Interactions between polystyrene nanoplastics and bovine lactoferrin in simulated gastric fluids: Aggregation kinetics and impact on protein digestion

Nanoplastics (NPs) pose emerging risks to gastrointestinal health by interfering with protein digestion. This study investigated the interactions of bovine lactoferrin (BLf) with negatively charged bare (BPS) and positively charged amino-modified (APS) polystyrene NPs (PSNPs) in simulated gastric fluids (SGF). In the absence of BLf, PSNPs exhibited pH-dependent aggregation governed by surface charge: BPS aggregated most rapidly at pH 2 (fasted state), while APS reached peak aggregation at pH 3.5 (late-fed state). BLf markedly altered aggregation kinetics, with rates following the order of pH 3.5 > 2 > 5, primarily through the formation of protein coronas with BLf and pepsin that stabilized PSNPs via electrostatic and steric interactions. Increasing PSNP concentrations generally promoted aggregation, except at pH 2 where inhibition occurred. Both PSNPs suppressed BLf digestion, with digestion degree ranking pH 3.5 > 5 > 2, and BPS exerting a stronger inhibitory effect than APS. PSNPs disrupted the secondary structures of pepsin and BLf through hydrogen bonding and electrostatic interactions, impairing enzymatic activity. Isothermal titration calorimetry (ITC) revealed distinct thermodynamic driving forces: BPS bound BLf via an enthalpy-driven mechanism, whereas APS promoted co-assembly through hydrophobic effects. These findings provide mechanistic insights into NP-protein interactions in gastric environments and highlight their potential to disrupt digestive processes, informing health risk assessments of ingested NPs.