Reduced Glutathione Peroxidase (GPx) Activity in Patients with Cutaneous Leishmaniasis

Leishmaniasis is a tropical parasitic infection caused by the genus Leishmania spp. This intracellular parasite is transmitted via the bite of a sand fly vector. Cutaneous leishmaniasis (CL), the predominant form of infection, has a worldwide distribution and affects about 1 million people each year in endemic countries (1). Cutaneous leishmaniasis is a significant public health and social issue in Iran due to its high incidence (77 per 100,000 population), the absence of an effective vaccine, and the lack of effective antileishmanial drugs (2, 3). Although the pathogenesis of CL is not fully understood, evidence has shown that the host immune system response plays a vital role in the disease outcome (4).

Oxidative stress is important in the pathogenesis of leishmaniasis. During infection, skin macrophages phagocytize the promastigote form of the parasite, causing it to convert into the amastigote stage (5). Leishmania amastigotes induce an increase in macrophage oxygen consumption and the generation of reactive oxygen species (ROS) in the phagosome, leading to the eventual death of phagocytosed microorganisms. Reactive oxygen species are highly reactive molecules that can cause cell and tissue damage due to an oxidant-antioxidant imbalance (6). The host body possesses antioxidant defense systems, including antioxidant enzymes, to protect cells from ROS damage. Some antioxidant enzymes require a sufficient quantity of trace elements (TE) to function (7).

Copper, zinc, and selenium (Se) are key elements vital to the immune system. These trace elements support antioxidant enzymes that help the body eliminate harmful free radicals by acting as cofactors. Antioxidant levels vary in a wide range of illnesses, including leishmaniasis. These alterations are part of the organism's defense strategies and are induced by different cytokines (8, 9). Glutathione peroxidase (GPx), an enzyme that recycles glutathione and catalyzes the oxidation of reduced glutathione by H₂O₂ and other hydroperoxides to generate oxidized glutathione and water, requires selenium, making it a vital nutrient for human health (10, 11). In the presence of reduced glutathione (GSH), this enzyme catalyzes detoxification (12). Previous research has found that CL patients have lower GPx enzyme activity and higher GSH concentrations (7).

However, while the significance of Se and its redistribution during Leishmania infection is well-documented, information regarding GPx activity is limited. The aim of this study was to measure GPx activity in patients with CL.

The matching process was strictly conducted by our team for age and two physical characteristics, including weight and body mass index, between the case and control groups to minimize bias in the outcomes. The data reveal that these parameters were comparable across the two groups (P > 0.05), indicating the absence of any confounding factors. In the study, a significant reduction in GPx activity was observed in CL patients compared to the control group (67.85 ± 12.67 vs. 100.80 ± 11.21 U/g Hb, P < 0.001).

Because antileishmanial drugs have significant adverse effects and there is no vaccine to prevent the disease, finding biomarkers that interfere with disease pathogenesis can aid in the diagnosis and treatment of CL (15). The current study discovered that GPx activity was considerably reduced in CL patients.

The immune system protects the host from Leishmania infection through various mechanisms, including NADPH oxidase, inducible nitric oxide synthase (iNOS), and myeloperoxidase activation. In response to parasite invasion, activated macrophages produce ROS such as superoxide (•O₂), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH) (16). Recently, there has been a surge of interest in identifying oxidative stress indicators in CL patients to estimate disease severity (17). Several investigations have shown an increase in ROS production and a decrease in antioxidant defense levels in leishmaniasis patients, indicating that oxidative damage plays a significant role in disease pathogenesis (18-20). Prolonged exposure to high levels of ROS leads to the development of leishmaniasis and tissue damage (17).

Minerals have been shown to affect the antileishmanial immune response through the production of antioxidant enzymes. Selenium has been linked to the pathophysiology of CL (21). This micronutrient element participates in free radical scavenging by functioning as a cofactor for many antioxidant enzymes such as GPx (22). Se deficiency can be caused by malnutrition or by various illnesses, including leishmaniasis (23). Prior studies have shown that patients with CL have reduced amounts of selenium in their serum (13, 24-26). Decreased serum Se during parasitic diseases has been related to the redistribution of Se from the plasma pool into the tissues, increased metabolism or consumption, stress, or hyperthermia (27). Furthermore, it has been proposed that Se deficiency, as a host defensive mechanism, kills parasites by lowering GPx activity and increasing the number of hydroperoxides (24). Several studies have found a link between low Se levels in the blood and the severity of leishmaniasis, suggesting that Se may have therapeutic potential in severe leishmaniasis (22, 26). We examined enzyme activity to test the hypothesis that lower Se levels would diminish GPx activity in patients.

Glutathione peroxidase, a selenoenzyme, plays a vital role in the antioxidant defense system by protecting membranes against peroxide damage (27). Numerous parasitic diseases, such as leishmaniasis, have been shown to modify antioxidant enzymes (5). In line with previous research, a decrease in GPx activity was observed in patients with CL, suggesting that this enzyme may be involved in the pathogenesis of CL (7, 18). Various studies have reported decreased GPx enzyme activity and elevated rGSH concentrations in CL patients, which were reversed after treatment (16-18). During the infection, increased production of ROS and reactive nitrogen species (RNS) leads to a decrease in GPx activity (28). The release of oxygen and nitrogen free radicals by neutrophils and macrophages reduces GPx activity and/or levels by consuming the enzyme during ROS scavenging (29, 30). The increased content of hydrogen peroxides in CL patients' erythrocytes was demonstrated in a previous study (16). In addition, the lack of selenium in CL patients may lead to diminished GPx activity or suppression of GPx expression, resulting in an accumulation of H₂O₂ and an increase in lipid peroxidation (24). This result is consistent with Kocyigit et al.'s study, which demonstrated a considerable decrease in serum Se and GPx activity in patients with CL (13). Furthermore, a decrease in the amount of GSH, the substrate of GPx, may result in decreased GPx activity in CL patients (31). Therefore, it seems that GPx enzyme activity can be considered a diagnostic and prognostic biomarker in CL patients, as well as a treatment monitoring tool.

The main findings and strength of the present study were the possible role of GPx enzyme activity in the development of CL. However, there were some limitations in this study. First, while the association between GPx enzyme activity and CL progression is established, a comparative analysis of GPx enzyme activity in other forms of leishmaniasis would have provided more context for understanding the current investigation's findings. Second, the study only examined a small sample size; as a result, larger studies examining a greater number of samples are required.