Protein oxidation, glycation, and carbamylation in patients with adrenal masses

Barbara Choromańska; Piotr Myśliwiec; Marta Lewoc; Jacek Dadan; Katarzyna Choromańska; Anna Zalewska; Mariola Matulewicz; Małgorzata Żendzian-Piotrowska; Mateusz Maciejczyk

doi:10.3389/fmolb.2026.1811270

. 2026 Jul 9;13:1811270. doi: 10.3389/fmolb.2026.1811270

Protein oxidation, glycation, and carbamylation in patients with adrenal masses

Barbara Choromańska ¹, Piotr Myśliwiec ¹, Marta Lewoc ¹, Jacek Dadan ¹, Katarzyna Choromańska ², Anna Zalewska ³, Mariola Matulewicz ⁴, Małgorzata Żendzian-Piotrowska ⁵, Mateusz Maciejczyk ^5,^*

PMCID: PMC13391313 PMID: 42495442

Abstract

Adrenal masses are the most common of all tumors in the adult population, and their size is associated with malignancy. The knowledge about the pathogenesis of adrenal tumors is limited. Objectives: Evaluation of the association between protein glycooxidation and carbamylation, as well as thiol-disulfide homeostasis and adrenal tumor size. The study group included 71 patients (39 women and 32 men) with adrenal masses, divided into two groups: 34 patients with adrenal masses <4 cm in diameter and 37 patients with adrenal masses >4 cm in diameter. Colorimetric and spectrophotometric methods were used in this study. The protein oxidative damage (increase in ischemia-modified albumin) and glycooxidation rate (increase in kynurenine, dityrosine, Amadori products, advanced glycation end products, and carbamyl-lysine [CBL]) were significantly higher in patients with adrenal masses than in comparison with the control group. Additionally, thiol-disulfide homeostasis (decrease in total thiols and native thiols and increase in disulfide) was disturbed in these patients. The plasma content of CBL seemed to express possible diagnostic utility. This parameter, with 75.68% sensitivity and 74% specificity, differentiates patients with adrenal tumors (both AM < 4 cm and AM > 4 cm) from the controls. Moreover, plasma content of CBL has a diagnostic value in distinguishing AM < 4 cm from those AM > 4 cm. In conclusion, protein glycoxidation and thiol-disulfide homeostasis are impaired in patients with adrenal tumors. Although oxidative stress is involved in the progression of carcinogenesis, oxidation and glycoxidation of plasma proteins, as well as thiol-disulfide homeostasis, were not dependent on tumor size. Only the evaluation of CBL plasma content seems helpful in predicting the malignant risk of adrenal masses. More research is needed to confirm our results.

Keywords: adrenal tumors, carbamylations, disulfide, glycoxidation, thiol

Introduction

Adrenal tumors, due to their non-specific symptoms, are a serious health problem. Although most of them are benign and hormonally inactive, some may have hormonal activity, with overproduction of cortisol (or other steroids), aldosterone, or catecholamines (Grumbach et al., 2003; Jason and Oltmann, 2019; Ebbehoj et al., 2020). These hormones play an important role in many physiological processes. Cortisol is primarily anti-inflammatory. Under stress, it affects the metabolism of carbohydrates, lipids, and proteins, increasing the concentration of glucose and intensifying lipolysis of adipose tissue (Jonczyk, 2014; Bin Rubaia’an et al., 2021). Catecholamines act as neurotransmitters and take part in the regulation of blood circulation, whereas aldosterone is involved in the regulation of water and mineral balance (Erlic and Beuschlein, 2019; Karwacka et al., 2021). Excessive secretion of these hormones by adrenal tumors may be associated with severe morbidity and even mortality (Clayton, 2022). The risk of malignancy depends on the size of the adrenal mass. In the report from the National Italian Study Group, selecting a 4 cm cutoff had a sensitivity of 93% in detecting adrenocortical carcinoma (Angeli et al., 2008). In most cases, complete surgical resection is the only potentially effective treatment for adrenal masses.

Until now, little has been known about the pathogenesis of adrenal tumors. Risk factors for developing these tumors include hormonal contraceptives in women, smoking, and genetic abnormalities. The mutation of the VHL/HIF axis as a risk factor for the development of pheochromocytoma has been the most extensively studied (Vaidya et al., 2018). It has been shown that deregulation of the hypoxia-inducible factor (HIF-1) may have a different effects on tumor growth and metabolism, as well as on metastases (Fishbein et al., 2017). HIF-1α exerts different effects depending on the amount of oxygen delivered to cells (Movafagh et al., 2015). Under conditions of normal cellular oxygen levels, it is degraded by the ubiquitin-proteasome pathway. However, hypoxia leads to its excessive accumulation and, consequently, to the disruption of many genes involved in cancer pathogenesis (Rashid et al., 2021). In our previous studies, we found that the pathogenesis of adrenal masses may be associated with an antioxidant/oxidative imbalance in patients with different types of adrenal tumors (non-functional incidentaloma, pheochromocytoma, and Cushing’s/Conn’s adenoma) (Choromańska et al., 2021a; 2021b). Excessive production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) is known to initiate the process of lipid and protein oxidation (Förstermann, 2008; Choromańska et al., 2020; Zińczuk et al., 2020). It is important to mention the detrimental effect of the glycooxidation process on cellular biomolecules. Glycooxidation exerts the most toxic effect on proteins, as it causes their aggregation, denaturation, and modification/loss of physiological function (Stadtman and Levine, 2003). Moreover, accumulation of highly cytotoxic products of protein glycooxidation leads to further cellular damage and consequently, to the dysregulation of multiple signaling pathways, initiation of inflammatory processes, and cell death (El-Bikai et al., 2010; Maciejczyk et al., 2020). The importance of the carbamylation process in the development of various diseases is also emphasized. Carbamylation is a post-translational modification of proteins resulting from a non-enzymatic reaction in which isocyanic acid binds to free amino groups of proteins. This process, similarly to glycooxidation, can lead to an abnormal structure and function of proteins. In contrast, thiol-disulfide homeostasis has a protective function against irreversible oxidation of protein cysteine residues. Thiol-disulfide homeostasis, which reflects the levels of thiols and disulfides, relies on the reversal of the thiol oxidation process in proteins (Erel and Erdoğan, 2020). Dysregulated thiol-disulfide homeostasis has been observed in many diseases such as coronary atherosclerosis, Alzheimer’s disease, type 2 diabetes, and lung cancer (Kundi et al., 2015; Dirican et al., 2016; Gumusyayla et al., 2016; Ergin et al., 2020). Until now, nothing has been known about protein glycooxidation and the carbamylation process, as well as thiol-disulfide homeostasis in patients with adrenal tumors. It is also unclear whether disorders in these processes are associated with the progression and growth of adrenal tumors. Tumor size is considered to be the best predictor of malignancy in adrenal masses (Barnett Jr et al., 2000). It was shown that 6% of adrenal tumors in the size range of 4.1–6 cm may become malignant, whereas tumors larger than 6 cm may be malignant in as many as 25% of cases (Bhoster et al., 1932). Therefore, the aim of our study was to evaluate protein glycooxidation and thiol-disulfide homeostasis in patients with adrenal masses depending on the tumor size.

Materials and methods

The study protocol conformed to the Guidelines for Good Clinical Practice and the Declaration of Helsinki, and was approved by the Bioethics Committee of the Medical University of Bialystok (permission code: R-I-002/66/2015, APK.002.341.2020). All patients provided informed consent to participate in this study.

The study group included 71 patients (39 women and 32 men aged 42–76 years) with adrenal masses, who had been diagnosed at internal medicine departments with an endocrinology profile. Preoperative workup included imaging with adrenal CT protocol, assessment of the daily cortisol rhythm, the dexamethasone suppression test, serum dehydroepiandrosterone, aldosterone and the aldosterone/renin ratio, electrolytes, as well as metanephrine and normetanephrine in daily urine. Patients underwent elective endoscopic adrenalectomy (via the lateral transperitoneal approach or the posterior retroperitoneal approach) at the First Department of General and Endocrine Surgery at the University Hospital in Bialystok, Poland. Patients were divided into two groups depending on the tumor size: 34 patients with adrenal masses <4 cm in diameter (AM < 4 cm) and 37 patients with adrenal masses >4 cm in diameter (AM > 4 cm). The cutoff value of 4 cm was used to divide the patients with adrenal tumors into two groups. This threshold was based on studies suggesting that adrenal incidentalomas larger than 4 cm should be considered for adrenalectomy because of the increased risk of malignancy (Mantero et al., 2000). These recommendations are consistent with the current Polish guidelines (Handkiewicz-Junak et al., 2024). Additional radiological features associated with a higher risk of malignancy include high unenhanced computed tomography attenuation values (>10 Hounsfield units), contrast washout below 60%, irregular shape, heterogeneous density, tumor calcifications, and rapid growth in consecutive imaging studies. Therefore, some adrenal masses smaller than 4 cm in our cohort were also classified as incidentalomas.

In the AM < 4 cm subgroup, 17 patients were diagnosed with incidentaloma, 5 with Cushing’s syndrome, 4 with Conn’s syndrome, and 8 with pheochromocytoma, whereas in the AM > 4 cm subgroup, 27 patients had incidentaloma, 5 had pheochromocytoma, 3 had Cushing’s syndrome and 2 had Conn’s syndrome. Patients suffering from Conn’s syndrome were treated with an aldosterone receptor blocker (spironolactone) and/or potassium supplementation in the preoperative period. Patients with pheochromocytoma received doxazosin (a selective alpha-1-adrenergic receptor blocker) for at least 14 days before surgery to prevent an intraoperative hypertensive crisis. The patients in the study group had creatinine results within the reference range.

Fifty healthy volunteers (25 women and 25 men aged 44–68) were assigned to the control group. They visited the Specialist Dental Clinic at the Medical University of Bialystok for routine dental follow-up examinations. These patients did not have periodontal disease or active caries. The full blood counts, INR, and biochemical blood parameters (creatinine, ALT, AST, K⁺, and Na⁺) were within the reference range.

The exclusion criteria for the control and study groups were: pregnancy among women, alcohol abuse, smoking, acute inflammation, infectious diseases (HIV/AIDS, hepatitis A, B and C) neoplastic diseases, cardiovascular diseases (other than hypertension), diseases of the respiratory, digestive and genitourinary systems, autoimmune diseases (ulcerative colitis, Crohn’s disease and Hashimoto’s disease) and metabolic diseases (gout, insulin resistance, type 1 diabetes, osteoporosis and mucopolysaccharidosis) and kidney diseases as well as intense physical activity within 24 hours before blood sampling. Patients in the study group had creatinine results within the reference range. Additionally, during the 3 months prior to sampling, all qualified patients did not take antioxidant supplements (including iron preparations), non-steroidal anti-inflammatory drugs, glucocorticosteroids, or antibiotics.

Blood collection

All blood samples from healthy controls and patients with adrenal masses were collected in a fasting state into serum and EDTA pre-coated tubes (SARSTEDT, S-Monovette) and centrifuged at 4,000 rpm for 10 min at 4 °C to obtain serum and plasma. A measure of 10 μL of 0.5 M butylated hydroxytoluene (BHT) was added to 1 mL of plasma to avoid oxidation of the samples, which were then stored at −80 °C until final assessments (Zińczuk et al., 2020). ischemia-modified albumin (IMA) was determined in the serum, and the remaining parameters were assessed in plasma.

Laboratory measurements

A Sysmex XN1000 automated blood analyzer was used to measure the full blood count. Biochemical tests (Na⁺, K⁺, cortisol before 10 a.m., serum aldosterone, urine metanephrine and normethanephrine, and glucose) were analyzed using an Abbott analyzer (Abbot Diagnostics, Wiesbaden, Germany). The laboratory tests of the controls and study group patients were assessed in the Laboratory of Biochemical Diagnostics, University Hospital in Bialystok, Poland. The clinical and laboratory characteristics of the controls and patients with adrenal masses are shown in Table 1.

TABLE 1.

Laboratory characteristics of the controls, patients with adrenal masses <4 cm in diameter (AM < 4 cm), and patients with adrenal masses >4 cm in diameter (AM > 4 cm).

	Controls (n = 50)	AM < 4 cm (n = 34)	AM > 4 cm (n = 37)	ANOVA
Size of the tumor (cm)	-	2.6 ± 0.89	5.2 ± 1.1^^^^	p < 0.0001
Aldosterone (ng/dL)	11.75 ± 5.35	20 ± 11.51**	15.64 ± 8.97	p = 0.0031
Serum cortisol before 10 a.m. (µg/dL)	11.43 ± 4.19	15.88 ± 5.21***	13.76 ± 5.89	p = 0.0008
Urine methanephrine (µg/24h)	115.8 ± 64.67	254.4 ± 241.5**	212.3 ± 445.3	p = 0.0044
Urine normethanephrine (µg/24h)	232 ± 79.67	435.8 ± 305.9***	353.8 ± 224.5**	p = 0.0002
Glucose (mg/dL)	77.46 ± 5	95.9 ± 14.72****	107.3 ± 53.28****	p < 0.0001
Na⁺ (mmol/L)	139.9 ± 2.74	139.5 ± 2.45	139.6 ± 2.85	p = 0.6395
K⁺ (mmol/L)	4.35 ± 0.45	4.32 ± 0.49	4.462 ± 0.41	p = 0.5266
WBC (10³/μL)	6.75 ± 1.1	7.39 ± 1.91	7.45 ± 2.4	p = 0.3043
RBC (10⁶/μL)	4.534 ± 0.38	4.6 ± 0.39	4.56 ± 0.53	p = 0.682
HGB (g/dL)	13.87 ± 0.85	13.89 ± 1.22	13.94 ± 1.47	p = 0.9007
PLT (10³/μL)	288.6 ± 10.66	216.5 ± 55.42****	240.1 ± 61.15****	p < 0.0001
BMI (kg/m²)	23.81 ± 1.44	29.73 ± 5.64****	29.8 ± 6.28****	p < 0.0001

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Results are presented as mean with standard deviation. **p < 0.01, ***p < 0.001 ****p < 0.0001 indicate significant differences from the controls. ^^^^ p < 0.0001 indicates significant differences from the (AM < 4 cm); white blood cell count (WBC), red blood cell count (RBC), hemoglobin (HGB), platelet count (PLT), and body mass index (BMI).

Glycation products

The formation of amyloid cross-β structure and Amadori products was evaluated colorimetrically using a thioflavin T assay for amyloid (LeVine, 1999) and a nitro blue tetrazolium assay for Amadori products (Johnson and Baker, 1987). The characteristic fluorescence of amyloid was measured at 435/485 nm, and that of Amadori products at 525 nm, and an extinction coefficient of 12,640 cm⁻¹ mol⁻¹ L for monoformazan was used. The content of plasma advanced glycation end products (AGE) was measured spectrofluorimetrically at 350/440 nm by evaluating AGE-specific fluorescence (Kalousová et al., 2002).

Glycoxidation products

Fluorescence evaluation of protein oxidative modifications was peformed. The tryptophan (TRY), dityrosine (DT), kynurenine (KYN), and N-formylkynurenine (NFKYN) contents were analyzed fluorimetrically. Immediately before measurement, blood plasma samples were diluted in 0.1 M H₂SO₄ (1:5, v/v). The characteristic fluorescence at 295/340 nm for TRY, 330/415 nm for DT, 365/480 nm for KYN, and 325/434 nm for NFKYN was measured in 96-well black-bottom microplates (Diplock and Rice-Evans, 1991). The results were expressed as arbitrary fluorescence units (AFU)/mg protein.

Oxidation products

The content of protein carbonyls (PC) was assayed spectrophotometrically using 2,4-dinitrophenylhydrazine (2,4-DNPH) (Reznick and Packer, 1994). The absorbance of a stable complex of PC - 2,4-DNPH was measured at 593 nm. Serum IMA content was determined colorimetrically at 470 nm. The method uses the ability to bind exogenous cobalt (Co^{2 +}) of human serum albumin (Bar-Or et al., 2000).

Thiol/disulfide homeostasis

The content of total thiols (TT) and native thiols (NT) was simultaneously measured in a paired test. The concentration of native thiol groups was measured spectrophotometrically at 412 nm, using a modified Ellman’s reagent. In the parallel run, in the reaction with NaBH₄, dynamic disulfide bonds were reduced to free thiol groups. The TT amount was measured after removing the unused reductant using formaldehyde. The amount of disulfide bonds (DS) was calculated as half the difference between the total amount of thiols and the NT (Erel and Neselioglu, 2014).

Carbamylation products

The content of N^ε-(carboxymethyl) lysine (CML) and carbamyl-lysine (CBL) was determined colorimetrically according to the manufacturer’s instructions using commercial enzyme-linked immunosorbent assay kits (Cell Biolabs, Inc. San Diego, United States).

Statistical analysis

The sample size was determined a priori based on our previous pilot study. An online sample size calculator (ClinCalc) was used, and the minimum number of patients was 41 (level of significance = 0.05; power of study = 0.8).

The statistical package GraphPad Prism 9 for Mac (GraphPad Software, La Jolla, CA, United States) was used for statistical analysis. The distribution of the data was checked using the Shapiro–Wilk test, and the homogeneity of variance was assessed using Levene’s test. Due to the lack of normality in the data distribution, we used a non-parametric analysis of variance (ANOVA), namely, the Kruskal–Wallis test. The Dunn test was used for multiple comparisons, and the multiplicity-adjusted p-value was calculated. In the case of a normal distribution, ANOVA with Tukey’s post hoc was used. Pearson’s correlation coefficient was used to assess the correlation between the dependent variables. The assessment of the diagnostic utility of plasma biomarkers was based on receiver operating characteristic curves. The results for p < 0.05 were considered statistically significant.

Results

Table 1 shows a comparison of the laboratory characteristics between the controls and patients with adrenal masses according to tumor size. Urine normethanephrine, serum glucose, and plasma PLT levels were significantly higher in patients with adrenal masses than in the controls. As there were more patients with hormonally active adrenal tumors in the AM < 4 cm group than in the AM > 4 cm group, we also observed increased serum aldosterone and cortisol concentrations, as well as urine methanephrine in patients with adrenal tumors in the AM < 4 cm compared with the controls. BMI was greater in patients with adrenal masses as compared with the controls.

There were no differences in plasma PC content in any of the subgroups with adrenal masses compared to the control group (Figure 1A).

Box plot graphic with panel A showing PC plasma and panel B showing IMA serum, each comparing three groups: control (C, blue), AM less than four centimeters (red), and AM greater than four centimeters (green). Both panels present nmol per mg protein on the y-axis, with panel B indicating significant increases for the AM groups with asterisks. — The plasma content of protein carbonyls (PC) **(A)** and serum content of ischemia-modified albumin (IMA) **(B)** in the controls **(C)** and patients with adrenal masses <4 cm in diameter (AM < 4 cm) and patients with adrenal masses >4 cm in diameter (AM > 4 cm). Results are presented as median with minimum and maximum. ****p < 0.0001 indicates a significant difference from the controls.

The content of serum IMA was significantly higher in patients with adrenal masses than in the controls (Figure 1B).

The plasma TT in patients with adrenal tumors AM < 4 cm (−2%, p < 0.0001) and AM > 4 cm (−5%, p < 0.0001) was decreased in comparison with the healthy controls (Figure 2A).

Six grouped bar charts labeled A through F display levels of various plasma components and their ratios, with bars color-coded for control, AM less than four centimeters, and AM greater than four centimeters groups. All comparisons against control indicate significant increases, denoted by four asterisks. Each y-axis denotes the respective measurement in either micromoles per milligram protein or as a component ratio. — Plasma content of total thiols (TT) **(A)**, native thiols (NT) **(B)** and disulfide (DS) **(D)** as well as ratio NT/TT **(C)**, ratio DS/TT **(E)** and ratio DS/NT **(F)** in the controls (C) and patients with adrenal masses <4 cm in diameter (AM < 4 cm) and patients with adrenal masses >4 cm in diameter (AM > 4 cm). Results are presented as mean and standard deviation. ****p < 0.0001 indicates a significant difference from the controls. Patients with adrenal masses: AM < 4 cm (−25%, p < 0.0001) and AM > 4 cm (−27%, p < 0.0001) had lower content of plasma TRY than the controls (Figure 3A).

Similarly to TT, plasma content of NT was lower in patients with adrenal masses: AM < 4 cm (−2%, p < 0.0001) and AM > 4 cm (−5%, p < 0.0001) than the controls (Figure 2B).

The plasma DS content in adrenal masses subgroups: AM < 4 cm (+25%, p < 0.0001) and AM > 4 cm (+16%, p < 0.0001) was higher compared to the control group (Figure 2D).

Ratio of NT/TT plasma content was diminished in patients with adrenal masses: AM < 4 cm (−1%, p < 0.0001) and AM > 4 cm (−1%, p < 0.0001) (Figure 2C), while DS/TT and DS/NT plasma content ratio were markedly higher in subgroups with adrenal masses: AM < 4 cm (+24%, p < 0.0001; +24%, p < 0.0001, respectively) and AM > 4 cm (+19%, p < 0.0001; +24%, p < 0.0001, respectively) in comparison with the controls (Figures 2E,F).

Patients with adrenal mases: AM < 4 cm (-25%, p < 0.0001) and AM > 4 cm (-27%, p < 0.0001) had lower content of plasma TRY than the controls (Figure 3A).

The greater content of plasma KYN was only in patients with adrenal masses >4 cm in diameter (+16%, p = 0.0257) in comparison with the healthy controls (Figure 3B).

Four box plot panels labeled A to D show plasma metabolite levels (TRY, KYN, NFKYN, and DT) between three groups: controls, AM≤4cm, and AM>4cm. TRY and DT levels are significantly higher in both AM groups compared to controls, KYN is modestly elevated in AM>4cm, and NFKYN shows no significant difference. Statistical significance is indicated by asterisks above relevant box plots. — The plasma content of tryptophan (TRY) **(A)**, kynurenine (KYN) **(B)**, N-formylkynurenine (NFKYN) **(C)** and dityrosine (DT) **(D)** in the controls (C) and patients with adrenal masses <4 cm in diameter (AM < 4 cm) and patients with adrenal masses >4 cm in diameter (AM > 4 cm). Results are presented as median with minimum and maximum. *p < 0.05, ***p < 0.001, ****p < 0.0001 indicate significant differences from the controls.

There were no statistically significant differences in the plasma content of NFKYN in groups of patients with adrenal masses as compared to the controls (Figure 3C).

In plasma of patients with adrenal tumors: AM < 4 cm (+17%, p = 0.0121) and AM > 4 cm (+24%, p = 0.0004), the content of DT was increased in comparison with the control group (Figure 3D).

Figure 4A shows greater plasma content of Amadori products in study groups: AM < 4 cm (+26%, p < 0.0001) and AM > 4 cm (+26%, p < 0.0001) than in the controls.

Four box plot panels labeled A through D display plasma levels of specific biomarkers in control (C), AM less than 4 centimeters, and AM greater than 4 centimeters groups. Panel A shows higher Amadori products in both AM groups compared to controls, with significant increases noted. Panel B presents Amyloid levels with no clear differences among groups. Panel C depicts CML levels with similar distributions across groups. Panel D shows AGE plasma significantly elevated in both AM groups compared to controls. Colors distinguish groups: blue for controls, red for AM less than 4 centimeters, and green for AM greater than 4 centimeters. — The plasma content of Amadori products **(A)**, amyloid **(B)**, N^ε-(carboxymethyl) lysine (CML) **(C)** and advanced glycation end products (AGE) **(D)** in the controls (C) and patients with adrenal masses <4 cm in diameter (AM < 4 cm) and patients with adrenal masses >4 cm in diameter (AM > 4 cm). Results are presented as median with minimum and maximum. ****p < 0.0001 indicates significant difference from the controls.

The plasma content of amyloid and CML in patients with adrenal masses did not differ from the control group (Figures 4B,C).

The plasma content of AGE was significantly increased in both subgroups of patients with adrenal masses: AM < 4 cm (+37%, p < 0.0001) and AM > 4 cm (+44%, p < 0.0001) as compared to the controls (Figure 4D).

The markedly higher plasma content of CBL was in patients with adrenal tumors: AM < 4 cm (+18%, p = 0.022) and AM > 4 cm (+51%, p < 0.0001) than the controls. Moreover, plasma content of CBL was higher in AM > 4 cm subgroup (+27%, p = 0.0347) in comparison with AM < 4 cm patients (Figure 5).

Box plot chart labeled "CBL plasma" compares pg/mg protein levels across three groups: control (C, blue), AM less than four centimeters (red), and AM greater than four centimeters (green), showing increasing median values with significant differences indicated by asterisks and symbols. — The plasma content of carbamyl-lysine (CBL) in the controls and patients with adrenal masses <4 cm in diameter (AM < 4 cm) and patients with adrenal masses >4 cm in diameter (AM>4 cm). Results are presented as median with minimum and maximum. *p < 0.05, ****p < 0.0001 indicate significant differences from the controls, ^ p < 0.05 indicates significant difference from the (AM < 4 cm).

Correlations

Correlations between the analyzed markers of protein glycation, glycoxidation, oxidation, and carbamylation products, as well as thiol-disulfide homeostasis, are shown in Figure 6.

Correlation matrix heatmap with variables including tumor size, various hormones, biochemical markers, and products, with values ranging from minus one to one. Blue indicates positive correlation, red indicates negative correlation, and white represents no correlation. — Correlations between the analyzed markers of protein glycation, glycoxidation, oxidation, and carbamylation products, as well as thiol-disulfide homeostasis; protein carbonyls (PC), ischemia-modified albumin (IMA), total thiols (TT), native thiols (NT), disulfide (DS), tryptophan (TRY), kynurenine (KYN), N-formylkynurenine (NFKYN), dityrosine (DT), N^ε-(carboxymethyl) lysine (CML), advanced glycation end products (AGE), and carbamyl-lysine (CBL).

In patients with adrenal masses, we found that tumor size positively correlated with plasma content of CBL (R = 0.496, p < 0.0001), and negatively with IMA (R = −0.252, p = 0.034) and plasma concentration of aldosterone (R = −0.313, p = 0.009).