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Ankara Üniv Vet Fak Derg, 66, 163-169, 2019
DOI: 10.33988/auvfd.423491
The effect of olive leaf extract on digestive enzyme inhibition and
insulin production in streptozotocin-induced diabetic rats
Mehmet Ali TEMİZ1, Atilla TEMUR2
1Karamaoğlu Mehmetbey University, Vocational School of Technical Sciences, Programme of Medicinal and Aromatic Plants,
Karaman; 2Van Yuzuncu Yil University, Faculty of Education, Department of Mathematics and Sciences, Van, Turkey.
Summary: Olive leaf has natural bioactive compounds, mainly oleuropein, that are widely considered to have potentially
beneficial effects on health. This study aimed to evaluate the effects of olive leaf extract (OLE) on the inhibition of carbohydrate
digestive enzymes, and immunohistochemical study of insulin in the pancreas of in vivo streptozotocin-induced diabetic rats. Blood
glucose levels, insulin, glycated hemoglobin (HbA1c), α-amylase and α-glucosidase activities, and an immunohistochemical study were
performed at the end of the experiment. In the OLE treated group, blood glucose levels and HbA1c significantly decreased while insulin
levels increased. Besides this, OLE treated group showed remarkable inhibitory activities on α-amylase and α-glucosidase compared
with the Acarbose treated group. It was observed that OLE exhibited partial positive immunoreaction for insulin in β-cells through
immunohistochemical analysis. Considering that OLE is more tolerable for digestion system compared to acarbose, it may be a better
fitoformulation for antidiabetic medications. OLE could offer an additional beneficial effect for the treatment of diabetes.
Keywords: α-amylase, α-glucosidase, hypoglycemic, immunohistochemistry, olive leaf.
Streptozotosin ile indüklenmiş diyabetik sıçanlarda zeytin yaprağı ekstraktının sindirim enzimi
inhibisyonuna ve insülin üretimine etkisi
Özet: Zeytin yaprağı potansiyel olarak sağlık üzerine yararlı etkileri olduğu düşünülen başlıca oleuropein olmak üzere
biyoaktif bileşenlere sahiptir. Bu çalışma, streptozotosin (STZ) ile indüklenmiş diyabetik sıçanlarda zetin yaprağı ekstraktının (OLE)
in vivo karbonhidrat sindirim enzimleri inhibisyonunun ve pankreasta insülin mevcudiyetinin araştırılmasını amaçlamaktadır. Deney
sonunda kan glukoz seviyeleri, insülin seviyeleri, glikozillenmiş hemoglobin (HbA1c), α-amilaz ve α-glukozidaz aktiviteleri analizi ile
immunohistokimyasal çalışma yapıldı. OLE tedavi grubunda kan glukoz seviyeleri ve HbA1c anlamlı şekilde azalırken insülin
seviyeleri arttı. Bunun yanında OLE, Akarboz grubuna göre dikkat çekici şekilde α-amilaz ve α-glukozidaz aktivitelerinde inhibitör
etki gösterdi. Immunohistokimyasal analizde OLE’nin β-hücrelerinde insülin için kısmi pozitif immunoreaksiyon gösterdiği gözlendi.
OLE akarboz ile karşılaştırıldığında sindirim sistemi bakımından daha tolere edilebilir olduğu düşünüldüğünde antidiyabetik ilaçlara
göre daha iyi bir fitoformülasyon olabilir. OLE, diyabet tedavisi için ek bir faydalı etki sunabilir.
Anahtar sözcükler: α-amilaz, α-glukozidaz, hipoglisemi, immunohistokimya, zeytin yaprağı.
Introduction
Diabetes mellitus is a heterogeneous metabolic
syndrome characterized by hyperglycemia caused by a
relative or absolute deficiency of insulin (11). Insulin
action deficiency, a common case of diabetes, leads to
impairment of carbohydrate, lipid and protein metabolism.
Furthermore, insulin resistance plays a role in the
pathogenesis of hyperglycemia and diabetes in tissues and
cells (10). Hyperglycemia developing with impaired
fasting glucose and glucose tolerance leads to serious
macrovascular and microvascular diabetic complications.
In addition, increased reactive oxygen species play a
substantial role in the development of diabetes
complications (25). Therefore, the control of blood
glucose levels in diabetes is very important.
Recently, scientific studies have proven that many
phytochemicals are effective both to prevent and treat
diseases. The health benefits of phytochemicals depend on
the amount consumed and on their bioactivity, for this
reason, nutraceutical and therapeutical usage have become
popular (20). The pharmacological properties of the fruit
of the olive tree and its products have been defined as
important components of a healthy diet due to their
bioactive phenolic content (21). Nevertheless, olive leaves
contain higher amounts of polyphenols than olive oil. For
example, oleuropein is the main phenolic compound and
the most active phenolic compound in olive leaf (23).
Although, several studies revealed that olives and
olive leaf have an antihyperglycemic effect, but there is no
information with respect to the inhibition effects of α-
Mehmet Ali Temiz - Atilla Temur
164
amylase and α-glucosidase and amelioration of β-cells in
vivo. Therefore, this study aimed to investigate the effect
of olive leaf extract (OLE) on α-amylase and α-
glucosidase enzymes inhibition and insulin production of
β-cells on in vivo experimental diabetes.
Materials and Methods
All chemicals and reagents used in the study were
analytical grade and obtained from Sigma Inc. (St. Louis,
MO).
Olive leaf extract: Olive (Olea europaea L.) leaves
were collected from Antalya, Turkey, in August 2013.
They were then dried and powdered to make the olive leaf
extract. The extract was composed of 1 g of powder
combined with 50 mL of distilled water. The extraction
process was carried out on a magnetic hot plate (Wisd
WiseStir MSH-20D) for a period of 12 hours at 80°C and
750 rpm. Then, it was filtered and placed in a centrifuge
(Hettich Universal 320r) for 5 minutes at 4oC, 3500 rpm
in a falcon tube. The final extract obtained was transferred
to vials in order to carry out content analysis. The
extraction was carried out in duplicate. OLE was
evaporated under reduced pressure to administer for study.
Furthermore, 1.5 g of leaf sample was infused in 100 ml
of water at 80oC for 5 minutes to create a homemade
extract.
Determination of extract content by HPLC: The
amounts of oleuropein, hydroxytyrosol, tyrosol and
verbascoside in the sample were measured quantitatively
against external standards in extracted olive leaves. A
Waters 1525 binary HPLC pump and Waters 2487 dual
absorbance detector were used for content analysis. The
measurement was made by adjusting chromatogram to
280 nm wavelengths for the determination and assignment
of polyphenolic compounds.
Animals: Experiments were performed on 40 male
rats (Wistar albino, 250–350 g and 5–6 months of age)
obtained from Experimental Application and Research
Center, Yuzuncu Yil University (Turkey). The rats were
housed in five groups (n=8) at 20 ± 2°C with 12:12 h
reverse light/dark cycle in stainless cages and fed standard
chow ad libitum and water for 21 days. This investigation
was approved by the Yuzuncu Yıl University Animal
Researches Local Ethic Committee (no. 2015/08).
Experimental protocol: Blood glucose levels were
measured by using a glucometer (Accu Check Nano,
Germany) in tail bloods, and monitored periodically every
week. Streptozotocin (STZ) was administered at 45 mg/kg
intraperitonally (i.p.) (13). Blood glucose levels were
measured in tail bloods at 3 days after STZ injection. Rats
with glucose levels ≥200 mg/dL were considered diabetic.
Control Group (CG): Rats were given a single dose of 1
mL citrate buffer. Diabetic Group (DG): Rats were given
a single dose of 45 mg/kg body weight (bw) i.p STZ.
Diabetic+OLE (OLE): Diabetic rats were treated with 25
mg/kg bw OLE daily using an intragastric tube.
Diabetic+Infusion (Inf): Diabetic rats were treated with 1
mL infusion solution daily using an intragastric tube.
Diabetic+Acarbose (Ac): Diabetic rats were treated with
150 mg/kg bw Glucobay (Bayer, Turkey) daily using an
intragastric tube.
Infusion and acarbose group designed for
comparison of only biochemical analysis and monitoring
blood glucose level.
The rats were anesthetized with ketamine+xylazine
and then blood and tissue samples were collected at the
end of the experiment. The pancreas and intestinal tissues
were removed and rinsed with physiological saline.
Biochemical analysis: Insulin levels were measured
with electrochemiluminescence immunoassay (ECLIA)
method (Architect İ4000SR, Abbott Laboratories Inc.).
HbA1c was determined using automatic glycohemoglobin
analyzer based on HPLC (ADAMS A1c HA-8180T,
Akray Inc.).
Determination of α-amylase and α-glucosidase
activities: α-Amylase activity was measured with Alpha-
Amylase Assay kit (Abnova) by using the
spectrophotometric method (Boeco S-22 UV-Vis) as
specified by the supplier. An insoluble dye-coupled
substrate amylose azure was cleaved by α-amylase into
soluble colored products. The color intensity was
measured at 595 nm in the sample.
α-Glucosidase activity was measured with Alpha-
Glucosidase Assay kit (Assay bioTech) by using the
spectrophotometric method (Biochrom Anthos Zenyth
200) as specified by the supplier. α-Glucosidase reacts
with 4-nitrophenyl α-D-glucopyranoside and a yellow
complex is formed. This complex was measured at 405
nm. Immunohistochemistry analysis: Pancreatic tissues
were fixed and embedded in paraffin.
Immunocytochemical reactions were performed by ABC
(avidin biotin complex) (14). Endogenous peroxidase
activity was inhibited by 3% H2O2 for 30 min at the
deparaffinized section and washed with tap water. The
section was blocked by incubation with normal goat serum
(DAKO, X 0907, Denmark) with PBS, diluted 1:4 to block
non-specific binding. They were incubated with
monoclonal insulin (18-0066, Zymed, San Francisco,
CA), diluted 1/40 overnight and washed with PBS for 30
min. The sections were incubated with biotinylated anti-
mouse IgG (DAKO LSAB2 Kit) for 30×2 min and washed
with PBS. They were incubated AEC
(Aminoethylcarbazole Substrate Kit, Zymed
Laboratories) for 10 min and washed with tap water.
Counterstaining was performed with hematoxylin and the
sections were mounted. The tissue preparations were
examined by light microscopy (Leica ICC 50).
Ankara Üniv Vet Fak Derg, 66, 2019
165
Statistical analysis: Data are expressed as mean
(X
̅) and standard deviation (±SD). Significant differences
between groups were assessed using one-way analysis of
variance followed by Tukey’s test and Tamhane’s T2. p
value ≤ 0.05 was accepted as statistically significant.
Results
The amounts of oleuropein, hydroxytyrosol, tyrosol
and verbascoside was determined at 15335.55, 461.05,
41.6 and 357.6 µg/g respectively in OLE by HPLC
analysis (Figure 1). The values of blood glucose levels
(BGL), insulin, HbA1c, α-amylase and α-glucosidase
activities are shown in Table 1. Blood glucose levels
increased in STZ administered groups. The results are
shown in Figure 2A. According to the results, BGL was
significantly decreased in the OLE-treated group
compared to the diabetic group, but it did not decrease
significantly in the infusion group. Glycated hemoglobin
(HbA1c) is a form of hemoglobin that was determined
9±0.4 % in the diabetic group. However, OLE and
infusion administration resulted in a decrease of HbA1c (p
< 0.05) as shown in Figure 2B. In this context there was a
strong positive correlation between BGL and HbA1c in the
groups. Although insulin levels decreased in the diabetic
group, it remarkably increased in the OLE-treated group
(Figure 2C). Activities of the carbohydrate digestive
enzymes, α-amylase and α-glucosidase, are shown in
Figure 3A-B. As concerns α-amylase and α-glucosidase
activities, the results of the present study showed that both
activities significantly decreased in the OLE group
compared with the diabetic group. Remarkably, OLE
showed a much more effective reduction in α-glucosidase
and α-amylase activities compared with acarbose.
Infusion administration significantly induced a decrease in
α-amylase activity in comparison with the diabetic group,
but there was not a significant decrease in α-glucosidase
activity. However, it did not display an efficient activity
on both enzymes compared with acarbose.
Figure 4 shows the presence of insulin in pancreatic
β-cells. Sections were counterstained with
immunoperoxidase-hematoxylin. Insulin positive cells
were approved by immunostaining for insulin antibodies.
Insulin showed normal expression with respect to the
control group (As shown Figure 4A). However, a negative
reaction was determined for insulin in Langerhans islets
(arrows) in the diabetic group (Figure 4B). On the other
hand, the OLE-treated group exhibited partial positive
immunoreaction (arrows) for insulin in β-cells (Figure
4C).
Table 1. Various parameters of control and STZ-induced diabetic rats.
Tablo 1. Kontrol ve STZ ile indüklenmiş diabetik sıçanların çeşitli parametreleri.
Analysis
CG
DG
OLE
Inf
Ac
Glucose (mg/dL)
112±9
566±37a
444±41a,b
520±15a
511±28a
Insulin (ng/mL)
0.65±0.10
0.22±0.04a
0.43±0.06a,b
0.32±0.04a
0.27±0.03a
HbA1c (%)
4.5±0.2
9±0.4a
7.4±0.2a,b
7.7±0.4a,b
8.1±0.3a,b
α-Amylase (U/L)
1268±135
1950±182a
1063±87b
1534±129a,b
1296±184b
α-Glucosidase (mU/mL)
3166±119
3475±180a
2696±201a,b
3364±104
3132±122b
a: Significantly different from control (p<0.05). b: Significantly different from the DG (p<0.05).
Figure 1. HPLC phenolic profile of OLE at 280 nm. (1) Hydroxytyrosol; (2) Tyrosol; (3) Verbascoside; (4) Oleuropein.
Şekil 1. OLE’nin 280 nm’de HPLC fenolik profili. (1) Hidroksitirozol; (2) Tirozol; (3) Verbaskozit; (4) Oleuropein.
Mehmet Ali Temiz - Atilla Temur
166
Figure 2. Blood glucose levels (A), HbA1c percentages (B), and Insulin levels (C) of control and STZ-induced diabetic rats.
Şekil 2. Kontrol ve STZ ile indüklenmiş diabetik sıçanların kan glukoz seviyeleri (A), HbA1c yüzdeleri (B) ve İnsulin seviyeleri (C).
a: Significantly different from control (p<0.05). b: Significantly different from the DG (p<0.05).
Figure 3. α-Amylase (A) and α-Glucosidase activities (B) of control and STZ-induced diabetic rats.
Şekil 3. Kontrol ve STZ ile indüklenmiş diabetik sıçanların α-Amilaz (A) ve α-Glukozidaz aktiviteleri (B).
a: Significantly different from control (p<0.05). b: Significantly different from the DG (p<0.05).
Ankara Üniv Vet Fak Derg, 66, 2019
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Figure 4. The Control group exhibited positive immunohistochemical reaction of pancreatic β-cells (A); negative immunohistochemical
reaction of pancreatic β-cells (arrows) in the Diabetic group (B); partial immunohistochemical reaction of pancreatic β-cells (arrows)
in the OLE group (C). Immunoperoxidase-Hematoxylin, Bar=50µm.
Şekil 4. Kontrol grubu pankreatik β-hücrelerinin pozitif immunohistokimyasal reaksiyonu (A); Diabetik grubta pankreatik β-
hücrelerinin (oklar) negaitf immunohistokimyasal reaksiyonu (B); OLE grubunda pankreatik β-hücrelerinde (oklar) kısmi
immunohistokimyasal reaksiyon (C). İmmunoperoksidaz-Hematoksilen, Bar=50µm.
Discussion and Conclusion
Recently, plant-based treatments have been
considered effective for the prevention and control of
diabetes because of their specific biological activities and
low toxic effects. Due to these characteristic effects, they
may be preferred over various antidiabetic medications (5,
22). In the present study, the α-amylase and α-glucosidase
inhibitory effects and β-cell ameliorative activity of OLE
were investigated on STZ-induced diabetic rats in vivo for
the first time.
The results showed that a dose of 25 mg/kg OLE was
effective in controlling blood glucose level, which
decreased by about 20%. Olive leaf was reported to have
a hypoglycemic effect on diabetic rabbits, rats, and
humans (3, 12, 26) which is in agreement with both current
findings and previous studies. In the previous study,
pancreatic islet cells isolated from allaxon-induced rats
were incubated with crude oleuropeoside (0.2 mg/mL)
which purified from olive leaves. As a result, the presence
of oleuropeoside in the islet incubation medium together
with 2.7 mmol/L glucose (basal) raised insulin levels (9).
Bock et al. (4) reported that OLE improves both insulin
sensitivity and pancreatic β-cell secretory capacity after
oral glucose challenge on the overweight males. The
findings of current study are consistent with the
aforementioned studies in that OLE may be effective in
insulin production. Besides, partial positive
immunohistochemical findings in the present study
corroborate OLE’s effect on insulin production. On the
other hand, treatment of diabetic rats with OLE
significantly decreased HbA1c. In a previous investigation,
Wainstein et al. reported that HbA1c significantly
decreased from 10% to 8.0±1.5% in OLE-treated subjects
at the end of the 14 weeks in the randomized clinical trial
(26). However, Wainstein et al. (26) did not measure the
physical activities and diet type of the participants in their
Mehmet Ali Temiz - Atilla Temur
168
study, so the independent effect of OLE alone could not
be determined. These data confirmed the hypoglycemic
and antidiabetic effects of OLE which might be mediated
through its α-glucosidase and α-amylase inhibitory
activity.
Previous in vitro studies have indicated that olive oil,
olive mill wastewater, and olive leaf extract were effective
at the inhibitory concentration 50% (IC50) against α-
glucosidase and α-amylase (6, 16, 18). Furthermore, olive
oil exhibited efficient inhibitory activity compared to
acarbose because of a richer phenolic content (18). On the
basis of these literatures, the results obtained in the present
study provided positive support in vivo. A positive
correlation was observed in the OLE group when
compared to α-glucosidase and α-amylase enzyme
activities with blood glucose level. It was indicated that
the blood glucose levels markedly attenuated while the
enzyme activities decreased in OLE group. OLE is
considered to be effective to decrease blood glucose level
by (i) inhibiting activity of carbohydrate digestive
enzymes, α-glycosidase and α-amylase, or (ii) down-
regulating gene expressions of these enzymes.
Antidiabetic medications cause undesirable symptoms due
to undigested starch in the colon (7). Thus, tending toward
alternative α-glucosidase inhibitors that are derived from
natural sources and nutrients may be more effective, safe,
tolerable and cheaper. Although acarbose administration
is known to cause bloating and diarrhea, these symptoms
were not observed in the OLE treatment in the present
study.
Impaired β cells function or structure causes
alteration in blood glucose and insulin levels. Based on
various immunohistochemical studies conducted on
diabetes demonstrated that the density of insulin positive
reaction area (24), amount (17) and percentage of β-cells
(15) of diabetic subjects were lower compared to non-
diabetic subjects. The antidiabetic activities of medicinal
plants depend on the degree of β cell destruction and the
presence of bioactive contents which show attenuation in
the blood glucose level (19). The results obtained in this
study revealed that monitored partial positive
immunoreaction for insulin in β cells at the OLE group
may play a role in the reduction of blood glucose due to
oleuropein, which is the most bioactive phenolic. In
previous studies, phytochemicals have been reported to be
effective with different mechanisms in the regeneration of
β-cells (1, 2, 8). For this reason, the results suggested that
OLE may regenerate β cells and/or up-regulate insulin
expression in intact β cells. In this way OLE may exhibit
a hypoglycemic effect.
OLE administration significantly improved glycemic
status and showed efficient activity for inhibitions of α-
amylase and α-glucosidase. The biochemical and
immunohistochemical results revealed that OLE might
have a potential agonist and/or antagonist effect for the
development of new antidiabetic medications.
Considering these effects of OLE, it may be a better
alternative phytoformulation in comparison with
antidiabetic medications.
Acknowledgement
This research was supported by the Yuzuncu Yil
University Scientific Research Projects Foundation, Van,
Turkey (Grant no. FBE-D063).
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Geliş tarihi: 12.05.2018 / Kabul tarihi: 12.03.2019
Address for correspondence:
Dr. Mehmet Ali TEMİZ
Karamanoğlu Mehmetbey University,
Vocational School of Technical Sciences,
Programme of Medicinal and Aromatic Plants,
Karaman, Turkey
e-mail: matemiz@kmu.edu.tr
