Bioactive compounds of the Opuntia monacantha fruit

Abstract Opuntia monacantha is a Cactaceae found in regions with few nutrients and water. This study aimed to identify bioactive compounds in the fruit of O. monacantha in the southern region of Brazil. In the fruit of O. monacantha, five phenolic compounds were identified: protocatechuic acid, ferulic acid, p-coumaric acid, catechin and quercetin. The ferulic acid and catechin were found in higher concentration in the fruit. The terpenes α-amyrin, β-amyrin, oleanolic acid and cycloartenol were also identified in the hexane fractions of the fruit. Unsaturated fatty acids were found in the fruit seed, with the highest concentration of linoleic acid (69.0%). Graphical Abstract


Introduction
Cactus (Cactaceae) are plants adapted to arid and semi-arid environments due to their distinct morphology and metabolism. They are rustic plants that need little management and are resistant to long periods without rain. Within the Cactaceae family, the genus Opuntia has a large number of species, about 125, with wide geographic distribution (Aruwa et al. 2018). In general, the genus Opuntia has succulent cladodes, edible fruits. Opuntia spp contains carotenoids, vitamin C, phenol compounds, sterols, coumarins, terpenoids, and alkaloids bring about several health benefits such as hypoglycemic, antimicrobial and hepatoprotective (Aragona et al. 2018, Kandan et al. 2021Katani c et al. 2019;Surup et al. 2021).
Opuntia monacantha, is a specie native to Brazil, common in the coastal plains of Rio Grande do Sul and other Brazilian states. It produces abundant fruits but little consumed, except by indigenous populations, who also use the species as an anti-inflammatory. The fruit of O. monacantha is a berry with a green base and a red tip. The fruit peel has large amounts of glochids and the pulp is succulent in green color with many seeds, gelatin consistency and a pleasant flavor. O. monacantha contain flavonoid compounds (kaempferol and isorhamnetin.), antioxidant activity and moderate antitumoral activity (Valente et al. 2010).
The species is still poorly studied chemically, and no phytochemical study is related to the fruit. It takes into account that this fruit is underused in human consumption and that other fruits within the genus present compounds with biological activity, the interest in knowing more about the chemical composition of the fruits, contributes to its use as potential functional food.

Results and discussion
The crude extracts (CE), chloroform (CF) and acetate (AF) fractions obtained from the peel and pulp of the fruit of the O. monacantha were submitted to HPLC-DAD analysis to determine their phenolic composition (Table 1S). In the pulp of O. monacantha were identified only the ferulic acid (CF) and p-coumaric acid (AF). The peel contains more phenolic compounds than pulp, these results is similar to the found to Opuntia ficus (Ativa et al. 2021). Protocatechuic acid was found only in the peel of the fruit in the CE and AF. The phenolic acids present in the O. monacantha have already been found in other Opuntia sp. and these compounds have already been investigated by several authors regarding their antioxidant capacity (Guevara-Figueroa et al. 2010). Catechin and quercetin were the flavonoids found only in the fruit peel the O. monacantha.
The hexane fraction (HF) of peel (analyzed in GC/MS,) showed three terpene compounds (Table 2S), a-amyrin, b-amyrin, and cycloartenol, corresponding to 73.0% of the peaks present in the chromatogram. The presence of these terpenes in the fruit of the species O. monacantha is unprecedented. Amyrins are bioactive pentacyclic triterpenes with anti-inflammatory, analgesic, anxiolytic and anti-platelet aggregation activity (Silva et al. 2020). The phytosterol cycloartenol also has anti-inflammatory properties (Kerrihard and Pegg 2015). From HF of peel was isolated and identified the oleanolic acid. The H 1 NMR spectrum of this compound showed characteristics of triterpene, with signals of olefinic hydrogen at d 5.5 ppm and methyl hydrogens in the region between d 0.9 and 1.5 ppm (Table 3S). This compound, as well as the other triterpenes, was identified for the first time in this species.
The seed of the O. monacantha fruit showed mainly unsaturated fatty acids (Table  4S), the linoleic acid (omega 6) was found in the highest percentage (69.0%). These are unprecedented results for the seed of the fruit of the species O. monacantha.

Experimental
Please refer to the supplementary data file.

Conclusions
This study with the fruit of O. monacantha, described for the first time in the literature, demonstrated the potential use of the fruit as a functional food, as a result of the profile rich in bioactive compounds present. Among the bioactive compounds present, the phenolic acids and flavonoids already have antioxidant activity described in the literature, indicating the use of the fruit as an antioxidant potential.
The pulp seed is rich in linoleic acid, which makes the fruit of O. monacantha a food source of omega 6.
Pentacyclic triterpenes were described for the first time in the literature for the fruit of the O. monacantha. These compounds are known for their anti-inflammatory activity, corroborating the popular knowledge of the use of this species.
The fruit of O. monacantha is a species that grows in poor soil conditions and produces in abundance. In a scenario of pandemic and hunger in Brazil, the discovery of nutritious foods with high biological value with potential benefits to human health which makes the fruit of O. monacantha a relevant alternative, showing the importance of scientific studies involving unconventional food plants.

Disclosure statement
No potential conflict of interest was reported by the author(s).