Enzymatically-Driven Mineralization Potential of a Calcium-Polyphosphate Bleaching gel

Objectives: The current research determined the alkaline phosphatase enzyme (ALP) activity and the effects of incorporating it in a highly concentrated hydrogen peroxide-based bleaching gel containing calcium-polyphosphate sub-microparticles (CaPP) on the orthophosphate (PO₄³⁻) amounts, bleaching efficacy and enamel microhardness.
Methods: ALP activity was evaluated at different pH levels, H₂O₂-concentrations, and in H₂O- and Tris-based thickeners. Circular dichroism (CD) was used to examine changes in the secondary structure of ALP in water-, Tris-, or H₂O₂-based mediums. The PO₄³⁻ levels were assessed in two thickeners with and without ALP. Enamel/dentin specimens were allocated into the following groups: control (saliva only); commercial (Whiteness-HP-Maxx, FGM); Exp-H (H₂O-based); CaPP-H (H₂O-based, CaPP); ALP-H (CaPP+ALP); Exp-T (Tris-based); CaPP-T (Tris-based, CaPP); and ALP-T (CaPP+ALP). Three bleaching sessions were performed. During and after bleaching, the total color change of the specimens (ΔE/ΔE₀₀) and their bleaching index (ΔWID) were determined, and surface (SMH) and cross-sectional microhardness (CSMH) assessed. The two-way ANOVA and Tukey’s post-hoc tests were used to compare ALP and PO₄³⁻ levels via pH and H₂O₂ concentration. Generalized linear models were used to examine ΔE, ΔE₀₀, SMH, and CSMH; and Kruskal–Wallis and Dunn’s tests were used to examine ΔWID (α=5%).
Results: The findings showed that ALP activity was higher at pH 9, lower in H₂O₂-based mediums, and similar in both thickeners. The CD spectra alterations indicated denaturation of the enzyme upon contact with H₂O₂. The PO₄³⁻ levels were higher after ALP incorporation. The ΔE, ΔE₀₀, and ΔWID values were similar among the bleached groups. After bleaching the SMH was lower in the Exp-H group, while CSMH was higher in the ALP-T group compared to the rest of the groups. Compared to gels containing CaPP only, the higher PO₄³⁻ levels in the ALP-containing group (ALP-T) increased microhardness without decreasing bleaching efficacy.
Conclusions: The incorporation of the alkaline phosphatase in a bleaching gel containing hydrogen peroxide and calcium polyphosphate demonstrated an improvement of the mechanical properties of the enamel, associated to its bioinspired remineralizing potential, without interfering with the bleaching effectiveness.