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Traditional medicines are used by about 60% of the world’s population. These are not only used for primary health care in rural areas in developing countries, but also in developed countries as well where modern medicines are predominantly used [1].

The genera of Leucas, Marrubium, Salvia and Origanum, which belongs to the family Lamiaceae, play an important role in folk medicine, cosmetics, culinary, and for the commercial production of essential oils [2]. It is considered as a significant resource for traditional medicine in Yemen which is used to cure diseases related to kidney disease, diabetes, cough, wounds, stomachache, dysentery, diarrhea etc [3,4].

Diabetes mellitus is a major and emerging public health problem all over the world. It is growing at an alarming rate and is considered as the fifth leading cause of death in the world. The first WHO Global report on diabetes demonstrates that the number of adults living with diabetes has almost quadrupled since 1980 to 422 million adults. This dramatic rise is largely due to the rise in type 2 diabetes and factors driving it include overweight and obesity. In 2012, diabetes caused 1.5 million deaths. Its complications can lead to attack, stroke, blindness, kidney failure and lower limb amputation. WHO estimates that, globally, 422 million adults aged over 18 years were living with diabetes in 2014 [5]. According to the World Health Organization (2016), 2% of the mortality percentage in Yemen is due to diabetes [6].

Inhibition of α-glucosidase is an important factor to control postprandial hyperglycemia in type 2 diabetes mellitus [7] and the uses of medicinal plants are recommended by World health organization, particularly for patient in rural regions of poor countries who are unable to purchase the synthetic medication [8]. Therefore, extensive research has been directed toward the use of medicinal plants to control DM and its complications [9,10].

Recently there has been some research works on the chemical composition, antioxidant and antimicrobial activities of essential oils of the four mentioned plants [11-14]. However, to the best of our knowledge there is no information on the antidiabetic properties of these essential oils. So, this study was designed to investigate both the chemical constituents and the antidiabetic properties of the essential oils isolated from Leucas, Marrubium, Salvia and Origanum leaves.

Plant material

Four medicinal Lamiaceae taxa were collected during the rainy season in 22-2/8/2015 from different Location in Bani Matar District, Sana’a governorate, Yemen. The identification of the specimens was done by utilizing the available taxonomic and floristic literatures [15-18]. Voucher specimens have been deposited at the Herbarium of Faculty of Science, Ain Shams University and a duplicate of each herbarium specimen was kept at the Herbarium of Biology Department, Faculty of science Sana’a University.

Isolation of the essential oil

The fresh leaves and green branches of the four medicinal Lamiaceae taxa were chopped into small pieces. The essential oil was isolated from each part by hydrodistillation for 5 hr using a Clevenger-type all glass apparatus. Each oil was transferred to a screw-capped glass vial, dried (Na₂SO₄) and stored at 4°C in the dark until analysis.

Analysis of essential oils by GC and GC-MS

GC analysis was carried out using a GC HP 5890 Hewlett Packard equipped with FID and HP-5 fused silica capillary column “30 m × 0.25 mm i.d., film thickness 0.25 μm”, using a sample volume of 0.03 μL. Oven temperature was programmed from 60°C to 240°C at 3°C/ min; injector temperature, 250°C; detector temperature, 280°C; carrier gas, helium (1.0 mL/min); automatic sample injection, 0.02 μL of the oil; split: 1/70. The relative proportions of the essential oil constituents were expressed as percentages obtained by peak area normalization. GC–MS analysis was performed on a Perkin-Elmer quadrupole MS system (Model 5) coupled with the GC HP 5972, equipped with a HP-5 capillary column. Oven temperature was programmed from 45°C to 240°C at 3°C/min; injector temperature, 250°C; carrier gas, helium (0.5 mL/min); automatic sample injection, 0.02 μL of the oil; split: 1/70. The MS operating parameters were: interface temperature: 300°C, ion source temperature: 200°C, EI mode: 70 eV, scan range: 41-400 amu.

Identification of the components

Mass spectra of the individual GC peaks were identified by a computer search of the commercial libraries (WILEY, NIST), as well as matching with published mass spectra [19]. The identification was further confirmed by the calculation of the retention indices (RI) relative to (C6–C22) n-alkanes [20].

α-Glucosidase inhibition assay

Fifty microliter from the essential oil and 100 μL of α-glucosidase solution (1.0 U/mL in 0.1 M phosphate buffer (pH 6.9) was incubated at 25°C for 10 min. Then, 50 μL of 5 mM p-nitrophenyl-α-Dglucopyranoside solution in 0.1 M phosphate buffer (pH 6.9) was added. The mixtures were incubated at 25°C for 5 min, before reading the absorbance at 405 nm in the spectrophotometer. The α-glucosidase inhibitory activity was expressed as percentage inhibition [21].

There are several essential oils which are screened for antidiabetic potential that inhibit α-glucosidase enzyme [7,22,23]. The inhibition of α-glucosidase by the essential oils of Leucas inflata, Marrubium vulgare, Salvia schimperi and Origanum majorana are shown in Figure 1 and Table 1. The results indicated that Salvia schimperi and Marrubium vulgare essential oils exhibited dose-dependent inhibitory activities against α-glucosidase with IC₅₀ of 14.26 μL and 35.47 μL respectively, compared to acarbose (IC₅₀ value at 12.87 μL). Leucas inflata essential oil exhibited weak dose-dependent inhibitory activity against α-glucosidase with IC₅₀ of 159.66 μL and Origanum majorana leaves essential oil had no effect.

Figure 1: α-Glucosidase inhibition by acarbose and essential oil from of 4 selected medicinal Lamiaceae plants.

Table 1: Anti-dibetic activity of the four selected lamiaceae plants.

The higher inhibitory effects observed in Salvia schimperi could be attributed to the presence of major components; 4,8-α-epoxy caryophyllene (26.06%), caryophyllene oxide (20.7%), bisabolol (11.67%), cadinene ether (7.46%) and cubenol (5.35%). The inhibitory effects observed in Marrubium vulgare could be attributed to the presence of major components as caryophyllene (10.95%), octadecanol (10.44%), α-bisabolene (9.72%), β-farnesane (6.91%), geranyl linalool (5.86%) and octadecenol acetate (5.36%) (Table 2).

Table 2: Essential oil composition of 4 medicinal plants of Lamiaceae in Yemen.

The inhibition of α-glucosidase by both plants could be attributed to the presence of previously mentioned compounds which reported with their activity against this enzyme. Β-caryophyllene has been proved to have high inhibitory effects against α-glucosidase enzyme [22]. α-Bisabolol along with α-farnesene isolated from Matricaria chamomilla L., showed also significant α-glucosidase inhibitory activity [24].

So the amount and the synergistic effect of these compounds in both oils are responsible for that activity. However, further test could be carried out to characterize the active principles responsible for these activities.