Ahmed Elkhadem

Dihydrofolate reductase has been purified 40-fold to apparent homogeneity from a trimethoprim-resistant strain of Escherichia coli (RT 500) using a procedure that includes methotrexate affinity column chromatography. Determinations of the molecular weight of the enzyme based on its amino acid composition, sedimentation velocity, and sodium dodecyl sulfate gel electrophoresis gave values of 17680, 17470 and 18300, respectively. An aggregated form of the enzyme with a low specific activity can be separated from the monomer by gel filtration; treatment of the aggregate with mercaptoethanol or dithiothreitol results in an increase in enzymic activity and a regeneration of the monomer. Also, multiple molecular forms of the monomer have been detected by polyacrylamide gel electrophoresis. The unresolved enzyme exhibits two pH optima (pH 4.5 and pH 7.0) with dihydrofolate as a substrate. Highest activities are observed in buffers containing large organic cations. In 100 mM imidazolium chloride (pH 7), the specific activity is 47 mumol of dihydrofolate reduced per min per mg at 30 degrees. Folic acid also serves as a substrate with a single pH optimum of pH 4.5. At this pH the Km for folate is 16 muM, and the Vmax is 1/1000 of the rate observed with dihydrofolate as the substrate. Monovalent cations (Na+, K+, Rb+, and Cs+) inhibit dihydrofolate reductase; at a given ionic strength the degree of inhibition is a function of the ionic radius of the cation. Divalent cations are more potent inhibitors; the I50 of BaCl2 is 250 muM, as compared to 125 mM for KCl. Anions neither inhibit nor activate the enzyme.

DATA SOURCES: Medline, Embase, Cochrane Oral Health Group's Trials Register and the Cochrane Central Register of Controlled Trials (CENTRAL). Unpublished data were sought by searching ClinicalTrials.gov, the National Research Register and Pro-Quest Dissertation Abstracts and Thesis database. There were no language restrictions.

STUDY SELECTION: Randomised and controlled clinical trials (including split mouth) directly comparing self-etch and acid-etch primers including patients with full-arch, fixed and bonded orthodontic appliances (not banded) with follow-up periods of at least 12 months were included.

DATA EXTRACTION AND SYNTHESIS: Two authors abstracted data independently, with disagreements being resolved by a third. The Cochrane Risk of Bias tool was used to assess study quality. A random effects meta-analysis was undertaken.

RESULTS: Eleven studies were included in the qualitative summary with five studies contributing to a meta-analysis. These five studies (n =3444 brackets, 1721 acid-etch, 1723 self-etch) had relatively low statistical and clinical heterogeneity. Meta-analysis demonstrated a tendency for a higher risk of failure (odds ratio 1.35; 95% CI, 0.99-1.83; P 5 0.06) with self-etch primers. The use of self-etch techniques was also associated with a small but statistically significant time saving (weighted mean difference 23.2 seconds per bracket; 95% CI, 20.7-25.8; P \0.001). There was insufficient evidence to assess the effect of bonding modality on demineralisation rates.

CONCLUSIONS: There is weak evidence indicating higher odds of failure with self-etch primer than acid-etch over 12 months in orthodontic patients, and there is strong evidence that a self-etch primer is likely to result in modest time savings (eight minutes for full bonding) compared with acid-etch.