Photoaging amplified the inhibition of microplastics on salivary amylase function via affecting the structure of α-helix: From in vitro digestive models to human samples

• Photoaged microplastics (MPs) selectively inhibited salivary α -amylase (AMS) activity. • AMS inhibition by aged MPs was consistent in simulated body fluids, cell models, and human samples. • Children’s AMS exhibited significantly greater susceptibility to MPs than adults. • Photoaging enhanced MPs’ binding to and disruption of the AMS α -helical structure. Microplastics (MPs), as ubiquitous environmental contaminants, inevitably undergo photoaging processes. Critically, the predominant route of human exposure to environmental MPs is oral ingestion, thereby establishing the digestive system as their key target site. However, the effects and underlying mechanisms of photoaged MPs on different types of key digestive enzymes were still unclear. In this study, significant physicochemical properties alterations, including surface crumpling, detachment, and the generation of more oxygen-containing functional groups, were observed in polystyrene and polylactic acid MPs after UVA or UVC-induced photoaging. Compared with pristine MPs, photoaged MPs selectively inhibited α -amylase (AMS) activity by nearly 50% at the estimated maximum realistic human exposure concentration (700 μg/mL), without affecting the protein expression of AMS. In contrast, neither pristine nor photoaged MPs exerted significant effects on the activities of pepsin, lipase, or trypsin. Moreover, human samples revealed that the inhibition rates of AMS activity in healthy children (aged 0–6 years) were 1.5-fold greater than those observed in healthy adults (aged 18–35 years), demonstrating a higher susceptibility to photoaged MPs in children, which may be associated with physiological development or exposure levels. Mechanistic analysis further demonstrated that new byproducts generated following photoaging might be more prone to bind to α -helical domains, resulting in a reduction of α -helix content and consequently disrupting the function of AMS. Our findings established a structure–activity relationship between photoaged MPs and dysfunction of digestive enzymes, which may provide novel insights into understanding the relationships between MP exposure and the onset/progression of diseases in the digestive system, as well as disorders related to sugar metabolism.