Transformation of biodegradable polylactic acid microplastics in a simulated human gastrointestinal system: key role of pepsin and trypsin

Human exposure to microplastics (MPs) has raised growing concern, yet the transformation behavior of MPs in the gastrointestinal tract remain unclear. To address this knowledge gap, we investigated the aging of polylactic acid (PLA) MPs in a simulated human gastrointestinal system. After 96 h incubation, the O/C ratio of PLA MPs increased from 0.497 to 0.614 in gastric fluid and 0.602 in intestinal fluid, evidencing the degradation and depolymerization behavior of PLA MPs during prolonged retention in the gastrointestinal system. In addition, XRD intensity of PLA MPs at 16.5° decreased by 25.76% in the enzyme-containing system, which was greater than the 17.63% reduction in the enzyme-free electrolyte solution and similar as the trend in intestinal fluid. This suggests that pepsin in gastric fluid and trypsin in intestinal fluid were key contributors to the PLA MPs aging in simulated human gastrointestinal system. Furtherly, molecular docking and molecular dynamic analysis revealed that both pepsin and trypsin can form stable complexes with PLA through hydrogen bonds and hydrophobic interactions, thereby promoting chain scission and fragmentation. These findings provide essential insights into the transformation behavior and mechanism of biodegradable MPs in the simulated human gastrointestinal system.