The influence of polystyrene nanoparticles on enzyme clusters of fumarate, malate dehydrogenase and citrate synthase: a fractal analysis study

Abstract

In the context that plastic debris could degrade into smaller and smaller particles especially at the nanoscale, concerns about their ecotoxicological effects are raised. The purpose of this study was to determine the biophysical effects of neutral polystyrene nanoparticles (NPs) on the activity of fumarase-malate dehydrogenase (MDH)-citrate synthase (CS) cluster both in vitro and in mussels exposed to NPs. At first, commercial preparations of fumarase, MDH, CS were examined in the presence of NPs to determine changes in the fractal environment based on time dependence MDH and CS rates and changes in the spectral dimension (sD) of the cluster. In second part, these effects were also examined in mussels exposed to either polystyrene NPs or in mussels exposed to municipal effluents, which are suspected environmental sources of plastic NPs. The results showed that the presence of NPs decreased the time dependence of MDH rate and increased significantly the sD. A sD=0.55 of the in vitro reaction revealed a close organization of the enzyme cluster, which was equivalent to a random percolation cluster with 74% of space occupancy. The addition of 50 and 100 nm in the reaction media reduced the space occupancy to 50 and 33% respectively. Exposure of mussels to 50 nm NPs for 24h lead to increased levels of NPs in the digestive gland and revealed similar effects in the digestive gland subcellular fraction where the sD was increased from 1.25 to 1.48 at the highest concentration of 5 mg/L reducing the space occupancy of 35% compared to 46% in control mussels. Exposure of mussels to a municipal effluent for 3 months also lead to increased polystyrene-like NPs in the digestive gland and increased fumarase-MDH-CS activity and the sD even further to 23% space occupancy. In conclusion, polystyrene NPs have the capacity to increase the sD of the cluster decreasing the normal time-dependence of enzyme activity of the fumarase-MDH-CS cluster thereby maintaining energy metabolism for longer times which could deplete energy reserves.