High Permeability of Polyunsaturated Lipid Bilayers As Applied to Attoliter Enzyme Reactors

Confinement of Metalloenzymes in PEGylated Liposomes to Formulate Colloidal Catalysts for Antioxidant Cascade

Antioxidant cascade reactions detoxifying reactive oxygen species are of significance to control oxidative stresses-triggered diseases. In the present work, the antioxidant catalysts were prepared through the confinement of dual metalloenzymes in liposomes. The amino groups of superoxide dismutase (SOD) were conjugated to the carboxyl groups-bearing liposomes encapsulated with the catalase (CAT) to formulate a spatially organized antioxidant reaction network. The activity of SOD and CAT in the liposomal system was evaluated in detail on the basis of the prolonged xanthine oxidase/xanthine reaction producing superoxide anion radicals (O₂̇^-) and hydrogen peroxide (H₂O₂) coupled with redox reactions of cytochrome c. The liposome-confined SOD and CAT molecules were clearly demonstrated to catalyze the sequential disproportionation of O₂̇^- and H₂O₂ at 25 °C in a potassium phosphate buffer solution (pH = 7.8) under moderate transfer resistance with respect to the intermediate product (H₂O₂) within the liposomes. Furthermore, the liposomal catalysts were modified with the poly(ethylene glycol) (PEG)-conjugated lipids with the molecular mass of the PEG moiety of about 5000 through the post-PEGylation approach. The mean hydrodynamic diameter of the PEGylated liposomal catalysts was 140-150 nm. The dual enzyme activity in liposomes and the thermal stability of the encapsulated CAT were practically unaffected by the PEGylation. The above liposome-based antioxidant catalysts are highly biocompatible, PEG-modifiable, and reactive, thereby making the catalysts potentially applicable to therapeutic materials exhibiting functionality similar to cellular peroxisomes.

Efficient Entrapment of Carbonic Anhydrase in Alginate Hydrogels Using Liposomes for Continuous-Flow Catalytic Reactions

A versatile approach to entrap relatively small enzymes in hydrogels allows their diverse biotechnological applications. In the present work, bovine carbonic anhydrase (BCA) was efficiently entrapped in calcium alginate beads with the help of liposomes. A mixture of sodium alginate (3 wt %) and carbonic anhydrase-liposome conjugates (BCALs) was dripped into a Tris-HCl buffer solution (pH = 7.5) containing 0.4 M CaCl2 to induce the gelation and curing of the dispersed alginate-rich droplets. The entrapment efficiency of BCALs, which was defined as the amount of catalysts entrapped in alginate beads relative to that initially charged, was 98.7 ± 0.2% as determined through quantifying BCALs in the filtrate being separated from the beads. When free BCA was employed, on the other hand, a significantly lower entrapment efficiency of 27.2 ± 4.1% was obtained because free BCA could pass through alginate matrices. Because the volume of a cured alginate bead (10 μL) entrapped with BCALs was about 2.5 times smaller than that of an original droplet, BCALs were densely present in the beads to give the concentrations of lipids and BCA of 4.6-8.3 mM and 1.1-1.8 mg/mL, respectively. Alginate beads entrapped with BCALs were used to catalyze the hydrolysis of 1.0 mM p-nitrophenyl acetate (p-NA) at pH = 7.5 using the wells of a microplate or 10 mL glass beakers as batch reactors. Furthermore, the beads were confined in a column for continuous-flow hydrolysis of 1.0 mM p-NA for 1 h at a mean residence time of 8.5 or 4.3 min. The results obtained demonstrate that the conjugation of BCA to liposomes gave an opportunity to achieve efficient and stable entrapment of BCA in alginate hydrogels for applying to catalytic reactions in bioreactors.

Cell mimicry as a bottom-up strategy for hierarchical engineering of nature-inspired entities

Artificial biology is an emerging concept that aims to design and engineer the structure and function of natural cells, organelles, or biomolecules with a combination of biological and abiotic building blocks. Cell mimicry focuses on concepts that have the potential to be integrated with mammalian cells and tissue. In this feature article, we will emphasize the advancements in the past 3-4 years (2017-present) that are dedicated to artificial enzymes, artificial organelles, and artificial mammalian cells. Each aspect will be briefly introduced, followed by highlighting efforts that considered key properties of the different mimics. Finally, the current challenges and opportunities will be outlined. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Rapid leakage from PEGylated liposomes triggered by bubbles

Liposomes are applicable to fabrication of colloidal carriers of drugs and proteins. Physicochemical stimuli-triggered leakage from liposomes offers a wide variety of applications in biochemical and biomedical fields. In this work, effects of bubbles on the characteristics of PEGylated liposomes encapsulating 5(6)-carboxyfluorescein were examined. The liposomes were composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-10 mol% 1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated with poly(ethylene glycol) (DSPE-PEG). The mean molecular mass M_r,PEG of the PEG moiety was 550 or 5000. A bubble column was used for generating air bubbles at a superficial gas velocity of 0.58-0.88 cm s^-1. Leakage from the PEGylated liposomes was remarkably accelerated at 25 or 40 °C by introducing air to a liposome suspension at pH 7.4, whereas the dye molecules practically remained encapsulated in the liposomes being suspended in static liquid. The apparent rate constant for the dye release from the liposomes composed of DOPC and 1 mol% DSPE-PEG (M_r,PEG = 5000) being suspended in the gas-liquid flow was 168 times larger than that obtained with respect to the same liposomes in static liquid. Leakage from non-PEGylated liposomes was not pronounced even in the gas-liquid flow. Furthermore, the release rate of the dye from the PEGylated liposomes in liquid shear flow (no bubble) was clearly smaller than that in the gas-liquid flow, meaning that the interaction between bubbles and the liposomes was responsible for the observed rapid leakage. Adsorption of the PEGylated lipids to bubbles was indicated to induce leaky lipid bilayers, which was discussed on the basis of the conformational state of the PEG moiety.