A comparison of maintenance of canal curvature using balanced-force instrumentation with three different file types.

Forty-five extracted human molar teeth with curved facial or mesial canals were instrumented to a #45 file with three different file types using balanced-force (BF) instrumentation. Pre- and postoperative X-rays were taken, projected, and file location traced to determine the canal curvature according to the method of Schneider. Results showed statistically less reduction in canal curvature with nickel-titanium (NT) files compared with either Flex-R (FR) (p < 0.05) or K-Flex (KF) files (p < 0.0021). There was no significant difference comparing FR with KF files (p > 0.05). Under the conditions of this study, NT files used with the BF technique were significantly less likely to change canal curvature than either FR or KF files when instrumenting curved root canals.

Kinetic and structural characterization of spinach carbonic anhydrase.

We have carried out kinetics studies of spinach carbonic anhydrase (CA) using stopped-flow spectrophotometry at steady state and 13C-NMR exchange at chemical equilibrium. We found that the rate of CO2<-->HCO3- exchange catalyzed by spinach CA at pH 7.0 to be 3-5 times faster than the maximal kcat for either CO2 hydration or HCO3- dehydration at steady state, suggesting a rate-determining H+ transfer step in the catalytic mechanism. Correspondingly, we measured a pH-independent solvent deuterium isotope effect on kcat of approximately 2.0, and found that the rate of catalysis was significantly decreased at external buffer concentrations below 5 mM. Our results are consistent with a zinc-hydroxide mechanism of action with for spinach CA, similar to that of animal carbonic anhydrases. We have also collected X-ray absorption spectra of spinach CA. Analysis of the extended fine structure (EXAFS) suggests that the coordination sphere of Zn in spinach CA must have one or more sulfur ligands, in contrast to animal CAs which have only nitrogen and oxygen ligands. The models which best fit the data have average Zn-N(O) distances of 1.99-2.06 A, average Zn-S distances of 2.31--2.32 A, and a total coordination number of 4-6. We conclude that animal and spinach CAs are convergently evolved enzymes which are structurally quite different, but functionally equivalent.