Agricultural and Food Chemistry
Pyrrolizidine Alkaloids in Honey
by Beth Ashby Mitchell
August 1, 2016
Plants and animals often come equipped with marvelous methods for self-defense. Skunks, rattlesnakes, cacti, and poison ivy are just a few self-evident examples. Some plants, however, harbor a subtle chemical defense against herbivores: pyrrolizidine alkaloids (PAs). The structure of a relatively simple PA, retronecine, is shown in Figure 1.
In 1981, L. W. Smith and C. C. J. Culvenor at CSIRO (Parkeville, Australia) reported that PAs had been identified in more than 6000 plants, including about 3% of the world’s flowering plants. (J. Nat. Prod. DOI: 10.1021/np50014a001) For resources on the characterization and decomposition of PAs, see “Complexity of Pyrrolizidine Alkaloids”.
Alas, PAs are not toxic just to insect or small animal herbivores. Plants that contain PAs can affect the livestock industry, particularly horses and cattle. Typically, pyrrolizidine alkaloidosis is a long-term poisoning that results in liver failure.
PAs are also passed down the human food chain from various sources; certain herbal teas and honey contain large amounts of PAs. Long-term consumption of low levels of PAs in food can lead to liver cirrhosis and cancer. A 2002 study of honey from PA-containing plants by John A. Edgar* at CSIRO Livestock Industries (Geelong, Australia), Erhard Roeder at the University of Bonn (Germany), and Russell J. Molyneux at the USDA Western Regional Research Center (Albany, CA) revealed some of the potential threats to human health. (J. Agric. Food Chem. DOI: 10.1021/jf0114482)
Pollen or nectar? That is the question
Concern about the levels of PAs in honey has led to research in Australia, New Zealand, Germany, and Switzerland. The German Federal Institute for Risk Assessment (BfR) recommends an intake of not more than 7 ng of 1.2-unsaturated PAs per kg bodyweight per day. Studies of monofloral honey have detected PA concentrations up to 2 orders of magnitude above the BfR limit from Echium vulgare, E. plantagineum, and Senecio jacobaea. (See references in Kast, et al. below.)
Bees feeding on these plants collect pollen and nectar, but it wasn’t known which of these actually carries the PAs to the honey. Previous studies focused on pollens that are known carriers of PAs. The authors of the 2005 Colegate investigation listed in the reference box studied pure pollen from E. vulgare to determine whether it was the potential source of PAs in honey from that plant. They found high levels of echivulgarine PAs in the pollen and pointed to it as the possible source of PAs in honey.
Complexity of Pyrrolizidine Alkaloids
PAs are not simple entities, and only some of the components are toxic. The N-oxides have been known culprits for a long time. Here are a few references to studies that identify and analyze PAs.
- Pyrrolizidine Alkaloids from Echium setosum and Echium vulgare (1996). A. El-Shazly, M. Wink, et al. J. Nat. Prod. DOI: 10.1021/np9600661
- Solid-Phase Extraction and LC−MS Analysis of Pyrrolizidine Alkaloids in Honeys (2004). Steven M. Colegate, et al. J. Agric. Food Chem. DOI: 10.1021/jf049102p
- Pyrrolizidine Alkaloids of Echium vulgare Honey Found in Pure Pollen. (2005) Michael Boppré, Steven M. Colegate*, and John A. Edgar. J. Agric. Food Chem. DOI: 10.1021/jf0484531
- Structure of Echivulgarine, a Pyrrolizidine Alkaloid Isolated from the Pollen of Echium vulgare (2015). Robert Keyzers, et al. J. Agric. Food Chem. DOI: 10.1021/acs.jafc.5b02402
- Lysine Adduction by Reactive Metabolite(s) of Monocrotaline (2016). Ying Peng, Jiang Zheng, et al. Chem. Res. Toxicol. DOI: 10.1021/acs.chemrestox.5b00488
In 2010, T. Beuerle and co-workers at the University of Würzburg (Germany) reported that “It could clearly be demonstrated that the PA content of honey was directly proportional to the amount of PA pollen in honey and that the transfer of PAs from pollen to honey was a rather quick process. Consequently, PA pollen represents a major source for the observed PA content in honey.” (Food Addit. Contam.: Part A DOI: 10.1080/19440049.2010.521771)
Honey, however, is made up mostly of nectar with only traces of pollen. Christina Kast and coauthors at the Swiss Bee Research Center (Bern), the University of Neuchâtel (Switzerland), and Quality Services International GmbH (Bremen Germany) studied nectar and pollen to determine whether the PA content of nectar is high enough to contaminate honey. The group conducted qualitative and quantitative analyses of honey derived from E. vulgare plants in two distinct regions near the Alps.
Using LC–MS/MS, the group detected six types of PAs or PA-N-oxides in the honey samples; they calculated the PA concentration as the sum of the six. The nectar and pollen were tested separately for PA concentration. The pollen contained >500 times as much PA as the nectar.
They then compared the proportions of the Echium-type PAs in the honey, nectar, and pollen. In the nectar, half of the PAs were echimidine and its N-oxide. In the pollen, 63% of the PAs were the echivulgarine type, supporting Colegate et al.’s findings.
Taking the analysis one step further, the authors analyzed “ripened” honey—honey allowed to age so that its water can be extracted. During the ripening process, the nectar concentration increased by ≈4:1, and the pollen concentration was diluted by at least 1:5000. The authors concluded that the PAs in honey are derived primarily from the nectar, and not the pollen.
This presents a dilemma. Pollen can be filtered out of honey, but filtration will not remove the PAs in nectar.
No simple solution
Kast and her group suggest that to reduce the contaminants in honey, apiaries should not be placed near PA-producing forage plants. Other authors agree. The PAs that protect the plants from herbivores are transmitted through the plants to other creatures, and for humans and livestock that spells trouble. (J. Agric. Food Chem. DOI: 10.1021/acsjafc.6b02320)
Honey: What’s Not to Love?
Humans have been using honey for thousands of years. An ≈8000-year-old cave painting (Figure 2) shows a figure clinging to three vines to retrieve honey from a cliffside hive. Archaeological records show that the Egyptians used honey for embalming and medicinal purposes as well as a sweetener. The Israelites were led to a land rich in milk and honey.
Today, many naturalists promote honey as an almost perfect substance with numerous benefits that include enhancing athletic performance, healing burns, and curing ulcers, as well as the more traditional uses to soothe sore throats and provide relief for colds and flu. The Mayo Clinic lists many of the claims about honey, along with notes about the evidence to support those claims. Researchers are investigating the antibacterial aspects of Manuka honey from New Zealand for multiple uses.