Boswellia

Al-Harrasi, Ahmed, Csuk, Rene, Khan, Ajmal & Hussain, Javid, 2019, Distribution of the anti-inflammatory and anti-depressant compounds: Incensole and incensole acetate in genus Boswellia, Phytochemistry 161, pp. 28-40 : 29-30

2.1. Genus Boswellia as a major source for incensole and its acetate

Resins obtained from a few species belonging to the genus Boswellia are considered the major source for incensole and its acetate. This genus possesses a huge structural diversity and it is the sole source for the medicinally important boswellic acids ( Al-Harrasi et al., 2018a,b).

Taxonomy-wise, the genus Boswellia (named after a Scottish botanist named John Boswell) belongs to the Burseraceae family of approximately 700 species originating from 18 genera ( Langenheim, 2003; Watson, 1992). The genus Boswellia has 21 known species but according to Eslamieh there are only 19 different species, and the other appear to be synonyms of the original species ( Eslamieh, 2017). Their distribution stretches from the Ivory coast to the Horn of Africa, from Yemen to Iran, Pakistan and India ( Eslamieh, 2017). These species are summarized in Table 1 .

In south Arabia there are eight species. It is worthwhile mentioning that the island of Socotra in Yemen hosts seven rare species of this important genus ( Table 1 ) whereas Boswellia sacra is the only species available in Oman. Fig. 2 depicts some pictures of the different Boswellia trees growing in Oman and Yemen.

Frankincense is obtained by careful incisions made in the trunk of the tree to allow the milky substance to ooze out which then gets hardened by its exposure to air ( Fig. 3 ) ( Al-Harrasi et al., 2018a,b).

Boswellia trees produce resins known as frankincense which varies significantly in its physical ( Fig. 4 ) and chemical properties. This variation is reflected on its commercial value and is also seen in different grades of the same species. The content of the secondary metabolites in these resins also varies significantly. While some species are known to have more monoterpenes, others are characterized by their abundance of diterpenes or triterpenes. In this genus, Boswellia elongata from Socotra in Yemen and Boswellia papyrifera growing in Ethiopia, Eritrea and Sudan are characterized with the high content of incensole (1) and incensole acetate (2).

Despite the relatively small number of species in genus Boswellia , taxonomic controversy was clearly observed in several literature reports. The debatable argument about the synonymy of Boswellia sacra (syn B. carteri ) ( South Arabia) and Boswellia carteri (Africa) was well pronounced and evidenced. While some reports regarded the two species as the same ( Culioli et al., 2003), others suggested they are two distinct species ( Al-Harrasi and Al-Saidi, 2008; Al-Harrasi et al., 2013; Hamm et al., 2005; Thulin and Warfa, 1987; Woolley et al., 2012). Based on the recent publications ( DeCarlo et al., 2018; Eslamieh, 2017; Niebler and Buettner, 2016; Paul et al., 2012) and the clear differences in morphology and chemical profile of both the oil and resins, we now consider Boswellia carteri and Boswellia sacra as the same species but differing in their habitat. Boswellia carteri is native to Africa and Boswellia sacra is native to South Arabia ( Oman and Yemen).

The argument to consider Boswellia microphylla to be a variety of Boswellia neglecta , and Boswellia popoviana to be synonymous to Boswellia nana based on ambiguous analysis lacks supported evidences. We adopt here Eslamieh's opinion ( Eslamieh, 2017) by considering all the four species as independent, and they will be treated in this review as such.

Another confusion in chemistry literature was seen when Boswellia papyrifera was referred to as Boswellia carteri ; this finding was unambiguously disproved by isolating chemotaxonomic markers (Paul, 2012).

Incensole (1) was first isolated by Corsano and Nicoletti (1967) in 1967 from plant material which was thought to be from Boswellia carteri . Recent studies suggested the species of Boswellia used by Corsano was in fact Boswellia papyrifera . Ten years later, Obermann (1977) reached a completely different conclusion with regard to the Boswellia species that Corsano and Nicoletti had used. Obermann and his team have studied two essential oils of Boswellia named as “Aden” and “ Eritrea ” during which he concluded that “Aden” type is mainly the fraction of neutral triterpenes published by Snatzke and Vértesy (1967). Furthermore, the GC-MS analysis revealed that incensole (1) was detected in infinitesimal amounts in the “Aden” type. In contrast, 1 and its acetate (2) were found in larger amounts in the “ Eritrea ” type. Interestingly, the percentage of incensole (1) found in the sample was comparable to that published by Corsano and Nicoletti (1967). Moreover, Obermann (1977) found commercial “Aden” type to be similar to the commercial grade of Somalian Boswellia carteri . Another supporting evidence proposed by Obermann was his observation that the odor of the commercial Somalian frankincense resembles to a great extent the “Aden” type whereas “ Eritrea ” type differed significantly. A similar conclusion was drawn by Paul (2012) in his chemotaxonomic investigation on resins of the different frankincense species. These observations have unambiguously concluded that the “Aden” type analyzed by Obermann was in fact a resin from Boswellia carteri or Boswellia sacra given the fact that they both belong to the same species but differ in their habitat. We - in this review - support the hypothesis proposed by Paul (2012) that the “Aden” type could have been transported from Somalia to the port of Aden in Yemen which lead to its trade name “ Aden ”. Furthermore, the resin used by Corsano and Nicoletti from which they have isolated incensole is the “ Eritrea ” type which appears to be Boswellia papyrifera . This conclusion is evidenced by several published reports ( Camarda et al., 2007; Hamm et al., 2003; Obermann, 1977). Alemika ( Alemika et al., 2004) isolated incensole from stem bark of Boswellia dalzielii collected in Nigeria.

Figures

Fig. 2. Boswellia trees from Oman and Yemen.

Fig. 3. Frankincense oozes out of Boswellia sacra tree in Dhofar, Oman. Pictures are from the authors' lab.

Fig. 4. Resins of the two main grades of Boswellia sacra Hougarai grade (mountainous) a and Sha'abi grade (coastal) b, Boswellia papyrifera c, Boswellia elongata d, Boswellia serrata e, Boswellia socatrana f. Pictures are from the authors' lab.