# Portuguese Grapevine Leaves: A Neglected By-Product with High Potential
## 2 Chemical Composition of Portuguese Grapevine Leaves  
The composition of the vine leaves of Portuguese wines is compiled in Tables 1-6. These tables also contain the detection methodologies, the bioactive properties, and the toxicities of each compound found in this neglected by-product. To date, there are studies on the composition of grape leaves from red cultivars Red Cornifesto (RC), Bastardo (B), Cornifesto (C), Merlot (Me), Mourisco (Mo), Rufe (Ru), Tinta Gorda (TG), Tinto Cao (TC), Touriga Francesa (TF), Touriga Nacional (TN), Trincadeira (T), Tinta Roriz (TR), Baga (Ba), Tinta Barroca (TB), Castelao (Ca), and white cultivars Carrega Branco (CB), Codega (Cd), Gouveio (G), Malvasia Fina (MF), Malvasia Rei (MR), Rabigato (Ra), Samarrinho (Sa), Viosinho (V), Fernao Pires (FP), Encruzado (E), Siria (Si), Verdelho (Ve), all of Portuguese origin. Generally, vine leaves are made up of flavonoid and non-flavonoid polyphenols. Among flavonoids, flavonoids derived from quercetin, myricetin and kaempfered stand out, as well as rutin and anthocyanins. Among non-flavonoids, hydroxycinamic acid derivatives were found. The leaves are also rich in fatty acids, terpenoids, chlorophyll pigments, and carotenoids. The levels of the flavonoids (Table 1) quercetin-3-O-glucose and quercetin-3-O-galactosidase are, in general, higher for the leaves of red cultivars, while myricetin-3-O-glucose shows the opposite behavior [41]. Saadaoui et al. (2023) [42] reported values for quercetin-3-O-glucose concentration between 2.83 and 1,48 mg/g dry matter for leaf extracts of two Tunisian _Vitis vinifera_ varieties. Another study has found high levels of quercetin-3-O-glucose and quercetin-3-O-galactoside, which together represent 64% to 73% of the total phenolic compounds present in white and red grapevine leaves from the species _Vitis labrusca_ and _Vitis vinifera_ L. from Brazil [43]. The authors also found, in lesser quantities, rutin, cyanidin-3-O-glucose, and penoidin-3-O-glucose. The rutin content, quantified by Jordao et al. (2019) [44] at between approximately 12 and 46 mg/L, does not seem to be influenced by the colour of the variety, showing higher values for Baga (red) and Verdelho (white). The antioxidant properties of grape leaves are, in part, related to the concentration of flavonoid glycosides and rutin, which show hepatoprotective activity [42]. These flavonoid glycosides also exhibit antimicrobial activity. For example, quercetin-3-O-galactoside showed antiviral effects against hepatitis B and C virus (HBV) [45]; kaempferol-3-O-glucose showed antibacterial activity against _S. aureus_, _E. faecalis_, _K. pneumoniae_, _P. aeruginosa_, among others, and antiviral effects against _A. niger_, _A. flavus_, _P. camemberti_, and others [46]. Quercetin-3-O-galactoside neutralized inflammatory markers and reduced liver inflammation resulting from high fat and alcohol consumption, revealing significant anti-inflammatory activity [45]. Flavonol glycosides also demonstrated anticancer properties, for example, inhibiting DLD-1 and E0771 [47] and human Caco-2 intestinal cancer cells [48]. Flavonoids are generally considered safe molecules for human intake, and toxicity studies in vivo and in vitro generally show minimal toxicity, if any. Several studies have demonstrated that myricetin derivatives are nontoxic and appropriate for human consumption [49]. Although myricetin is capable of inhibiting human cytochrome P450 activity in hepatic microsome models, its inhibitory constant value is higher than physiologically relevant concentrations, suggesting it remains a safe compound [50]. No data was found for myricetin-3-O-glucose, but all myricetin derivatives appear to be safe for human consumption. The same conclusions can be drawn for the other flavonoids: either no toxic effects were observed, or a dose higher than therapeutic is necessary for these compounds to display acute or chronic toxicity.