Discussion
We demonstrated that total and native thiol levels were significantly lower in patients with CKD (stage 3-5) and patients receiving HD than healthy subjects. However, disulfide levels were significantly higher only in patients receiving HD. Moreover, TrxR enzyme levels were significantly higher both in patients with CKD (stage 3-5) and patients receiving HD than healthy subjects. Our study also revealed that IMA and TOS levels were significantly higher in both CKD (stage 3-5) and HD groups compare to controls. However, oxidative stress index (OSI) levels which are calculated by TOS/TAS ratio were significantly higher only in the HD group compare to control.
Coskun et al. showed that the native and total thiol levels of the patients receiving HD treatment were significantly lower than the control group. They hypothesize that low native and total thiol levels occurred as a result of oxidative stress and chronic inflammation in HD patients. In another study, Ates et al. reported that native and total thiol levels were lower in HD patients compared to the control group and they associated this decrease with the reduced total thiol reserves in the organism [11].
In the same line with previous studies, we found that native and total thiol levels in plasma samples of HD patients were lower than both CKD and control groups. Our results were consistent with the literature. One reason of the decrease in plasma thiol levels may be the continuous depletion of sulfhydryl-containing antioxidant molecules, particularly glutathione, to remove ROS as previously suggested [11]. However, although the levels of glutathione as one of the antioxidants are known to be high in the cell, the contribution of other low molecular weight thiol compounds to the plasma sulfhydryl pool is relatively low compared to albumin [14]. Therefore, reduced glutathione levels may not be sufficient to explain the total thiol decrease alone in CKD (stage 3-5) patients.
Another reason contributing to decrease in thiol levels can be explained by the dynamic relationship between albumin and thiol balance. It is known that albumin constitutes most of the thiol pool in plasma (70-90%). Albumin is known to be irreversibly converted into end products as a result of prolonged oxidative damage. One of these albumin-transformed products is sulfenic acid (RSOH), which results in an increase in the presence of oxidant, resulting in sulfinic (RSO2H) or sulfonic (RSO3H) acid formation, and these products have been suggested to be removed from the circulation through the liver. We may speculate that uremic toxins cause chronic elevation of oxidative stress in CKD patients, albumin is exposed to a constant oxidative stress.
As a result, albumin may be irreversibly converted and withdrawn from the circulation into oxidation products such as sulfenic, sulfinic, and sulfonic acid as previously shown under long-term oxidative stress in CKD patients. In addition, the liver’s depletion of plasma glutathione and sulfhydryl sources due to this increased detoxification metabolism may also contribute to low thiol depletion in plasma [12].
As it is well known, GFR is an important diagnostic and follow-up parameter used in predicting renal function loss in renal diseases and is used in the staging of CKD [4]. In this study, we examined the correlations between thiol groups and GFR and found a positive and strong correlation between both native and total thiol levels and GFR. Plasma native and total thiol levels positively and highly correlated with GFR which suggests that thiols can be used as a test parameter related to disease prognosis in CKD patients.
We also evaluated the effect of HD session on native and total thiol levels and disulfide parameters. Total and native thiol levels of samples measured after dialysis were significantly higher compared to the ones before dialysis. However, there was no significant difference between two groups in terms of disulfide level after the correction with albumin, there was no significant difference between the native and total thiol values. On the other hand, the decrease in disulfide levels of the samples after HD was statistically significant. In other words, single HD session did not have a significant effect on total and native thiols, but resulted in a significant decrease in disulfide levels. We consider that volume correction, which may be caused by dialysis, may be especially important in comparing thiol values ​​associated with albumin. In addition to that, this decrease in disulfide level may be related to the regeneration of plasma thiol redox status by hemodialysis as stated in the previous studies [10].
In the literature, there are only two studies evaluating the effect of hemodialysis on plasma dynamic thiol balance by using Erel method [10]. In these studies, a correction for a possible volume change due to hemodialysis was not mentioned. It is known that during the HD procedure, different degrees of hemoconcentration can occur in the blood due to volume withdrawal from the patients after HD treatment. The resulting increase in analyte concentrations which was proportional to the volume withdrawn may be an important cause of interference, especially for large molecular weight albumin or albumin-related compounds. In the present study, unlike the previous two studies, we determined albumin levels in blood against a possible hemoconcentration before and after the dialysis. Albumin values were significantly higher in the samples after the dialysis.
TrxR enzyme is a selenoprotein that effectively converts the oxidized thioredoxin protein (Trx) to its reduced form. Therefore, they are responsible for the regeneration of Trx and play a role in maintaining the antioxidant effect. In the present study, increased serum TrxR enzyme levels in CKD patients may be explained by the over-expression of the enzyme to increase the antioxidant effect against increased oxidative stress, as suggested in previous studies. As a result, serum TrxR values were significantly higher in CKD (stage 3-5) and HD group compared to healthy controls. This increase was more prominent in the HD group.
IMA is a modified form of albumin due to oxidative stress. Elevated plasma levels of IMA have been shown in diseases associated with increased oxidative stress, particularly in ischemic heart disease [23,24]. In this study, we found that IMA levels were higher in CKD and HD groups compared to the control. However, there was no significant difference between HD and CKD groups. In the literature, Turedi et al. reported that IMA levels of patients receiving HD were found to be higher compared to healthy controls [25]. In our study, in accordance with the literature, increased IMA levels support the view that increased oxidative stress may lead to albumin modification.