Tuning Cerium(IV)‐Assisted Hydrolysis of Phosphatidylcholine Liposomes under Mildly Acidic and Neutral Conditions

Abstract

With the goal of designing a lysosomal phospholipase mimic, we optimized experimental variables to enhance Ce(IV) -assisted hydrolysis of phosphatidylcholine (PC) liposomes. Our best result was obtained with the chelating agent bis-tris propane (BTP). Similar to the hydrolytic enzyme, Ce(IV) -assisted hydrolysis of PC phosphate ester bonds was higher at lysosomal pH (∼4.8) compared to pH 7.2. In the presence of BTP, the average cleavage yield at ∼pH 4.8 and 37 °C was: 67±1 %, 5.7-fold higher than at ∼pH 7.2 and roughly equivalent to the percent of phospholipid found on the metal-accessible exo leaflet of small liposomes. No Ce(IV) precipitation was observed. When BTP was absent, there was significant turbidity, and the amount of cleavage at ∼pH 4.8 (69±1 %) was 2.1-fold higher than the yield obtained at ∼pH 7.2. Our results show that BTP generates homogenous solutions of Ce(IV) that hydrolyze phosphatidylcholine with enhanced selectivity for lysosomal pH.

Metal-Assisted Hydrolysis Reactions Involving Lipids: A Review

“…The lanthanides were then shown to quench the fluorescence of membrane embedded carboxyfluorescein, leading the authors to propose a hydrolytic mechanism in which densely packed Ln(III) Lewis acid centers assembled at the lipid-water interface of the PC liposomes accelerate phosphodiester hydrolysis in a cooperative fashion. Dissimilar to Williams et al's data (Kassai et al, 2011; Williams et al, 2015), metal-assisted cleavage of unactivated bilayer phosphatidylcholine molecules was not reported.…”

“…In order for such an enzyme mimic to be optimal, cleavage should occur at physiological temperature and should be greatly diminished in neutral environments. To further enhance and tune cerium(IV) chemistry, Williams et al turned to the chelating ligand bis-tris propane (BTP; Figure 1J) (Williams et al, 2015). Upon the addition of BTP to optimized 37°C reactions, cerium(IV) hydrolyzed unactivated PC liposomes to release 5.7 times more inorganic phosphate at ~pH 4.8 than at ~pH 7.2, a major enhancement compared to the ~2.1 fold increase that was observed in ligand free controls (20 h).…”

“…In the presence of BTP, the yield of inorganic phosphate at pH 4.8 and 37°C was 67%, a value that is roughly equivalent to the percent of phospholipid molecules found on the metal-accessible exo surface of small liposomes (Entry 11 in Table S1). NMR studies indicated that the pH-dependent “on-off switch” of the BTP ligand is related to the p K a values of its nitrogen donor atoms (p K a1 = 6.8, p K a2 = 9.1) (Maldonado and Yatsimirsky, 2005; Williams et al, 2015). At pH 4.8, near-complete donor atom protonation minimizes interactions between cerium(IV) and BTP.…”

Kinetic Characterization of Cerium and Gallium Ions as Inhibitors of Cysteine Cathepsins L, K, and S

“…Certain cerium complexes have been shown to possess hydrolase activity [ 38 , 39 ]. Therefore, we also investigated whether the cerium and gallium salts had detectable hydrolytic activity toward the substrate Z-Phe-Arg-AMC at a metal ion concentration of 0.5 mM, the highest concentration used in the kinetic experiments.…”

Bis-tris propane as a flexible ligand for high-nuclearity complexes

“…The possibility of buffer interference by 3d metal complexation has been studied [30]. More recently, Bis-tris propane has been employed to lower the redox potential of the Mn(II)/Mn(III) couple in the study of electron transfer processes in native anoxygenic bacteria [31], as a ligand in the Ce IV -assisted hydrolysis of phosphatidylcholine liposomes [32] and when dissolved in DMSO it has been studied for CO 2 capture via amine scrubbing [33].…”