*2.1.1.2 Extracting methods*

There are various extracting methods of phenolic compounds from olive leaves (after drying and milling), including solid-liquid extraction by maceration and soxhlet extraction utilizing water-methanol blends or hexane to yield OLE [1, 35, 36]. For more explanation on one of the most common techniques, mixing the specific quantity of dried olive leaf powder with an aqueous alcohol solution, incubating there to produce an alcohol extract. after a draining process, the crude extract will be dried again and treated with alcohol and water solution at least two more times. Then, by distilling the mixed extract under vacuum, the OLE will be produced [6, 12]. OL can be chemically decomposed into two different products, including hydroxytyrosol (HT) and elenolic acid by distinct factors such as high temperature, acid, base, light,

#### **Figure 3.**

*Oleuropein chemical structure. Source: pubChem. URL: https://pubchem.ncbi.nlm.nih.gov/compound/5281544. Description: data deposited in or computed by pubChem.*

*The Effect of Olive Leaf Extract on Systolic and Diastolic Blood Pressure in Adults: A Systemic... DOI: http://dx.doi.org/10.5772/intechopen.102769*

metal ions, etc. [37]. This process assembles the enzymatic hydrolysis of this phenolic compound that occurs in human body. However, the studies conducted to exactly specify what happens to this phenolic compound extracted from olive leaves during absorption from small intestine and colon to the blood circulation, have mentioned scattered findings. Therefore, we go directly to the mechanism of its action in the body.

#### *2.1.1.3 Mechanisms of action*

The studies performed in human models to show the mechanisms of action for anti-hypertensive property of OL are scarce and have been conducted much more in vitro. This mysterious compound is endowed with anti-hypertensive property which is thought to be due to its influence on membrane receptors and/or enzymes involved in cell signalling, including ACE, L-type Ca2+ channels, nitric oxide (NO) and reactive oxygen species (ROS), or to clarify, the metabolite of OL inhibits ACE. Another mechanism is that a degraded product of OL (3,4-dihydroxy-phenyl-ethanol) directly affects L-type Ca2+ channels as an antagonist resulting in blocking the channels [29, 38, 39]. In fact, OL has synergic effects with other active substances in OLE to present ACEI and CCB activity in the body. Also, the Vasodilator effect of OLE justifies its anti-hypertensive activity [1, 29]. This phenolic compound performs a particular task to increase NO bioavailability and expression of the inducible form of endothelial NO synthase (e NOS) studied in animal subjects [9, 25, 29, 40]. As a matter of fact, OL reacts with NO and its noxious derivative peroxynitrite (−OONO). There is a possibility that OL increases NO production via modifying two specific enzymes: nicotinamide adenine dinucleotide phosphate-oxidase (NADPH-oxidase) and NO synthase [41]. These mechanisms modulate NO bioavailability, thus improving vascular function and ultimately reducing blood pressure [25]. The last one influences ROS. ROS play a significant role in the development of oxidative stress, which also encompasses the principal role in the pathology of HTN. ROS are produced permanently in the human body. They are indispensable for several mechanisms happening in the cells, such as chemical signalling, immune performance and energy production [24]. When the balance between ROS and antioxidants upsets, meaning ROS level more than the other, the cell makes oxidative stress [2, 9]. Indeed, an excess in the production of ROS which could be controlled by a number of enzymes, including glutathione peroxidase, catalase (CAT) and superoxide dismutase (SOD), enable to damage lipids, proteins and DNA in the cells particularly cardiovascular cells, are even able to ruin the vascular function and structure [2, 42, 43]. So oxidative injury increases the risk of CVD. In this regard, the OL molecule consists of some active components that have determined scavenging functions [44]. So, there is a potential antioxidant property that is suggested to be related to the H-atom donation from the OL phenolic groups [8, 33, 45]. In other words, OL preserves paraventricular nucleus (PVN) of the hypothalamus from oxidative stress. OL activates the Nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated signalling pathway and finally, it improves mitochondrial function. Thus provides an exquisite way to treat HTN [1, 8, 46]. Hence, antioxidant property of OLE enhances its antihypertensive yield.

#### **2.2 OLE safety**

In spite of the beneficial health properties of OLE in human body, it is essential to be determined what dosage of this extract will be safe for the body. Many studies aimed at this indicated acute OLE toxicity (2000 mg/kg) and also 4-week OLE toxicity (100–400 mg/kg) revealed no symptoms of toxicity in subjects [47, 48]. However, another study reported the opposite result by noticing bleeding in the liver and kidneys of rats when using OLE [49].
