Assessment of serum phenylalanine and tyrosine isomers in patients with ST‐segment elevation vs non‐ST‐segment elevation myocardial infarction

Abstract Background Under conditions of oxidative stress, hydroxyl radicals can oxidize phenylalanine (Phe) into various tyrosine (Tyr) isomers (meta‐, ortho‐, and para‐tyrosine; m‐, o‐, and p‐Tyr), depending on the location of the hydroxyl group on the oxidized benzyl ring. This study aimed to compare patients with ST‐segment elevation myocardial infarction (STEMI) and non‐STEMI (NSTEMI) and the serum levels of Phe and Tyr isomers at the aortic root and distal to the culprit lesion in both groups. Methods Forty‐four patients participated in the study: 23 with STEMI and 21 with NSTEMI. Arterial blood samples were taken from the aortic root through a guiding catheter and from the culprit vessel segment distal from the primary lesion with an aspiration catheter, during the percutaneous coronary intervention. Serum levels of Phe, p‐Tyr, m‐Tyr, and o‐Tyr were determined using reverse‐phase high‐performance liquid chromatography. Results Serum levels of Phe were significantly higher distal to the culprit lesion compared to the aortic root in patients with STEMI. Serum p‐Tyr/Phe and m‐Tyr/Phe concentration ratios were both lower distal to the culprit lesion than at the aortic root in patients with STEMI. There were no statistically significant differences with respect to changes in serum Phe and Tyr isomers distal to the culprit lesion compared to the aortic root in patients with NSTEMI. Conclusion Our data suggest that changes in serum levels of different Tyr isomers can mediate the effects of oxidative stress during myocardial infarction.

Reactive oxygen species (ROS) play a vital role in vascular inflammation during atherogenesis, from the onset of fatty streak development to plaque rupture. 3 ROS or free radicals can be any chemical species (atom, ion, or molecule) that contains a single, unpaired electron in its outer orbit conferring very high reactivity; examples include hydrogen peroxide (H 2 O 2 ), singlet oxygen ( 1 O 2 ), superoxide radical (O· −2 ), and hydroxyl radical ( · OH). [4][5][6][7][8][9][10][11][12] Oxidative stress refers to conditions caused by an imbalance between ROS and antioxidant systems, in which either excessive amounts of free radicals are produced or the antioxidant capacity is decreased; such conditions can result in the oxidation of proteins, lipids, carbohydrates, and DNA. [4][5][6][7][8]10 Under conditions of oxidative stress in which the levels of free radicals are elevated, hydroxyl radicals can oxidize the benzyl ring of phenylalanine (Phe), producing various tyrosine (Tyr) isomers (meta-tyrosine, ortho-tyrosine, and para-tyrosine; m-, o-, and p-Tyr). These Tyr isomers differ, depending on the location of the hydroxyl group on the benzyl ring ( Figure 1). [13][14][15][16][17][18] According to previous studies, [13][14][15][16][17][18] Phe can be enzymatically transformed into physiological p-Tyr. Additionally, p-Tyr can also be nonenzymatically produced by reactions involving hydroxyl radicals during oxidative stress. However, the enzymatically produced p-Tyr is more plentiful than the free radical-derived p-Tyr. Therefore, p-Tyr is viewed as the physiological isoform. [13][14][15][16][17][18] Furthermore, in humans, m-and o-Tyr amino acids cannot be formed enzymatically; instead, they are produced as a result of the reaction between the hydroxyl free radical and the benzyl ring of Phe. Therefore, m-Tyr and o-Tyr are viewed as free radical markers, [13][14][15][16][17][18] which may play a role in chronic inflammation during the initiation and progression of ACS. 16,17,19 This study aimed to compare patients with STEMI and NSTEMI, and the serum levels of Phe and Tyr isomers at the aortic root and distal to the culprit lesion in both patient groups.  [20][21][22] The extent of CAD was ascertained by coronary angiography.

| Study population
The extent of CAD was determined according to the number of coronary arteries with obstructive CAD (defined as ≥50% angiographic stenosis): 0-, 1-, 2-, or 3-vessel disease. On angiography, of course an occlusive lesion easily recognizable as infarct-related artery.
Occasionally, in cases of NSTEMI, to define the culprit lesion is not F I G U R E 1 Oxidation of phenylalanine into different tyrosine isoforms due to the activity of the phenylalanine hydroxylase enzyme or under conditions of oxidative stress (mainly via hydroxyl free radicals) so easy in patient with multivessel coronary artery disease. Thus, the identification of the culprit lesion is usually achieved by a combination of factors, including angiographies characteristics and information from non-invasive examinations (ECG, echocardiography).
In case of non-occlusive MI, the ESC/AHA definition was used the determine existing of CAD.

| Clinical and biochemical parameters
Personal and medical histories of all study patients were recorded. Arterial blood samples were taken from the aortic root using a guiding catheter and from the culprit vessel segment distal from the primary lesion using an aspiration catheter, during the percutaneous coronary intervention.
The database has been published previously. 23

| Baseline characteristics of patients with ACS
Demographics and patient data are summarized in Table 1 Note: Data are expressed as mean ± SD for the continuous variable and percentages (%) and frequencies (n).  (Table 2).
There were no statistically significant differences with respect to changes in serum levels of Phe and Tyr isomers distal to the culprit lesion compared to the aortic root in patients with NSTEMI (Table 3).
As shown in Table 4, there were no significant differences between patients with STEMI and NSTEMI with regard to serum levels of Phe and Tyr isomers, whether distal to the culprit lesion or at the aortic root.

| Correlation between serum amino acid parameters and baseline patient characteristics
We examined the associations of the amino acid parameters with demographics and clinical data for patients, according to their diag-

| D ISCUSS I ON
As mentioned in the literature, the pathological processes underlying vascular diseases are not fully understood; however, there is increasing evidence that oxidative stress and inflammation are positively associated with the rupture of atherosclerotic plaques and the incidence of ACS. 11 The findings from this study suggest that certain oxidative stress markers may be associated with the extent of myocardial damage located proximally to the aortic root or distally from the culprit lesion in patients with STEMI and in those with NSTEMI.
In the current study, there were significantly higher levels of serum Phe distal from the culprit lesion compared to the aortic root in patients with STEMI; while there were slightly higher levels in patients with NSTEMI, this difference was not significant. Similar .248 Serum o-Tyr (nmol/L) 16.6 (6.6-32.6) 11.9 (6.9-36.7) .927 reports have shown that elevations in some tyrosine isomers correspond to the duration of ischemia, indicating that hydroxyl radical production is associated with prolonged periods of ischemia. 14,17,30 Another possible explanation is that only a small number of patients were studied, which may bias the results obtained during the study. The results of this study showed no significant association between serum amino acid parameters and baseline patient characteristics except for serum m-Tyr levels; they are negatively correlated with the extent of CAD at the aortic root in patients with NSTEMI.  The clinical significance of this study is to discover that changes in the Phe and Tyr isomers (m-, o-, and p-Tyr) are associated with oxidative stress after myocardial injury, which may play a role in chronic inflammation during initiation and progression of ACS. The limitation of this study is that only a small number of patients have been studied and a time period of ischemia is not specified. This research has thrown up many questions in need of further investigation.

| CON CLUS IONS
Our data suggest that changes in serum levels of different Tyr isomers can mediate the effects of oxidative stress during myocardial infarction. The contribution of this study is to confirm the association of changes in the Phe and Tyr isomers with oxidative stress following myocardial injury.