The effect of cold atmospheric plasma on diabetes-induced enzyme glycation, oxidative stress, and inflammation; in vitro and in vivo

Cold atmospheric plasma (CAP) is known as the versatile tool in different biological, and medical applications. In this study, we investigated the effect of cold plasma on diabetes via in vitro and in vivo assessments. We performed the in vitro assay to evaluate the impact of CAP on glycated glutathione peroxidase (GPx) through enzyme activity measurement as a function index and far- and near-UV circular dichroism (CD) and fluorescence analysis as structure indices. The result of in vitro assessment showed that the exposure of glycated GPx to plasma causes a considerable increase in enzyme activity up to 30%. Also, the evaluation of far- and near-UV CD and fluorescence analysis indicated a modification in the protein structure. According to obtained result from in vitro assessment, in vivo assay evaluated the effect of CAP on diabetic mice through analyzing of blood glucose level (BGL), advanced glycation end products (AGEs), antioxidant activity, oxidative stress biomarkers such as malondialdehyde (MDA), advanced oxidation protein products (AOPP), and oxidized low-density lipoprotein (oxLDL), and inflammation factors including tumor necrosis factor (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6). The result of in vivo experiment also showed a 20% increase in antioxidant activity. Also, the reduction in AGEs, oxidative stress biomarkers, and inflammatory cytokines concentrations was observed. The result of this study revealed that CAP could be useful in diabetes treatment and can be utilized as a complementary method for diabetes therapy.


Results
In vitro. OES. The emission spectrum of argon plasma jets in the air medium has been shown in Fig. 1. The spectrum line of argon plasma jet indicates the existence of hydroxyl radical, nitrogen, argon, and oxygen species.
Effect of the cold plasma on enzyme activity. The activity of the enzyme (as a protein) is affected by protein conformational changes such as glycation. So, for studying the impact of CAP on protein conformation, enzyme activity can be analyzed. Figure 2 shows the effect of the argon plasma jet treatment on GPx activity for different plasma exposure times. As inferred from Fig. 2, enzyme activity is a function of the cold plasma treatment time. Such that, after 600 seconds of treatment, enzyme activity increased from 63.51 (for 100% glycated GPx) to 82.33 U mL −1 .
Fluorimetry analyze. The effect of CAP on glycated GPx structure was investigated by fluorimetry analyze (Fig. 2). According to results, the improvement of protein denaturation, which is a reverse process of enzyme glycation has been occurred through receiving plasma treatment. Figure 2 shows that with the enhancement of plasma exposure time from 60 to 600 seconds, the naturation of GPx increased so that the 600-second imposing argon plasma jet to the solution leads to about 30% naturation.
Circular Dichroism (CD) analyze. The ellipticity measurement is the most popular method used for monitoring protein structural changes due to destabilizing agents such as glycation. The results of molar ellipticity analysis show a significant change after the cold plasma treatment for different plasma exposure times (Fig. 2); such www.nature.com/scientificreports www.nature.com/scientificreports/ that, by increasing treatment time, the molar ellipticity decrease, and after 600 seconds of treatment, its amount reduced from 307617 to 246139 Θ nm −1 .

In vivo. Effect of CAP on BGL, AGEs, antioxidant activity, and H 2 O 2 concentration. An increase in BGL leads
to producing AGEs due to direct exposure of lipid or protein to sugar. Moreover rising glucose level results in enzyme glycation, a decrease in antioxidant activity, and oxidants accumulation. Table 1 illustrates the mean values and standard divisions of BGL, AGEs index, GPx activity, and H 2 O 2 concentration for normal mice (group 1), diabetic mice (group 2), and CAP treated diabetic mice (group 3). Figure 2 also shows the mentioned indices in each group, normalized to that in group 1. As indicated in Table 1 and Fig. 3, BGL of plasma treated diabetic mice is lower than diabetic mice, and its amount is reduced from 391.5 to 388.1 mg dL −1 for 600 seconds applying argon plasma. Also, by comparing the AGEs index of Group 2, and 3, it is clear that the plasma has a positive effect on the reduction of this index. Moreover, the obtained data revealed that CAP is effective in antioxidant activity enhancement. Such that the plasma treatment leads to increasing GPx enzyme activity, and decreasing H 2 O 2 concentration. On average, a 20% increase in enzyme activity was observed at 600 seconds plasma exposure time that this result was in accordance with in vitro ones (Fig. 3).     www.nature.com/scientificreports www.nature.com/scientificreports/ ones (group one) relative to untreated mice and its reduction amounts for all indices was about 1%. These results confirm the cold plasma influence on the reduction of inflammation.
The overall effect of the cold plasma treatment on diabetic mice. The ANOVA test is a general analyze for studying and comparing mean vectors of the three groups. So, for the exact determination of pairwise differences for each variable based on the ANOVA test, post-hoc tests can be considered (Tables 2-4). Obtained results from Tukey-HSD test with 95% family-wise confidence level for BGL, AGEs index, GPx activity, and H 2 O 2 concentration has been shown in Table 2, oxidant biomarkers concentrations (oxidative stress) has been illustrated in Table 3, and inflammation factors indices has been presented in Table 4. According to the confidence interval resulting from Tukey's post-test, the possibility of comparing variable mean between two groups has been provided that this is a criterion to discuss about significant differences in three groups. For all factors, the means difference between group one and two (1-2: group 1 minus group 2) are negative (except to GPx) due to enhancing variable after STZ injection. So, the diabetes-induced process was carried out successfully. Also, for groups two and three, all mean difference (3-2) is negative except to GPx, as a variable was descended after CAP treatment. Therefore, this method is effective for complications arising from diabetes. By comparing the means difference (1-2), and (1-3) together, the same results can be obtained.

Discussion
In this study, the effect of CAP on glycated GPx as in vitro and diabetic mice as in vivo is investigated. To the best of our knowledge, we pioneered to introduce a basic nonclinical research for evaluating CAP impact either on glycated protein or diabetes.
As illustrated in Fig. 6, the argon plasma jet is containing both hydroxyl, and singlet oxygen radicals. RONS has two-edged sword nature in biology, as it could be either beneficial or harmful to biological systems depending on plasma properties (such as plasma source type, mode of energy dissipation into the system, feeding gas, plasma plum, and types and concentrations of reactive species), type of organism, and exposure time 3,6 . However, in the  www.nature.com/scientificreports www.nature.com/scientificreports/ present study, the physiological characteristic of plasma is used as a positive induction to modify denatured protein structure and enzymatic activity. The results of the in vitro assessment showed that plasma treatment led to a modification of protein conformation and enzymatic activity improvement (Fig. 2). It is clear that these factors strongly correlated; such that the improvement of protein conformation directly affects the enzyme activity. Also, these effects have been augmented over CAP treatment time.
Many studies have been focused on the interaction of plasma with various proteins. Attri et al. have shown that CAP could lead to a modification in protein oxidation, dimerization, nitration, and dehydrogenation of amino acids 25 . On the other hand, Takaei et al. reported that nitration, hydroxylation, sulfonation, amidation, and disulfide linkage formation in amino acids could occur during the cold plasma treatment 26 . For example, Choi et al. concluded that the reduction in the fluorescence intensity of the plasma-treated lysozyme was due to the modification of the Trp group or Trp surroundings 27 . Altogether, quenching of intrinsic fluorescence and CD spectra intensity of denatured GPx after exposure to CAP could be the result of modifications of amino acids and variations in the secondary and tertiary structures in the vicinity of the modified amino acids. Such that these modifications shifted the denatured GPx toward its native structure and subsequently enzymatic activity enhanced.
Also, in vivo result is consistent with data obtained from in vitro assay, which this accordance is notable (Fig. 3). The slight difference observed in enzymatic activity between in vitro and in vivo assessments could be due to the direct treatment of glycated GPx in exposure to plasma plum and short-lived reactive species. In the gas phase, plasma generates different types of ions and radicals that can interact with an aqueous solution to produce biodiverse reactive species with short/long lifetime in a liquid phase. Some of the short-lived RONS such as hydroxyl (OH·), superoxide ( − O 2 ), and singlet oxygen (O·) have been measured in an aqueous solution 28 . These species decay to produce long-lived species such as ozone (O 3 ), hydrogen peroxide (H 2 O 2 ), nitrite ( − NO 2 ), and nitrate ions ( − NO 3 ) 28 . So, in direct treatment, the glycated GPx is affected by either short-lived species or long one, whereas glycated GPx in the bloodstream could receive only the effect of the long-lived species.
Reduction of H 2 O 2 concentration in the blood of affected mice shows an increase in GPx activity, which implies a modification in protein structure, due to the fact that activity and structure of a protein are correlated to each other. As a result of this phenomenon, the oxidative stress reduced and inclined the body to the balanced  www.nature.com/scientificreports www.nature.com/scientificreports/ state between oxidants and antioxidants. The decrease in the oxidative stress led to the reduction in the oxidation of proteins, lipids, and lipoproteins. Therefore, the oxidative stress biomarkers, including MDA, AOPP, and oxLDL diminished after plasma treatment.
Also, It is widely proved that low-density lipoprotein (LDL) particles are extremely susceptible to oxidative damage 29 . In other words, ROS could attack to LDLs and oxidize the lipids and protein components of LDL particles 30 . oxLDL plays a vital role in the initiation, development, and progression of atherosclerosis, and causes endothelial inflammation, and endothelial dysfunction 31,32 . Uptake of oxLDL by lectin-like oxLDL receptor-1 (LOX1), the receptor on endothelial cells, and macrophages via the scavenger receptors leads to the promotion of foam cell formation and increases the release of inflammatory cytokines including IL-1, IL-6, and TNF-α 30,31,33,34 . So, in our work, the decrease in oxLDL due to the reducing of either oxidant or oxidative stress can be related to the declined expression of inflammatory cytokines such as IL-1, IL-6, and TNF-α.
It is well known that the presence of RONS, as the essential cellular second messengers, are necessary for the physiological performance of insulin but, it turns out to that RONS is also associated with resistant of body cells to insulin 35 . That is (to say), they can have a physiological or pathological effect on insulin functions 6 . It seems the extension of RONS along with induction of oxidative stress alters the structure of the protein that acts as insulin signaling molecules or otherwise affects insulin signaling pathways in a harmful way 35 . Therefore, the decrease in oxidative stress could reduce the resistance of cell and body to insulin and improve insulin performance. One of the results in the following of these phenomena could be the reduction in the BGL.
On the other hand, rising BGL content leads to non-enzymatic glycation process or Millard reaction involves unstable Schiff 's bases formation through the attachment of carbonyl group of reducing sugar (such as glucose) to free amino group in proteins. As a result, rearrangement to stable ketoamine products, or Amadori compounds occur 36 . These products can undergo further reactions, such as cyclization, and condensation, to form AGEs 37 . It is well known that reduction of BGL diminishes non-enzymatic glycation process of protein or lipid as a result of exposure to glucose. These phenomena result in decreasing of AGEs concentration of the plasma-treated diabetic mice.
Moreover, the effect of the cold plasma light also should be considered. The mitochondria rotary motor that is known as ATP synthase is the most efficient molecular motor 38 . It consists of two parts: a largely hydrophilic and a hydrophobic part 39 . So, a local change in nanoscopic interfacial water viscosity alters the performance and the dynamics of the nanomotor system 38 .
Mitochondria produces both ROS and ATP compounds that ROS can affect the intramitochondrial space. These species can improve hydrophilicity and thus the viscosity of the interfacial water layers bound to exposed surfaces of intramitochondrial. So oxidative stress situation leads to an increase in interfacial viscosity, along with www.nature.com/scientificreports www.nature.com/scientificreports/ an increase in viscous friction, which contributes to the drop in ATP production related to performance reduction of the rotary motor 38 .
It has been shown that the nanoscopic interfacial water layers structure can be modified by different wavelengths of laser lights such as 633 and 670 nm 38,40,41 . The modification involves expansion of the volume and reduction of viscosity, particularly on hydrophilic surfaces, that are known as promoting high viscosities 42,43 . As illustrated in Fig. 1, CAP spectrum includes different emissions; in ultraviolet, infrared, and visible regions. These emissions might be effective in modulation of interfacial water, promotion of mitochondrial ATP synthase and procession of glucose metabolism. So, the reduction in BGL can be the result of emitted light-spectra by CAP. This reduction might contribute directly to reduced GPx glycation (enhancement of antioxidant activity) and AGEs index, and might have an indirect outcome in the favorable change of MDA, AOPP, oxLDL, and inflammation factors indices.
Metformin is primarily utilized for Type 2 diabetes treatment as well as oral anti-diabetes agent 44 . Although, it has considerable benefits, including low risk of hyperglycemia, weight neutrality, cost-effective, and possible cardiovascular advantages, up to 25% of patients treated with metformin suffer metformin-associated gastrointestinal (GI) side-effects, and 5% are unable to tolerate metformin due to the severity of these side effects 45 .
As a conclusion, the result of this study revealed that the CAP could be effective in the modification of glycated GPx (as denatured protein) and improvement of its enzymatic activity. Also, the obtained data showed that the cold plasma treatment has a positive effect on the reduction of oxidative stress, AGEs, and inflammatory cytokines concentrations in diabetic mice. So, the cold plasma treatment, as a novel and complementary method, can be utilized with metformin for diabetes therapy to lower metformin dosage and reduce metformin's side effects.

Materials. Atmospheric Pressure Plasma Jet (APPJ) device and instrumentation. As illustrated schematically
in Fig. 6, the APPJ consisted of a dielectric, powered, and ground electrodes, and a high voltage power supply. A Pyrex tube (L: 150 mm, ID: 4 mm, OD: 6 mm) utilized, as the dielectric barrier and the nozzle. A copper rod (L: 30 mm, D: 1 mm) and a thin copper cylindrical tube (L: 15 mm) used as the powered and ground electrodes, respectively. The powered electrode was inserted in the tube from one end, while the other tube end was surrounded by the ground electrode, such that the distance between the nozzle tip and the lower edge of the ground electrode was 5 mm. The plasma was generated by a 10 kHz pulsed DC power supply with an amplitude up to 10.0 kV. Also, argon gas with a purity of 99.999%, and 3 l min −1 flow rate used as feeding gas.
Assay kits. Activity assay kit of GPx (D-89075) was purchased from Biocore Diagnosik Ulm GmbH Co (Germany). Quantity assay kits of IL-1β (MBS175967), AGEs (MBS704846), AOPP (MBS263319), MDA (MBS264973), and oxLDL (MBS2512757) were acquired from MyBioSource Co (USA). Quantity assay kits of IL-1α (BMS611), IL-6 (LMC0061), and TNF-α (BMC607-3) were obtained from Termo Fisher Chem Co (USA). Also, quantity assay kits of H 2 O 2 (E-BC-K102) and glucose (81692) were supplied from Elabscience and Crystal Chem companies (USA), respectively. Streptozotocin (STZ) (S0130) and nicotinamide (NA) (N0636) were purchased from Sigma-Aldrich Co (USA). Methods. In vitro. Optical Emission Spectroscopy (OES). CAP applications mostly are based on the generation capability of sufficient amounts of various reactive species. So, optical emission spectroscopy (OES) was used to analyze the existence, and intensity of any species. For this purpose, an ocean optic HR 2000 spectrometer was employed. The spectral range was chosen from 200 to 11000 nm with an optical resolution of 0.5 nm. Plasma optical emissions recorded at 20 mm distance from plasma stream. According to the atomic spectra database lines, the recorded spectrum was analyzed and different species were determined.
Preparation of glycated GPx solution. A pure solution of GPx (10 mg mL −1 ) was prepared by combining the protein with phosphate buffered saline (PBS) at pH 7.4, and a 50 mM l −1 glucose solution was made by mixing pure glucose with PBS. An aliquot of the prepared GPx solution was combined with glucose solution and named glycated GPx. Then, pure GPx and glycated GPx samples were filtered under the sterilized condition and incubated for 16 weeks at 37 °C 46,47 . At the end of 16th week, Glycated GPx was treated by cold plasma, and an aliquot of each of the three solutions (pure GPx, glycated GPx, and treated glycated GPx) was taken, and kept at −80 °C until could be analyzed by three methods: Circular Dichroism (CD), fluorometry, and activity assay 48 .
CAP treatment. CAP treatment was performed by direct exposing the prepared Glycated GPx to the plasma jet stream (Fig. 6). Two mL of the glycated enzyme solutions loaded in a 2 mL 96-well plate, and the the cold plasma applied at the 15 mm distance from the the solution surface with four different durations (60, 120, 300, and 600 seconds). Also, the untreated enzyme solution considered as the control sample.
Function analyze. Measurement of GPx Function was performed by related activity assay kit, enzymatic colorimetric method, and Tecan's Sunrise absorbance microplate reader (Switzerland). The enzyme activity was measured as U mL −1 . www.nature.com/scientificreports www.nature.com/scientificreports/ Fluorescence spectroscopy. Each sample with a concentration of 0.5 mg mL −1 was analyzed by Shimadzu spectrofluorometer RF-5000 (Japan, Kyoto). Wavelengths of 350, and 440 nm were considered as excitation, and emission wavelengths, respectively. The results have been presented as percentage 46,49 .
In vivo. Subjects. Sixty male BALB/c type mice aged 6 weeks with an average body weight of 30 g (bought from Pasteur Institute of Iran) were housed in a vivarium under controlled condition (a temperature of 23 ± 3 °C, and a relative humidity of 50 ± 10%) with a 12:12 h light-dark cycle, and had free access to rat chow and water ad libitum. After one week of acclimatization to these conditions, forty-five mice which had showed favorable growth were selected and randomly allotted into three groups (n = 15): (1) Nondiabetic control (Normal ones without any interference), (2) Diabetic control (Diabetic ones involved by STZ), (3) Affected diabetics (Diabetic ones involved by STZ, and affected by cold plasma). Diabetes was induced in mice (groups 2, and 3) by intraperitoneal (i.p.) injection of a single dose of STZ (50 mg kg −1 ) that after 15 min followed up by administration of nicotinamide (NA) (120 mg kg −1 ). One week after injection, serum glucose ≥200 g L −1 (checked by glucose assay kit (Crystal Chem Co, USA)) was considered as diabetes 52 . The animal ethics review committee (the Biological Research Institute of Cognitive, and Brain Sciences, Shahid Beheshti University) approved the study protocol under the international guidelines for the care and use of laboratory animals 53 .
CAP treatment. In vivo assay involved treatment of 15 diabetic mice (group 3) by APPJ. According to the information obtained from the analysis of in vitro processes, each mouse treated for 600 seconds. The back skin of the mice was chosen for treatment (Fig. 6).
Sampling. One week after exposure to plasma, blood samples of mice were obtained from the vein of their orbits. Related serum samples of each group were prepared to detect the biochemical parameters. Recent preparation was accomplished during 15 min centrifugation of blood at 5000 × g, clot separation, and storage at 70 °C for further studies.
Oxidants analyze. Oxidant parameters, including AGEs, AOPP, MDA, and oxLDL were measured by enzyme-linked immunosorbent assay (ELISA) techniques according to their assay kits and ELISA reader apparatus (MR-96A, Mindary Co) (Germany). H 2 O 2 was detected by the colorimetric method, quantity assay kit, and Tecan's Sunrise absorbance microplate reader (Switzerland).
Antioxidant analyze. The function of GPx was determined according to the method described in in vitro section.
Glucose analyze. The enzymatic colorimetric method, related kit, and Tecan's Sunrise absorbance microplate reader (Switzerland) were used to determine serum glucose of mice. Data processing. Statistical analyze. All data are expressed as the mean value of 45 mice blood analyze results. In this study, 11 variables were investigated for each of the groups. For this purpose, statistical analysis were performed through the ANOVA test with 0.001 significant level (p < 0.001) by the dplry package of R software (R-3.5.2 version). Also, based on the ANOVA test, the posthoc test (Tukey-HSD test) was applied for pairwise comparison of the three groups means.