# Sea Buckthorn Leaves as a Potential Source of Antioxidant Substances
## 3 Results and Discussion  
Phenolic compounds are a very large group of common, natural compounds and are secondary metabolites of plants. They belong to the group of antioxidants [48]. Evaluation of the antioxidant potential of raw materials is carried out by tests in different systems and using different reaction mechanisms [49; 50]. In this study, tests involving scavenging of free stable radicals were used, DPPH\({}^{\bullet+}\) and ABTS\({}^{\bullet+}\). The use of the DPPH\({}^{\bullet+}\) radical to assess antioxidant potential only allows for the evaluation of hydrophobic antioxidants, in contrast to the ABTS\({}^{\bullet+}\) radical, which reacts with both hydrophobic and hydrophilic antioxidants. The PCL method was also used in this study. PCL was first used to assess antioxidant potential by [PERSON] et al. [51]. This method has a high level of sensitivity and can therefore be used to determine the antioxidant properties of both antioxidant-rich and antioxidant-poor products. This method is used to test the antioxidant activity of both the hydrophilic fraction (water-soluble antioxidant capacity ACW) and the lipophilic fraction (lipid-soluble antioxidant capacity ACL) in plant samples. Table 1 shows the total polyphenol (TP) content and antioxidant activity (AA) of sea buckthorn leaves. SBT leaves from different years of this study had statistically different levels of total polyphenols (from 186.64 to 330.16 mg GAE/1 g DM), confirming that the quality of the raw material (leaves) can be significantly modified by habitat conditions. A trend was also observed (\(p\) = 0.239) indicating a variation in the TP content of the leaves of the SBT cultivars tested. The TP content in the leaves of the Ascola cv. was 307.02 mg GAE/1 g DM, while in the leaves of the Hergo cv., it was only 195.80 mg GAE/1 g DM. In the study by [PERSON] et al. [52], the TP content in sea buckhorn leaf tea and green tea extracts ranged only from 23 to 66 and from 33 to 118 mg GAE/1 g DM, respectively. [PERSON] et al. [53] also reported that 70% ethanol was optimal for extracting phenolic compounds from sea buckthorn leaves. The variability of TP has been recorded due to the influence of a number of factors, such as the type of locality, the year of harvest, and cultivars [11; 24]. This wide variation in the compounds responsible for antioxidant properties was confirmed by the different results of the antioxidant assays in the studies presented (Tables 1 and S1). The free-radical-scavenging activity of SBT leaf extracts was studied for their ability to bleach stable ABTS\({}^{\bullet+}\) and DPPH\({}^{\bullet+}\) free radicals, providing information on the reactivity of compounds with a stable free radical. Extracts of SBT leaves exhibited AA potential, as analysed by DPPH and ABTS assays (Tables 1 and S1). The leaf extracts showed a significant reduction in both the DPPH\({}^{\bullet+}\) and the ABTS\({}^{\bullet+}\) radicals in the samples tested (RSA, from 56.33 to 98.25%). The AA of the SBT leaves measured with the ABTS\({}^{\bullet+}\) radical was dependent on the cultivar. SBT leaf extracts from different cultivars showed antioxidant activity ranging from 490.47 to 493.32 \(\upmu\)M TRX/1 g DM, and these were the statistically different contents found in the leaves of the Ascola and Hego cv. Due to the lipophilic nature of the DPPH\({}^{\bullet+}\) radical, a slight difference in the experimental cultivars was observed in the AA measured by DPPH, in comparison to the ABTS assay. The simplicity of the ABTS method makes it a routinely used analysis for determining the AA of samples. However, it should be borne in mind that this radical has a much higher persistence than the radicals present in the raw materials, which may raise concerns as to whether such a method describes the actual free radical reactions well. On the other hand, a limitation of the DPPH method is that DPPH\({}^{\bullet+}\) is only soluble in organic solvents and does not allow for the determination of hydrophilic antioxidants [54].