Pyrrolizidine alkaloids (PA) – food safety at risk?

Pyrrolizidine alkaloids are currently making the headlines in Germany ("Death caused by herbal tea" (DAZ, 04/06/2009), "Poisonous leaves in rocket salad" (Spiegel online, 19/08/2009)) due to isolated cases of ragwort (Senecio) being found in leaf vegetables (e.g. rocket). People have been aware of the fact that certain foodstuffs may be contaminated by pyrrolizidine alkaloids for some time now; one example being honey from particular parts of the world.

Pyrrolizidine alkaloids are currently making the headlines in Germany ("Death caused by herbal tea" (DAZ, 04/06/2009), "Poisonous leaves in rocket salad" (Spiegel online, 19/08/2009)) due to isolated cases of ragwort (Senecio) being found in leaf vegetables (e.g. rocket). People have been aware of the fact that certain foodstuffs may be contaminated by pyrrolizidine alkaloids for some time now; one example being honey from particular parts of the world.

While restrictive limits had already been defined for herbal medicinal products as early as the beginning of the 1990s (see below), limits have only been imposed in isolated cases in the food sector so far. As far as analysis is concerned, pyrrolizidine alkaloids present a special challenge by virtue of their structural diversity and the low concentrations and complex matrix of composite foodstuffs in which they are found.

Numerous exponents of the Asteraceae daisy family (e.g. the Senecio species, familiarly known as ragwort) contain pyrrolizidine alkaloids (PAs) in not inconsiderable quantities, as do Boraginaceae and Fabaceae. Pyrrolizidine alkaloids and their N-oxides have also been found in medicinal plants, however, including comfrey (Symphytum officinale), butterbur (Petasites) and coltsfoot (Tussilago farfara). Such pyrrolizidine alkaloids as senecionine, seneciphylline, retrorsine, lycopsamine, lycopsamine N-oxide and echimidine may also be found in honey, depending on where the honey is collected and the plants on which the bees feed. The Echium, Senecio and Borago genera are the primary contributors towards the PA contamination of honey. High PA levels are known to occur in corn harvested in Afghanistan and Ethiopia, caused by a proliferation of plants of the Heliotropium genus in corn fields. The recent cases of rocket contamination with ragwort, which is known to contain PAs, in Germany gave rise to postulations of an increased risk of such contamination as a consequence of the intensified propagation of Senecio species favoured by global warming. We are unable to make a conclusive assessment of this scenario at the moment. In this respect, sensitising the parties involved in cultivating vegetables and herbs can bring about a pronounced improvement in safety and security. The conspicuous nature of Senecio species makes them easy to identify in most cultures and they can therefore be controlled effectively by means of suitable fostering strategies.

In chemical terms, a pyrrolizidine alkaloid is an ester alkaloid with a pyrrolizidine skeleton. A distinction is made between the retronecine / heliotridine type and the otonecine type (Figure 1). The combination of non-toxic necines with necine acids gives rise to a very large number of possible alkaloids, whereby around 200 of these have been discovered and their structures explained so far.

Some pyrrolizidine alkaloids, those with a double bond at the 1,2 position of the pyrrolizidine ring, have hepatotoxic, mutagenic and carcinogenic effects. Typical examples of these are echimidine, senecionine and senkirkine, which are also characterised by the presence of an allyl ester in the side chain. Following ingestion and resorption in the intestine, most of the alkaloids get into the liver cells, where they are metabolised to produce pyrrolopyrrolidine derivatives, the actively toxic species, which react with the nucleophile groups of proteins and enzymes, as well as DNA or RNA.

Germany's graduated pharmacovigilance plan [Stufenplan zur Abwehr von Arzneimittelrisiken] drawn up in 1992 states that herbal medicinal products may only be introduced into the market if measures have been taken to ensure that the maximum daily intake of toxic pyrrolizidine alkaloids is within specific limits (German Federal Gazette (BAnz.) No.: 111, page 4805, dated 17/06/1992):

-       100 µg per day for external administration
-       1 µg per day for internal administration
-       10 µg per day when using coltsfoot leaves as an infusion

Medicinal products containing pyrrolizidine alkaloids should not be taken for longer than 4 to 6 weeks per year.

Although no limits have yet been defined for PAs in food at EU level, refined echium oil is one exception, with 0.4 µg/kg as the specified limiting value (Commission Decision of 27 June 2008; 2008/558/EC). While this cannot be applied to other foods, it may provide a point of reference for the evaluation of positive PA findings in other foodstuffs, allowing for the assumed quantities consumed.

Suitable analytical methods, which can be used to detect even traces of pyrrolizidine alkaloids, include GC, GC-MS, HPLC and HPLC-MS. The most interesting examples of these PAs are the highly toxic senecionine, senkirkine, seneciphylline, retrorsine and monocrotaline, followed by lasiocarpine and echimidine.

PhytoLab can offer assay detection of pyrrolizidine alkaloids and their N-oxides, calculated as senecionine or senkirkine, in your herbs and extracts by means of GC-MS. Our highly effective method registers all of the toxic pyrrolizidine alkaloids by detecting the characteristic 1,2-unsaturated skeleton. This method is acknowledged by the authorities within the framework of the registration and marketing authorisation procedures for medicinal products.

Our phyproof® reference substances offer you a broad spectrum of suitable primary standards for your analytical procedures. Apart from senecionine, senkirkine, retrorsine and monocrotaline, our range also includes seneciphylline, heliotrine, lycopsamine and echimidine. We would be happy to submit a quotation for these reference substances or for the determination of pyrrolizidine alkaloids in your samples.

Bibliography:

Nuhn, P. Naturstoffchemie, 4. Auflage, S. 79f./591, 2006, S. Hirzel Verlag Stuttgart

DAZ, 04.06.2009

Spiegel online, 19.08.2009

Röder, E. Pharmazie in unserer Zeit, 13. Jahrg. 1984, Nr. 2, Verlag Chemie Weinheim

Prakash, A. S. et al. Mutation Research 1999, 443, 53-67

Pharmazeutische Zeitung Nr. 32, 137. Jahrg., 1992

Edgar, J.A. J.Agric. Food Chem. 2002, 50, 2719-2730

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