Background
Biosynthesis of noble metallic nanoparticles (NPs) has attracted significant interest due to their environmental friendly and biocompatible properties.
Methods
In this study, we investigated syntheses of Au, Ag and Au–Ag bimetallic NPs using protein extracts of
Deinococcus radiodurans
, which demonstrated powerful metal-reducing ability. The obtained NPs were characterized and analyzed by various spectroscopy techniques.
Results
The
D. radiodurans
protein extract-mediated silver nanoparticles (Drp-AgNPs) were preferably monodispersed and stably distributed compared to
D. radiodurans
protein extract-mediated gold nanoparticles (Drp-AuNPs). Drp-AgNPs and Drp-AuNPs exhibited spherical morphology with average sizes of 37.13±5.97 nm and 51.72±7.38 nm and zeta potential values of −18.31±1.39 mV and −15.17±1.24 mV at pH 7, respectively. The release efficiencies of Drp-AuNPs and Drp-AgNPs measured at 24 h were 3.99% and 18.20%, respectively. During the synthesis process, Au(III) was reduced to Au(I) and further to Au(0) and Ag(I) was reduced to Ag(0) by interactions with the hydroxyl, amine, carboxyl, phospho or sulfhydryl groups of proteins and subsequently stabilized by these groups. Some characteristics of Drp-AuNPs were different from those of Drp-AgNPs, which could be attributed to the interaction of the NPs with different binding groups of proteins. The Drp-AgNPs could be further formed into Au–Ag bimetallic NPs via galvanic replacement reaction. Drp-AuNPs and Au–Ag bimetallic NPs showed low cytotoxicity against MCF-10A cells due to the lower level of intracellular reactive oxygen species (ROS) generation than that of Drp-AgNPs.
Conclusions
These results are crucial to understand the biosynthetic mechanism and properties of noble metallic NPs using the protein extracts of bacteria. The biocompatible Au or Au–Ag bimetallic NPs are applicable in biosensing, bioimaging and biomedicine.
Understanding
the synthetic mechanisms and cell–nanoparticle interactions of biosynthesized
and functionalized gold nanoparticles (AuNPs) using natural products
is of great importance for developing their applications in nanomedicine.
In this study, we detailed the biotransformation mechanism of Au(III)
into AuNPs using a hydroxylated tetraterpenoid deinoxanthin (DX) from
the extremophile Deinococcus radiodurans. During the process, Au(III) was rapidly reduced to Au(I) and subsequently
reduced to Au(0) by deprotonation of the hydroxyl head groups of the
tetraterpenoid. The oxidized form, deprotonated 2-ketodeinoxanthin
(DX3), served as a surface-capping agent to stabilize the AuNPs. The
functionalized DX–AuNPs demonstrated stronger inhibitory activity
against cancer cells compared with sodium citrate–AuNPs and
were nontoxic to normal cells. DX–AuNPs accumulated in the
cytoplasm, organelles, and nuclei, and induced reactive oxygen species
generation, DNA damage, and apoptosis within MCF-7 cancer cells. In
the cells treated with DX–AuNPs, 374 genes, including RRAGC
gene, were upregulated; 135 genes, including the genes encoding FOXM1
and NR4A1, were downregulated. These genes are mostly involved in
metabolism, cell growth, DNA damage, oxidative stress, autophagy,
and apoptosis. The anticancer activity of the DX–AuNPs was
attributed to the alteration of gene expression and induction of apoptosis.
Our results provide significant insight into the synthesis mechanism
of AuNPs functionalized with natural tetraterpenoids, which possess
enhanced anticancer potential.
The translocation and assembly module (TAM) in bacteria consists of TamA and TamB that form a complex to control the transport and secretion of outer membrane proteins. Herein, we demonstrated that the DR_1462-DR_1461-DR_1460 gene loci on chromosome 1 of Deinococcus radiodurans, which lacks tamA homologs, is a tamB homolog (DR_146T) with two tamB motifs and a DUF490 motif. Mutation of DR_146T resulted in cell envelope peeling and a decrease in resistance to shear stress and osmotic pressure, as well as an increase in oxidative stress resistance, consistent with the phenotype of a surface layer (S-layer) protein SlpA (DR_2577) mutant, demonstrating the involvement of DR_146T in maintenance of cell envelope integrity. The 123 kDa SlpA was absent and only its fragments were present in the cell envelope of DR_146T mutant, suggesting that DR_146T might be involved in maintenance of the S-layer. A mutant lacking the DUF490 motif displayed only a slight alteration in phenotype compared with the wild type, suggesting DUF490 is less important than tamB motif for the function of DR_146T. These findings enhance our understanding of the properties of the multilayered envelope in extremophilic D. radiodurans, as well as the diversity and functions of TAMs in bacteria.
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