Summary of main findings

In this study, we evaluated the association between type 2 diabetes mellitus and the concentrations of MPO and PON-1 in patients with IHD. Our results suggest that significant differences in enzyme concentration levels exist between patients with and without diabetes. Diabetes is associated with higher concentration of MPO and at the same time with lower concentration of PON-1. Differences in MPO and PON1 levels were maintained despite the statin treatment.

Comparison with other studiesImpact of T2DM on MPO concentration levels in IHD patients.

The study revealed significantly higher MPO concentrations in the serum of diabetic patients. The results seem consistent with the data presented by Song et al., which reported higher plasma MPO levels in diabetic patients with IHD [19]. In the cited study the authors also observed a correlation between MPO levels and the progression of ischemic heart disease. The concentration of the enzyme was shown to be much higher in diabetic patients with advanced IHD and the disease progressed as a function of MPO levels. A stronger correlation was held for patients with higher HbA1c levels, what was not demonstrated in our study.

A study of Borato et al. showed a positive correlation between plasma MPO levels and classical inflammatory parameters and cardiovascular risk factors in diabetic patients [20]. Our study, however, did not reveal any significant correlations between MPO concentrations and the inflammatory indicator or selected parameters of carbohydrate and lipid metabolism. Interestingly, mean MPO levels were significantly higher in patients with heart failure, but only in the non-diabetic group.

A study published by Shao et al. showed that the levels of 3-chlorotyrosine and 3-nitrotyrosine, two typical products of MPO metabolism, tend to be elevated in HDL molecules isolated from patients with ischemic heart disease [21]. These products are likely to stand behind the oxidative damage to HDL particles. Our study showed a significantly higher concentration of MPO in patients with IHD and diabetes, which may be one of the mechanisms associated with HDL particles “dysfunction” in this patient group.

Zhang et al. demonstrated that in patients without T2DM, plasma glucose levels are positively correlated with MPO levels, which suggests that the latter may play an atherogenic role in patients with higher blood glucose [22]. Our study did not show such a correlation. The only parameter with an independent impact on MPO concentrations was the presence of diabetes alone. However, the actual impact of T2DM on MPO levels was minor, as evidenced by the low correlation coefficient in the model. In the other recently published observational study, it was shown that MPO levels were higher in obese compared with non-obese participants but did not differ between T2DM and control participants [23]. Our study did not reveal the correlation between MPO and patients’ BMI.

In the studies performed by Agarwal et al. and by Mahat et al. similar findings have been reported in patients with prediabetic state [24, 25]. The results of these studies showed statistically significant increase in the level of MPO in prediabetic subjects in comparison to control subjects.

A study by Heslop further confirmed that cardiovascular risk grows as a function of increasing MPO levels; the relationship is even stronger when hsCRP is considered [26]. It must be noted, however, that the study was conducted on patients who were not in treatment for T2DM .

Consistent with data from most recent studies, our results warrant the conclusion that plasma MPO levels, as an indicator of oxidative stress, may be useful for cardiovascular risk stratification in diabetic patient.

Impact of T2DM on PON-1 concentration levels in IHD patients

In our study, significantly lower concentrations of PON-1 were observed in patients with diabetes than those without the disease. A decrease in PON-1 levels may be responsible for compromising the protective role of HDL particles in terms of inhibiting LDL oxidation. Another publication, which investigated PON-1 activity rather than its concentration, likewise reported a decrease in the activity of PON-1 in patients with diabetes, irrespective of the presence or absence of IHD [27].

Jamuna et al. observed a significant reduction in PON-1 activity, accompanied by a concomitant drop in HDL-C levels in diabetic patients as compared to healthy volunteers [28]. In addition, the study demonstrated that the progression of diabetes correlates with a further decrease in PON-1 activity. The correlation between low PON-1 levels in the serum and higher incidence and mortality rates of cardiovascular complications in patients with type 2 diabetes also was suggested [29]. Another recently published study suggests that PON-1 deficiency in T2DM is a gender-specific phenomenon, and that female patients are more affected than men [30]. The authors suggested that this could contribute to the partial loss of female cardiovascular advantage associated with T2DM. The results of our study did not reveal similar trend.

Even though most studies published thus far focus on enzyme activity rather than concentration, the overall trend seems to be the same. With that in mind, it has been proposed that the activity or concentration of PON-1 may serve as a better indicator of atherosclerotic risk in diabetic patients than the concentration of HDL-C alone. No clinical trials have unequivocally substantiated that claim thus a study performed by Jornayvaz et al. showed that myeloperoxidase was an independent, negative determinant of paraoxonase-1 activity [31]. The authors concluded that this may be one of the mechanisms by which it promotes HDL particles dysfunction and increases cardiovascular risk. The study published by Ferretti G. et al. demonstrated that the increase in oxidative stress in LDL and HDL of obese subjects is associated with a decrease in HDL-PON-1 activity [32]. The authors concluded that lower PON-1 activity could contribute to the greater risk of cardiovascular disease associated with obesity. This was confirmed by Cervellati C et al. [33]. However, in the cited above studies the activity of the enzymes was measured.

Another recently published study, which measured the levels of the enzymes in question, showed that plasma MPO concentrations show a significant inverse correlation with PON-1 levels in patients with stable and unstable angina [34].

Some researchers have observed that an imbalance between pro-oxidants and antioxidants may contribute to the progression of the instability of atherosclerotic plaques [35]. The results of our study suggest that a similar relationship should also be considered in patients with stable ischemic heart disease and type 2 diabetes, but the hypothesis requires additional studies on a larger patient group.

The significance of study results

The results of our study suggest that T2DM is accompanied by elevated MPO levels. Since the latter may be associated with a greater cardiovascular risk in patients with type 2 diabetes, measuring MPO levels may aid risk stratification in this patient group. A significant reduction was also observed in the levels of PON-1, an enzyme with antioxidant and atheroprotective properties, in diabetic patients, which may further increase cardiovascular risk. As a result, the proportion of pro-oxidant and antioxidant enzymes becomes even more detrimental from the perspective of HDL-C function. All these factors increase the severity of the atherosclerotic process and may elevate cardiovascular risk despite normal HDL-C levels.

Strengths and limitations of the study

A clear advantage of the study lies in its clinical nature and the assignment of patients to study groups, which ensured that the diabetic and non-diabetic groups would not differ significantly in terms of potential confounding variables. The only significant differences had to do with anti-diabetic drugs, which is the natural consequence of the disease, and the treatment’s potential impact on the study parameters should always be kept in mind.

Another strength of the study had to do with the advanced analytical methods used to perform measurements with a high degree of well-documented reliability. The analysis of correlations between the enzymes under study and several variables allowed to shed light on key clinical and lab relationships associated with T2DM.

The main limitation of the study was the small size of the study group. However, the inclusion criteria allowed to eliminate the most important potential confounding variables. Moreover, we did not include the duration of T2DM, however we have checked the HbA1c levels which indicate the degree of control of the diabetes. Also, due to the small size of the DM and nDM groups, multiple regression analysis models could only be constructed for the entire patient group, with diabetes introduced as an independent variable.

It must also be noted that the enzymes were measured at a single point in time and, for financial reasons, never repeated, which represents yet another weakness of the analysis. Also, it cannot be ruled out that the reason why no significant correlation was observed between parameters such as glucose or HbA1c and the levels of the studied enzymes was the limited size of the study groups.

Importantly, other studies into the correlations and the impact of enzymes on other parameters tend to rely on enzyme activity measurements. Our study focused on enzyme concentration levels rather than activity because of the methodological difficulties associated with measuring the latter. It should be kept in mind, however, that studies published thus far indicate a strong correlation between the two parameters [22]. Therefore, it seems probable that the significantly increased MPO concentration levels in diabetic patients in this study correspond to a greater activity of the enzyme in the group. In the future, the measurements of activity of MPO and PON-1 may be easier to implement in clinical practice. Lastly, we need to indicate that we did not check the polymorphisms in PON-1 and MPO, which may affect the expression of these enzymes.