Aflatoxins

Aflatoxins are a family of difuranocoumarin secondary metabolites produced principally by Aspergillus flavus and Aspergillus parasiticus. The four major variants are named for their fluorescence under ultraviolet light: AFB1 and AFB2 fluoresce blue; AFG1 and AFG2 fluoresce green. Two hydroxylated metabolites (AFM1 and AFM2) appear in the milk of mammals that have ingested aflatoxin-contaminated feed. AFB1 is classified by the International Agency for Research on Cancer as a Group 1 carcinogen.

All aflatoxins share a coumarin-fused bisfuran skeleton. AFB1 differs from AFB2 by a single 8,9-vinyl bond (saturated in B2). AFG1 and AFG2 are the lactone-ring analogs of the B series. The metabolic activation of AFB1 by hepatic cytochrome P450 (primarily CYP3A4 in humans) produces the highly reactive aflatoxin-8,9-epoxide, which forms DNA adducts and is the proximate carcinogen.

The aflatoxin-8,9-epoxide reacts with the N7 position of guanine residues in DNA to form the AFB1-N7-Gua adduct. Unrepaired adducts produce G→T transversions, including the signature 249ser TP53 mutation observed in aflatoxin-associated hepatocellular carcinoma. Aflatoxins are also genotoxic in non-hepatic tissues at sufficient exposure.

Mechanism description here is cellular only; no clinical interpretation. The chemistry is the same regardless of exposure route, but indoor inhalation versus dietary ingestion have very different absorption, distribution, and metabolism profiles and the carcinogenicity data underpinning IARC's Group 1 classification is overwhelmingly dietary.

Aflatoxin biosynthesis requires water activity A_w ≥ 0.85, higher than the threshold for A. flavus / A. parasiticus growth itself^[1]. Production optimum is A_w 0.95–0.99 and temperatures of 25–37 °C^[2]. The principal substrates are starchy materials: peanuts, corn, cottonseed, tree nuts. Production indoors is comparatively poorly characterized; substrate-DNA detection of producer organisms does not establish that biosynthesis was active.

Aflatoxins are among the most studied food-contamination mycotoxins globally. Principal commodities are peanuts, corn (maize), cottonseed, tree nuts (Brazil nuts, pistachios, almonds), and dried fruits and spices. Warmer climates and drought-stressed crops have higher prevalence. Most national food-safety systems test for aflatoxin in these commodities at action levels in the parts-per-billion range.

IARC carcinogen classification: Group 1 (carcinogenic to humans). AFB1 specifically is classified as Group 1; the four major aflatoxins are classified together as Group 1 by mixture^[3].

Aflatoxins share a biosynthetic pathway with sterigmatocystin. Sterigmatocystin is the penultimate biosynthetic intermediate, and the same gene cluster produces both. A. versicolor's sterigmatocystin production indoors is therefore biosynthetically related to A. flavus's aflatoxin production, though the species and indoor contexts differ. See sterigmatocystin.