The Role of Hyperhomocysteinemia in the Pathogenesis of Thromboocclusive Diseases

Homocysteine is a sulfur-containing non-proteinogenic amino acid that is an intermediate product of methionine metabolism. With excessive accumulation of homocysteine in the body, a state of hyperhomocysteinemia occurs, which has attracted the attention of doctors since the middle of the last century and received a serious impetus for research after the publication of data on the role of hyperhomocysteinemia in the pathogenesis of thromboocclusive diseases. To date, there are more than 7.5 thousand scientific papers devoted to the study of hyperhomocysteinemia, monothematic international conferences on clinical and therapeutic aspects of hypermonocysteinemia are held every year. In the Russian Federation, a detailed study of the role of homocysteine and its side effects in excess is of particular interest due to the prevalence of cardiovascular pathologies Short Communication Yusupov et al.; JPRI, 33(48A): 130-136, 2021; Article no.JPRI.76381 131 (according to statistics of the Ministry of Health of Russia-49.27% of deaths of Russians fall on this cohort of diseases). This is an important fact in Russian medicine, which deserves close attention from researchers and scientists.


INTRODUCTION
Homocysteine is a sulfur-containing nonproteinogenic amino acid that is an intermediate product of methionine metabolism. That is, this amino acid is not part of proteins, it is not contained in food [1]. Metabolic transformations involving homocysteine occur in two ways: remethylation and transulfation. In the first case, the amino acid attaches a methyl group, forming methionine with the participation of folic acid and methylenetetrahydrofolate reductase and B12dependent methyltransferase. An additional route of homocysteine methylation is through an independent B12-independent betainehomocysteine methyltransferase [2]. Due to the limited tissue bioavailability of the latter, there is not a sufficient conversion of homocysteine, therefore, in the case of a defect in methylenetetrahydrofolate reductase or B12dependent methyltransferase, the amino acid is not able to compensate for the disturbed metabolism [3,4]. In the process of transulfation, homocysteine combines, with the participation of B6-dependent cystation-β-synthase, with serine in a reaction to form a diaminodicarboxylic aliphatic sulfur-containing amino acidcestathione. Cystathionine is an S-substituted cysteine. By the way, genetically determined disorders of cystathionine metabolism are the cause of hereditary diseases-cystathioninuria and homocystinuria [4].
To date, more than 55 mutations of the CBS gene are known. More than 9 different mutations of methylenetetrahydrofolate reductase have been described [5]. Two alleles are of practical importance: the thermolabile C677T and the A1298C allele. The mutation in the allele N677-O is inherited most often, and according to an autosomal recessive type, it occurs both in North America and in Europe in a homozygous carrier in 10-20% of the population, in a heterozygous carrier-in almost 40-60% of residents. It can be assumed that the same ratios are found in the Russian Federation [6].
The results of the Augusta University researchers showed that oxidative stress, inflammation and epigenetic modifications are possible mechanisms of dysfunction of the external blood-brain barrier caused by hyperhomocysteinemia. The study also talks about hyperhomocysteinemia -induced inflammation of the brain, as a mechanism of dysfunction of the blood-brain barrier and the pathogenesis of Alzheimer's disease [7].

THE EFFECT OF HYPERHOMOCYSTEINEMIA ON THE CARDIOVASCULAR SYSTEM
The metabolism and "efficiency" of homocysteine metabolism depends on a number of factors: the amount of methionine consumed, the concentration of folic acid and B vitamins in the blood, the amount of S-adenosyl methionine in cells [8]. Since the kidneys are the organ of homocysteine excretion, in case of their damage, homocysteine increases in the blood [9]. Fact: in patients with type 2 diabetes mellitus, the level of homocysteine constantly exceeds the norm, and even the severity of microalbuminuria, a marker of kidney damage, depends on its initial concentration in the blood [10]. In patients with CRF who are on hemodialysis, hyperhomocysteinuria is detected in 100% of cases, and the concentration of the amino acid itself is from 25 mmol/l [11]. In general, the level of homocysteine in the blood is affected by a large number of various factors, including genetic mutations, age and gender (men, menopausal period in women, age over 55 years), nutrition, bad habits, lack of B vitamins, some nosologies (type 2 diabetes, malignant neoplasms, rheumatoid arthritis, SLE, hypothyroidism, cystic fibrosis, Alzheimer's disease, etc.), the effect of various pharmacological drugs [12].
Even a small excess of homocysteine cytotoxically affects the endothelial vascular lining. It inhibits cyclooxygenase activity in endotheliocytes, minimizing the synthesis of prostacyclin, and thereby stimulating the production of thromboxane A2, with an increase in the amount of the latter, the aggregation activity of platelets increases, which is already a prerequisite for vascular pathology.
Hyperhomocysteinemia is accompanied by an increase in the synthesis of tissue factor, a decrease in the activity of natural anticoagulants and a tissue activator of plasminogen (the primary form of plasminan enzyme that restricts blood coagulation) [13].
Endotheliocytes and the nitrogen monoxide synthesized by them (hereinafter NO) are of significant importance in the exchange of homocysteine) [14]. In healthy people, an excess of this amino acid binds to NO in the bloodstream with the formation, as a result, of Snitrogomocysteinea substance that lacks a priori cytotoxic properties and has a vasodilator effect. And how it should be concluded: with hyperhomocysteinemia, endothelial NOsynthetase is blocked, NO production decreases rapidly, the formation of S-nitrosohomocysteine is disrupted [15]. Homocysteine has the ability to induce the activity of 3-hydroxy-3-methylglutaryl-COA reductase, which, in turn, leads to increased synthesis of cholesterol and its deposition in the places of the damaged endothelial layer. In addition to this trouble, with the growth of homocysteine in the blood, the growth of asymmetric dimethylargine, an inhibitor of nitric oxide synthetase, also increases [16].
In researches of the effect of hyperhomocysteinemia on the cardiac system in laboratory conditions, it was found that in patients with coronary disease, this fact contributed to endothelial damage, which is confirmed by an increased level of endothelin 1 [17].
There is no doubt that hyperhomocysteinemia contributes to the formation of endotheliosis. Also, studies have shown that hyperhomocysteinemia affects the formation and maintenance of a systemic inflammatory response. This was shown by biochemical blood tests of cardiac patients, where overestimated markers of inflammation (interleukin-6, fibrinogen, CRP) were clearly demonstrated [18].
Next, we will consider clinical cases of coronary artery damage with obvious consequences for the myocardium, which were conducted on the basis of the Stavropol Regional Hospital.
When studying patients aged 46-65 years, 3-12 months before the study, who had suffered a myocardial infarction and concomitant diseases, such as arterial hypertension (stage 2-3), it was found that the average level of hyperhomocysteinemia in plasma was 24 microns/l. In the group of patients under 46 years of age, the average level of HZ was 17.6 microns/l. It was found that the latter values are close to those established for patients with unstable angina in the same age group. Samples with a methionine load (100 mg/kg) showed that the ratio of the resulting increase in the concentration of HC to its initial value fluctuated widely and the lower the level of post-loading HC, the less likely it was to repeat coronary disorders.
The significance of the concentration of HC in the blood of patients as a prognostic sign of postoperative complications and the adequacy of the vitamin profile after heart transplantation was demonstrated by the fact that the concentration of HC in patients of the examined group was higher than in the comparison group (20.1 ± 13.0 vs. 10.0 ± 4.0 microns/l; p < 0.01), and an increase in the level of HC was observed in 78% of recipients even before transplantation. The concentrations of folate and vitamin B6 in these patients before and after surgery were significantly lower than in the comparison group.
Post-transplant complications were noted in 69.3 % of patients with severe hyperhomocysteinemia. In our study, for most of these patients, an increase in the concentration of HC and a decrease in folate and vitamin B6 levels were observed (Table 1) before surgery. It is believed that the correction of these disorders could significantly reduce the number of posttransplant complications.
The HC concentration was also determined in 100 patients with confirmed coronary artery disease during subsequent angioplasty, compared with 110 subjects without coronary pathology. In this study, it was not possible to find a relationship between the level of HC in the blood and the classic manifestations of cardiovascular disease.  Additional blood tests were also conducted for platelet aggregation on the first day of the disease, depending on the number of white blood cells in the blood of patients ( Table 2).
The systemic inflammatory reaction observed in hyperhomocysteinemia manifests itself in the form of induction of a pro-inflammatory phenotype in the arterial membrane [19]. This leads to the activation of adhesive molecules in the endothelium, the adhesion of monocytes and platelets and, ultimately, is manifested by atherothrombotic complications [20].
Homocysteine, as an amino acid, has the property of mitogenicity in relation to smooth muscle cells of the vessels of the circulatory system, contributing to their accelerated proliferation, which is manifested by thickening of the intima and copper vessels [21]. Mitogens are peptides or low molecular weight proteins that induce mitosis. Mutagenesis is the entry of a cell into the process of division under the action of a certain mutagen. Their action is provided by the translation of signals that turn on mitogen-activated protein kinases and lead to mitosis [22].
Currently, there is no generally accepted norm for the level of homocysteine in blood plasma, so it varies depending on the age category, ethnicity, nutrition, and so on. But it was found that with age, both men and women have homocysteine levels increasing. If the level of homocysteine in children does not exceed 6 mmol/l before puberty, then during puberty the level of this amino acid increases markedly, especially in males [23][24][25].

DETERMINATION OF LATENT HYPERHOMOCYSTEINEMIA
There is no single classification for hyperhomocysteinemia, because it is an extremely variable phenomenon that takes into account many conditions. After analyzing various literature, some researchers have attempted to create such a classification [26]. Athrombosis of different localization was significantly more common than in those with a lower amino acid level [28][29][30].
Among the commonly used methods for the level of homocysteine is a high-performance liquid chromatography immunoenzyme analysis [31]. The advantage of this method is not only in sensitivity and high accuracy (up to 1 mmol/l of total homocysteine in blood serum), but also accessibility. This method can be used in any laboratories where there is an enzyme immunoassay.
To determine the latent form of hyperhomocysteinemia, some authors suggest a test with a methionine load [32,33]. This manipulation allows us to see a hidden defect in the exchange of amino acids, including due to a heterozygous mutation of cystation-β-synthase. This is an outdated method, which, nevertheless, is often used.
The method consists in the fact that the patient is given methionine inside at the rate of 100 mg/kg of body weight. Then, after 2, 4, 6 and 8 hours, the concentration of homocysteine in the blood is determined, comparing it with the initial values. The most optimal test option is a 2-hour one. If, after a couple of hours, the amino acid concentration is 2 times higher than the initial level (by 2 standard deviations), then the sample is considered positive.
This method allows to identify an additional average of up to 30% of patients with latent hyperhomocysteinemia.

THERAPY
For the treatment and reduction of homocysteine in the blood, combinations of group B drugs are used. The literature describes the ability to reduce high levels of homocysteine both by one representative of the vitamins of this group (this is B12), and in combination with folic acid [34]. Information on the positive use of B6 and B2 was reported. There is evidence that the use of folate even improves cognitive functions in people, especially in those with very high homocysteine levels [35].

CONCLUSION
Hyperhomocysteinemia is one of the most important factors contributing to endothelial damage. Endometriosis is accompanied by a systemic inflammatory reaction, activation of platelet and coagulation hemostasis, stimulation of mitotic activity of vascular smooth muscle cells, which leads to the development of atherosclerosis and associated thromboocclusive vascular damage. The latent form of hyperhomocysteinemia can be identified by modern laboratory research methods, which should be resorted to in the case of additional examination, if no obvious causes of "chronic" thrombococclusive diseases have been found. Also, for such a thorough work, the treating specialist must have a deep understanding of biochemistry and molecular interactions at the cellular level in order to find the initial etiological factor, and not to treat the consequences of the disease already.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.