased flexibility of the loops could disrupt both subunit interactions and the structure of the active site.