The mechanisms of heterolytic versus homolytic O−O bond cleavage of H2O2, tert-butyl hydroperoxide (t-BuOOH), 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), and m-chloroperoxybenzoic acid (m-CPBA) by iron(III) porphyrin complexes have been studied by carrying out catalytic epoxidations of cyclohexene in protic solvent. In these reactions, various iron(III) porphyrin complexes containing electron-withdrawing and -donating substituents on phenyl groups at the meso position of the porphyrin ring were employed to study the electronic effect of porphyrin ligands on the heterolytic versus homolytic O−O bond cleavage of the hydroperoxides. In addition, various imidazoles were introduced as axial ligands to investigate the electronic effect of axial ligands on the pathways of hydroperoxide O−O bond cleavage. Unlike the previous suggestions by Traylor, Bruice, and co-workers, the hydroperoxide O−O bonds were found to be cleaved both heterolytically and homolytically and partitioning between heterolysis and homolysis was significantly affected by the electronic nature of the iron porphyrin complexes (i.e., electronic properties of porphyrin and axial ligands). Electron-deficient iron porphyrin complexes show a tendency to cleave the hydroperoxide O−O bonds heterolytically, whereas electron-rich iron porphyrin complexes cleave the hydroperoxide O−O bonds homolytically. The heterolytic versus homolytic O−O bond cleavage of the hydroperoxides was also found to be significantly affected by the substituent of the hydroperoxides, ROOH (R = C(O)R‘, H, C(CH3)3, and C(CH3)2CH2Ph for m-CPBA, H2O2, t-BuOOH, and MPPH, respectively), in which the tendency of O−O bond heterolysis was in the order of m-CPBA > H2O2 > t-BuOOH > MPPH. This result indicates that the O−O bond of hydroperoxides containing electron-donating tert-alkyl groups such as t-BuOOH and MPPH tends to be cleaved homolytically, whereas electron-withdrawing substituents such as an acyl group in m-CPBA facilitates O−O bond heterolysis. Since we have observed that the homolytic O−O bond cleavage of hydroperoxides prevails in the reactions performed with electron-rich iron porphyrin complexes and with hydroperoxides containing electron-donating substituents such as the tert-alkyl group, we suggest that the homolytic O−O bond cleavage is facilitated when more electron density resides on the O−O bond of (Porp)Fe(III)-OOR intermediates. We also present convincing evidence that the previous assertion that the reactions of iron(III) porphyrin complexes with hydrogen peroxide and tert-alkyl hydroperoxides invariably proceed by heterolytic O−O bond cleavage in protic solvent and that the failure to obtain high epoxide yields in iron porphyrin complex-catalyzed epoxidation of olefins by hydroperoxides is due to the mechanism of heterolytic O−O bond cleavage followed by a fast hydroperoxide oxidation is highly unlike.

Host immune response is known to contribute to the progression of periodontitis, and alveolar bone destruction in periodontitis is associated with enhanced osteoclast activity. Therefore, we evaluated the roles of activated lymphocyte subsets in osteoclastogenesis. Osteoclast precursors were co‐cultured with activated lymphocytes (B, CD4+ T, CD8+ T) in the presence of either macrophage colony‐stimulating factor (M‐CSF) alone or M‐CSF plus soluble receptor activator of NF‐κB ligand (sRANKL), and subsequent differentiation into active osteoclasts was evaluatedby a resorption assay. The activated B and CD4+ cells, but not CD8+ T cells, induced osteoclast differentiation in the presence of M‐CSF alone. In the presence of M‐CSF and sRANKL, B cells induced the formation of small but highly active osteoclasts and increased resorption, while CD8+ T cells profoundly suppressed osteoclastogenesis. Co‐culture using an insert wellor supernatant suggested that both B and CD8+ T cells acted on osteoclasts mostly via soluble proteins. Activated B cells expressed many osteoclastogenic factors including RANKL, TNF‐α, IL‐6, MIP‐1α, and MCP‐3. CD8+ T cells expressed a substantial amount of osteoprotegerin (OPG) along with RANKL. However, blocking antibody to OPG did not reverse the suppression by CD8+ T cells, suggesting that other factor(s) are involved. Taken together, activated B cells promoted osteoclastogenesis, while CD8+ T cells inhibited the osteoclast formation via direct interaction. The results imply the importance of lymphocyte subpopulations in the development of periodontitis.

Mitophagy has been implicated in mitochondrial quality control and in various human diseases. However, the study of in vivo mitophagy remains limited. We previously explored in vivo mitophagy using a transgenic mouse expressing the mitochondria‐targeted fluorescent protein Keima (mt‐Keima). Here, we generated mt‐Keima Drosophila to extend our efforts to study mitophagy in vivo. A series of experiments confirmed that mitophagy can be faithfully and quantitatively measured in mt‐Keima Drosophila. We also showed that alterations in mitophagy upon environmental and genetic perturbation can be measured in mt‐Keima Drosophila. Analysis of different tissues revealed a variation in basal mitophagy levels in Drosophila tissues. In addition, we found a significant increase in mitophagy levels during Drosophila embryogenesis. Importantly, loss‐of‐function genetic analysis demonstrated that the phosphatase and tensin homolog‐induced putative kinase 1 (PINK1)‐Parkin pathway is essential for the induction of mitophagy in vivo in response to hypoxic exposure and rotenone treatment. These studies showed that the mt‐Keima Drosophila system is a useful tool for understanding the role and molecular mechanism of mitophagy in vivo. In addition, we demonstrated the essential role of the PINK1‐Parkin pathway in mitophagy induction in response to mitochondrial dysfunction.—Kim, Y. Y., Um, J.‐H., Yoon, J.‐H., Kim, H., Lee, D.‐Y., Lee, Y. J., Jee, H. J., Kim, Y. M., Jang, J. S., Jang, Y.‐G., Chung, J., Park, H. T., Finkel, T., Koh, H., Yun, J. Assessment of mitophagy in mt‐Keima Drosophila revealed an essential role of the PINK1‐Parkin pathway in mitophagy induction in vivo. FASEB J. 33, 9742–9751 (2019). http://www.fasebj.org