Recent trends and important developments in propolis research

Bees have been in existence for >125 million years and their evolutionary success has allowed them to become perennial species that can exploit virtually all habitats on Earth. This success is largely because of the chemistry and application of the specific products that bees manufacture: honey, beeswax, venom, propolis, pollen and royal jelly. As the most important ‘chemical weapon’ of bees against pathogenic microorganisms, propolis has been used as a remedy by humans since ancient times. It is still one of the most frequently used remedies in the Balkan states (1), applied for treatment of wounds and burns, sore throat, stomach ulcer, etc.

For this reason, propolis has become the subject of intense pharmacological and chemical studies for the last 30 years. As a result, much useful knowledge has been gathered. However, it is important to note that in the last decade, the paradigm concerning propolis chemistry radically changed. In the 1960s, propolis was thought to be of very complex, but more or less constant chemistry, like beeswax or bee venom (2,3). In the following years, analysis of numerous samples from different geographic regions led to the disclosure that the chemical composition of bee glue is highly variable. This circumstance was soon understood by seasoned chemists, such as Popravko (4) and Ghisalberti (5). Nevertheless, most of the scientists studying the biological properties of propolis continued to assume that the term ‘propolis’ was as determinative with respect to chemical composition as the botanical name for a medicinal plant. Numerous studies, carried out with the combined efforts of phytochemists and pharmacologists, led in recent years to the idea that different propolis samples could be completely different in their chemistry and biological activity.

To understand what causes the differences in chemical composition, it is necessary to keep in mind the plant origin of propolis. For propolis production, bees use materials resulting from a variety of botanical processes in different parts of plants. These are substances actively secreted by plants as well as substances exuded from wounds in plants: lipophilic materials on leaves and leaf buds, gums, resins, latices, etc. (6). The plant origin of propolis determines its chemical diversity. Bee glue's chemical composition depends on the specificity of the local flora at the site of collection and thus on the geographic and climatic characteristics of this site. This fact results in the striking diversity of propolis chemical composition, especially of propolis originating from tropical regions.

Nowadays, it is well documented that in the temperate zone all over the world, the main source of bee glue is the resinous exudate of the buds of poplar trees, mainly the black poplar Populus nigra (7). For this reason, European propolis contains the typical ‘poplar bud’ phenolics: flavonoid aglycones (flavones and flavanones), phenolic acids and their esters (8). Poplar trees are common only in the temperate zone; they cannot grow in tropical and subtropical regions. For this reason, in these habitats, bees have to find other plant sources of propolis to replace their beloved poplar. As a result, propolis from tropical regions has a different chemical composition from that of poplar type propolis. In the last decade, Brazilian propolis attracted both commercial and scientific interest. The main source of Brazilian bee glue turned out to be the leaf resin of Baccharis dracunculifolia (9,10). Among the main compound classes found in Brazilian propolis are prenylated derivatives of p-coumaric acid and of acetophenone. Diterpenes, lignans and flavonoids (different from those in ‘poplar type’ propolis) have also been found (9). However, in Brazil, several types of propolis were registered in recent studies (11,12), that come from plant sources different from B.dracunculifolia and containing compounds other than those mentioned above. Recently the chemistry of Cuban propolis caught the attention of scientists. Its main components are polyisoprenylated benzophenones, and this makes Cuban propolis different from both European and Brazilian bee glue. The plant source of this propolis type was detected to be the floral resin of Clusia rosea, from whence came the prenylated benzophenones (13). There is no doubt that in other ecosystems, propolis plant sources and the chemical composition of propolis will continue to surprise scientists.

The distinct chemistry of propolis from different origins leads to the expectation that the biological properties of different propolis types will be dissimilar. However, in most cases. this is not true! Actually, propolis is the defense of bees against infections, and the antibacterial and antifungal activity of all samples is not surprising.

Bankova, V. (2005). Recent trends and important developments in propolis research. Alternative Medicine. 2(1): 29-32.

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