Rational Design of Superoxide Dismutase (SOD) Mimics:The Evaluation of the Therapeutic Potential of New Cationic Mn Porphyrinswith Linear and Cyclic Substituents

摘要

Our goal herein has been to gain further insight into the parameters which control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP⁵⁺, MnTnHexOE-2-PyP⁵⁺, MnTE-2-PyPhP⁵⁺, and MnTPhE-2-PyP⁵⁺) that bear the same positive charge and same number of carbon atoms at meso positions of porphyrin core were explored. The carbon atoms of their meso substituents are organized to form either linear or cyclic structures of vastly different redox properties, bulkiness, and lipophilicities. These Mn porphyrins were compared to frequently studied compounds, MnTE-2-PyP⁵⁺, MnTE-3-PyP⁵⁺, and MnTBAP^3–. All Mn(III) porphyrins (MnPs) have metal-centered reduction potential, E1/2 for Mn^IIIP/Mn^IIP redox couple, ranging from −194 to +340 mV versus NHE, log kcat(O2^•–) from 3.16 to 7.92, and log kred(ONOO^–) from 5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol and water, log POW, was in the range −1.67 to −7.67. The therapeutic potential of MnPs was assessed via: (i) in vitro ability to prevent spontaneous lipid peroxidation in rat brain homogenate as assessed by malondialdehyde levels; (ii) in vivo O2^•– specific assay to measure the efficacy in protecting the aerobicgrowth of SOD-deficient Saccharomyces cerevisiae;and (iii) aqueous solution chemistry to measure the reactivity towardmajor in vivo endogenous antioxidant, ascorbate.Under the conditions of lipid peroxidation assay, the transport acrossthe cellular membranes, and in turn shape and size of molecule, playedno significant role. Those MnPs of E1/2 ∼ +300 mV were the most efficacious, significantly inhibitinglipid peroxidation in 0.5–10 μM range. At up to 200 μM,MnTBAP^3– (E1/2 = −194mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP⁵⁺ with 129 mV more positive E1/2 (−65 mV vs NHE) was fully efficacious at 50 μM. The E1/2 of Mn^IIIP/Mn^IIP redoxcouple is proportional to the log kcat(O2^•–), i.e.,the SOD-like activity of MnPs. It is further proportional to kred(ONOO^–) and the ability of MnPs to prevent lipid peroxidation. In turn,the inhibition of lipid peroxidation by MnPs is also proportionalto their SOD-like activity. In an in vivo S. cerevisiae assay, however, while E1/2 predominates,lipophilicity significantly affects the efficacy of MnPs. MnPs ofsimilar log POW and E1/2, that have linear alkyl or alkoxyalkyl pyridyl substituents,distribute more easily within a cell and in turn provide higher protectionto S. cerevisiae in comparison to MnP with bulkycyclic substituents. The bell-shape curve, with MnTE-2-PyP⁵⁺ exhibiting the highest ability to catalyze ascorbate oxidation,has been established and discussed. Our data support the notion thatthe SOD-like activity of MnPs parallels their therapeutic potential,though species other than O₂^•–, such as peroxynitrite, H₂O₂, lipid reactivespecies, and cellular reductants, may be involved in their mode(s)of action(s).