An Enzyme That Eats Plastic — Inside Your Blood

Complete monomer release: PETase fully degrades PET microplastics into TPA and MHET, as confirmed by HPLC and NMR.
Active in serum: The S238A PETase variant releases ~500 µM of monomers within 15 minutes in real human serum at 37 °C.
No cell toxicity: HEK293 cells remain fully viable across all tested concentrations and densities of PETase.
A medical proof of concept: This is the first demonstration of enzymatic microplastic depolymerization in a serum-like biological environment.

Degradation of PET microplastic particles to monomers in human serum by PETase: Lopez-Lorenzo et al., Faraday Discussions, 2024
Discovery and quantification of plastic particle pollution in human blood: Leslie et al., Environment International, 2022
A bacterium that degrades and assimilates PET: Yoshida et al., Science, 2016
Machine learning-aided engineering of hydrolases for PET depolymerization: Lu et al., Nature, 2022
A Review of Human Exposure to Microplastics and Insights Into Microplastics as Obesogens: Kannan et al., Frontiers in Endocrinology, 2021

Research from KTH, Sweden, demonstrates that a bacterial enzyme can break PET microplastics into harmless building blocks in human serum — without harming cells

Every year, humans ingest and inhale tens of thousands to millions of microplastic particles — and in 2022, researchers made it official: those particles get into our blood. A landmark study by Leslie et al. measured plastic in the blood of 17 out of 22 healthy adults, finding an average concentration of 1.6 µg/mL, with PET — the same polymer used in drinks bottles and food packaging — present in half the samples. We know these particles are circulating inside us. What we did not know until recently was whether anything could be done about it.

PET stands for polyethylene terephthalate, a plastic woven into daily life, making it hard to avoid. It is made by linking two chemical building blocks — terephthalic acid (TPA) and ethylene glycol — into long, tightly packed chains. When PET weathers and fragments into microscopic particles, those chains retain their structure, making the material stubborn and biologically inert in ways that are not well understood but may not be harmless. In 2016, Yoshida et al. discovered a soil bacterium, Ideonella sakaiensis, that produces an enzyme — PETase — capable of snipping those polymer chains apart. The enzyme works by hydrolysis, slipping water molecules into the chemical bonds that hold PET together, breaking them apart, and returning them to TPA and MHET (mono(2-hydroxyethyl) terephthalate), the same monomers the polymer is built from.

The key question for any potential medical application was whether PETase could function inside the body. Blood serum is a chemically complex environment — full of proteins, salts, antibodies, hormones, and enzymes — and enzymes that work in laboratory buffer conditions often fail in it. Researchers at KTH Royal Institute of Technology in Stockholm set out to answer this directly. In a study published in Faraday Discussions in 2024, Lopez-Lorenzo, Hueting, Bosshard, and Syrén tested an engineered variant of PETase — the S238A mutant — against microplastic-like PET particles in both simulated body fluid (SBF) buffer and real human serum, at 37 °C to mimic body temperature. The substrate they used had been pre-treated by microwave to enrich it in the crystalline, trans-conformation chains that dominate real environmental microplastics — the form that is notoriously resistant to enzymatic attack.

The enzyme works!

As shown in Panel A of figure below, the S238A PETase released measurable quantities of both TPA and MHET across a wide range of substrate concentrations in SBF buffer — from 0.25 mg/mL up to 10 mg/mL — with product release scaling with substrate load, reaching approximately 2,100 µM of TPA and over 4,100 µM of MHET at 10 mg/mL. Strikingly, the enzyme showed a clear preference for producing TPA, the more rapidly excreted and better-characterized of the two products. This selectivity matters: TPA has been shown to be non-toxic to human fibroblasts at concentrations up to 200 µg/mL and is rapidly cleared through urine in animal models.

The serum experiment, summarised in Panel C, delivered the most striking result. Starting from zero, around 500 µM of total monomers was released within the first 15 minutes of incubation, rising to approximately 850 µM by 60 minutes. The reaction was not stopped by the biological complexity of real human serum — it was catalytically competent under conditions closely resembling those in a living bloodstream. This rate of monomer release, in the authors’ assessment, is comparable to that observed in buffer, providing important proof of feasibility.

Chemical confirmation that the plastic was genuinely broken down to its constituent monomers — and not merely surface-eroded or partially digested — came from nuclear magnetic resonance (NMR) spectroscopy. Panel D shows stacked 1H NMR spectra for the products of the enzymatic reaction in the SBF buffer. After one hour with S238A PETase, a distinct new peak appears at 8.08 ppm, corresponding to the aromatic benzene ring protons of TPA — a signal absent in the no-enzyme control. A second new peak at 5.19 ppm indicates the ethylene glycol moiety of released MHET. Peaks at 4.30 ppm and 4.66 ppm correspond to inner-chain protons from residual oligomers. Together, these signals confirm that PETase cleaves PET all the way to its free monomer units, not simply nicking the surface of the particles.

A practical treatment would be useless — and dangerous — if the enzyme harmed the very cells it was meant to protect. To test this, the team exposed human HEK293 kidney cells to S238A PETase at three enzyme concentrations (1.5, 3.0, and 6.0 µg/mL) and four cell densities (1×10³ to 1×10⁶ cells per well), using an MTT viability assay. The data in Panel B show that relative cell viability remained at or above 100% across all conditions — the cells were entirely unaffected by the enzyme. This is a significant result: PETase is an esterase, a class of enzyme that can theoretically act on cell membrane lipids. The absence of any cytotoxic signal across all tested concentrations and cell densities indicates that the enzyme’s substrate specificity is sufficiently narrow to spare biological membranes, reinforcing earlier evidence that PETases are found naturally in the oral microbiota and are therefore likely to be compatible with human biology.

“Our work highlights the potential of applying biocatalysis in biomedicine and represents a first step towards finding a future solution to the problem that microplastics in the bloodstream may pose.”

— Lopez-Lorenzo et al., Faraday Discussions, 2024

This work sits within a broader effort to engineer increasingly capable PET-degrading enzymes. In 2022, Lu et al. used machine learning to design FAST-PETase, a variant capable of degrading untreated post-consumer PET almost completely within one week at temperatures below 50 °C. The KTH study complements that industrial focus by asking a different question: can this class of enzyme function inside a human body? The answer, based on the serum experiments, is yes — at least in vitro. The concentrations of microplastics used in the experiments were deliberately higher than those measured in real blood (1.6 µg/mL by Leslie et al.), meaning that the activity observed in the study represents a conservative baseline, not a ceiling. Real microplastic loads in blood would present a far easier enzymatic target.

There is still considerable distance between these results and a clinical application. The enzyme would need to be humanized to avoid immune clearance, encapsulated for delivery — perhaps within red blood cells, as some recent work on enzyme therapeutics has explored — and tested in animal models before any translation could begin. The current study did not address immunogenicity, biodistribution, or long-term stability in blood. What it does establish, convincingly, is the enzymatic principle: PETase is active in human serum at body temperature, it converts microplastic PET cleanly to monomers, and it does not harm human cells at biologically relevant concentrations. Given the mounting evidence for health consequences of microplastic exposure — including inflammation, endocrine disruption, and altered blood coagulation — an enzymatic clearance strategy deserves serious scientific attention.