Do Polystyrene Beads Contribute to Accumulation of Methylmercury in Oysters?

To clarify whether microplastics contribute to elevated bioaccumulation of methylmercury (MeHg) in aquatic organisms, we studied the sorption pattern of MeHg on polystyrene beads (PBs) and evaluated MeHg accumulation, via uptake of MeHg-adsorbed PB, in the oyster Crassostrea gigas. MeHg-cysteine conjugates were added to seawater at 10, 100, and 1000 µg/L as Hg. Polystyrene beads (φ = 0.02, 0.2, and 2 µm) were immersed in the seawater for 24 h. The concentrations of total mercury (T-Hg) adsorbed onto the PBs were then measured using the reduction vaporization method. T-Hg concentrations for the PBs with diameters of 0.02, 0.2, and 2 µm were 10.6 ± 0.4, 1.8 ± 0.1, and 1.3 ± 0.1 ng/mg-PBs, respectively, when immersed in 2 mL of MeHg-added seawater (100 µg/L as Hg). Thus, the adsorption efficiency of MeHg onto PBs was higher in the presence of smaller diameter PBs. Next, 1 mg of PBs immersed in 2 mL of seawater containing 100 µg/L of MeHg for 24 h was added to an oyster tank containing 1 L of seawater. The T-Hg concentration of the oysters was measured after 6 h of exposure. No significant difference was found in the T-Hg concentration of oysters in the presence of PBs (0.30 ± 0.01 to 0.37 ± 0.05 ng/mg as dry weight) with MeHg and in the absence of PBs (0.36 ± 0.03 ng/mg as dry weight). Our results suggest that the presence of PBs in seawater has little effect on MeHg uptake by oysters.

Bioactive polymeric metallosomes self-assembled through block copolymer-metal complexation.

Spontaneous formation of polymeric metallosomes with uniform size (~100 nm) was found to occur in aqueous medium through the reaction of an anticancer agent, (1,2-diaminocyclohexane)platinum(II) (DACHPt), with a Y-shaped block copolymer of ω-cholesteroyl-poly(L-glutamic acid) and two-armed poly(ethylene glycol) (PEGasus-PLGA-Chole). Circular dichroism spectrum measurements revealed that the PLGA segment forms an α-helix structure within the metallosomes, suggesting that secondary-structure formation of metallocomplexed PLGA segment may drive the self-assembly of the system into vesicular structure. These metallosomes can encapsulate water-soluble fluorescent macromolecules into their inner aqueous phase and eventually deliver them selectively into tumor tissues in mice, owing to the prolonged blood circulation. Accordingly, fluorescent imaging of the tumor was successfully demonstrated along with an appreciable antitumor activity by DACHPt moieties retained in the vesicular wall of the metallosomes, indicating the potential of metallosomes as multifunctional drug carriers.

Interaction of a magainin-PGLa hybrid peptide with membranes: insight into the mechanism of synergism.

The antimicrobial peptides magainin 2 and PGLa isolated from the skin of the African clawed frog Xenopus laevis show marked functional synergism. We have proposed that the two peptides form a heterodimer composed of parallel helices with strong membrane permeabilizing activity [Hara, T., Mitani, Y., Tanaka, K., Uematsu, N., Takakura, A., Tachi, T., Kodama, H., Kondo, M., Mori, H., Otaka, A., Fujii, N., and Matsuzaki, K. (2001) Biochemistry 40, 12395-12399]. In this study, to elucidate the molecular mechanism of the synergy, we synthesized a chemically fixed heterodimer and investigated in detail the interaction of the hybrid peptide with bacteria, erythrocytes, and lipid bilayers. The hybrid peptide showed antimicrobial activity and membrane permeabilizing activity against negatively charged membranes, similar to or even stronger than those of a physical equimolar mixture of magainin and PGLa, indicating that the synergy is due to the formation of a parallel heterodimer. The heterodimer assumed a more oblique orientation than the component peptides. In contrast, the cross-linking of the two peptides significantly strengthened the action against erythrocytes and zwitterionic lipid bilayers by enhancing the affinity for membranes without changing the basic mode of action. Thus, the separate production of mutually recognizing peptides without cross-linking appears to be a good way to increase selective toxicity.