Patulin

Patulin is an unsaturated lactone (4-hydroxy-4H-furo[3,2-c]pyran-2(6H)-one) produced principally by Penicillium expansum on damaged pome fruit (apples, pears, peaches). The molecule is highly reactive with thiol groups in proteins and depletes intracellular glutathione. P. expansum's indoor mycotoxin relevance has been strengthened by Salo et al. 2019 documenting strain CBS 145620 growing on gypsum board with guttation droplets containing chaetoglobosins and communesins, though notably, that specific study did not measure patulin in the droplets. Patulin biosynthesis on building substrate remains less directly characterized than its well-documented production on fruit.

Patulin's reactive feature is the conjugated enone system. The α,β-unsaturated carbonyl is electrophilic and forms covalent adducts with thiol nucleophiles via Michael addition, particularly cysteine residues in proteins and the free thiol of glutathione. This thiol reactivity is the molecular basis for the molecule's toxicity.

Patulin depletes glutathione via Michael addition and alkylates protein sulfhydryl groups, which disrupts cellular ion transport (particularly Na⁺/K⁺-ATPase), inhibits multiple enzymes containing essential cysteines, and at sufficient exposure produces apoptosis. Mechanism description is cellular only.

Optimal production at A_w ≥ 0.99; growth (without significant production) occurs at lower water activity^[1]. Patulin production peaks at 16 °C, notably below P. expansum's growth optimum of ~24 °C, which means cool indoor environments (basements, crawl spaces, refrigerated storage) favor toxin production relative to biomass accumulation^[2]. Principal substrates are damaged stored pome fruits. Indoor production on gypsum board has been documented at the substrate level for P. expansum (Salo et al. 2019), though specific patulin quantification on building substrate is not yet established in primary literature.

Apple juice and apple products (the principal regulatory concern globally) and damaged stored pome fruits. Indoor relevance is emerging via P. expansum on gypsum board: Salo et al. 2019 documented strain CBS 145620 growing on gypsum board indoors and producing guttation droplets, though that study measured chaetoglobosins and communesins specifically and did not measure patulin. Direct patulin emission from indoor building substrate is therefore inferred (the species is a well-established patulin producer on fruit) rather than directly measured. Tannous et al. 2016 separately established the physicochemical parameters for patulin production (peak at 16 °C), supporting that cool indoor conditions favor toxin biosynthesis.

IARC carcinogen classification: Group 3 (not classifiable as to human carcinogenicity)^[3].

Patulin is not directly biosynthetically related to other indoor mycotoxin families. Within P. expansum's metabolite profile, it co-occurs with chaetoglobosins, communesins, and roquefortine C, but those share producer organism rather than biosynthesis.