## 2 Natural Toxic Plant Alkaloids with Relevant Occurrence in Food
### Origin, Toxicity and Occurrence of Pyrrolizidine Alkaloids (PAs)  
Pyrrolizidine alkaloids (PAs) and their \(N\)-oxides (PANOS) are toxic secondary metabolites of plants synthetized as a defense against insects and herbivores. It has been estimated that these alkaloids may occur in more than 6000 plant species worldwide, representing 3-5% of all flowering plants. Accordingly, the main PA-producing plants are the ones belonging to five main families: Asteraceae (tribes Senecioneae and Eupatorieae), Fabaceae (genus _Crotalaria_), Boragnaceae, Orchidaceae and Apocynaceae [19]. The content of PAs in these plants is highly variable and depends on many factors, such as: the species, the plant organ, growing conditions, harvesting, storage, etc. The intake of PAs/PANOs involves a potential health risk because they can possess either acute or chronic toxic effects [20]. In accordance, the structure of PAs is an important prerequisite for their toxicity. PAs have a common backbone chemical structure, so-called necine, which includes two pyrrole rings fused to a nitrogen heteroatom at position 4 (Figure 2a). This necine part can be esterified with miscellaneous acids, leading to an acidic part called necic acid (Figure 2a). Depending on the type of necic acid bound to the necine base, many different structures can arise in nature. Accordingly, more than 600 different structures for PAs and PANOs are currently known [19]. Moreover, the necine base can either have one unsaturation in the 1,2-position or be saturated, leading to two main groups of PAs: 1,2-unsaturated PAs and saturated PAs, respectively (Figure 2a). In addition, depending on the esterification of one or both hydroxyl groups, 1,2-unsaturated PAs can present monoester, open chain diester and macrocyclic type forms. Figure 1: (a) Evolution in the number of food alerts that have noted high levels of pyrrolidine alkaloids (PAs), propane alkaloids (TAs) and opium alkaloids (OAs), from 2012 to 2022; Distribution of food alerts related to (b) PAs, (c) TAs and (d) OAs based on the food type contaminated (data obtained from Rapid Alert System Feed and Food (RASFP) portal, 2022 [b]). Figure 2: **(a)** Common chemical structure of pyrrolidine alkaloids and different forms for pyrrolidine alkaloids (R1 and R2 correspond to different neck acids). (b) General common chemical structure of tropane alkaloids (TAs), and chemical structure of the main TAs: atropine and scopolamine: (c) Chemical structures of the two families of optimum alkaloids (phenanthrenes and benzylisoquinolines). The unsaturation in the necine base increases the toxicity of these alkaloids. Hence, saturated PAs are not considered toxic compounds, as they do not undergo metabolic activation in the body to exert toxicity. In contrast, 1,2-unsaturated PAs are metabolically activated into highly reactive pyrrole intermediates that act as alkylating agents of proteins and nucleic acids forming cellular adducts, which are responsible of the hepatotoxic, hepatocarcinogenic and genotoxic effects associated with these alkaloids [6; 21; 22; 23]. Acute PA intoxication can produce abdominal pain, nausea, vomiting, ascites, fever, diarrhea, oedema and jaundice [18]. On the other hand, long-term exposure to PAs is mainly associated with hepatic veno-occlusive disease (HVOD), which involves obstruction of the small veins in the liver with the sudden onset of hepatomegaly and ascites, leading to liver cirrhosis and even liver failure [24]. Moreover, it can also affect the lungs (pulmonary hypertension) or the cardiovascular system (cardiac right ventricular hypertrophy) [21]. Furthermore, high doses of unsaturated PAs can produce fetotoxic and teratogenic effects [18]. In addition, the International Agency for Research on Cancer (IARC) has classified PAs as \"possibly carcinogenic to humans\" (category 2 B), particularly lasiocarpine, riddelline and monocotraline, because of their capacity to bind to DNA and cellular proteins [6; 18; 21].