**2.2 Role of B vitamins and enzymes**

B vitamins function as coenzymes in the synthesis of purines and thymidylate during normal DNA synthesis. Diminished levels of these vitamins may result in misincorporation of uracil into DNA, leading to chromosome breaks and disruption of DNA repair and both, folate and vitamin B12 levels are involved in DNA methylation. Deficient folate and vitamin B12 levels can reduce the availability of Sadenosylmethionine, the universal methyl donor, for DNA methylation and may thereby influence gene expression (Blount et al., 1997).

Some people have elevated homocysteine levels due to an unbalanced diet with suboptimal intake of B vitamins (B6, B12 and folate), which act as coenzymes in the metabolism of homocysteine (de Bree et al., 1997, Stanger et al., 2003). Several studies have found that high blood levels of B vitamins influence the integrity and function of DNA, and, correlate with a low concentration of homocysteine, while folate depletion has been found to change DNA methylation and DNA synthesis in both animal and human studies.

B vitamines are very important in the transformation of homocysteine in methionine and are cofactors to three important enzymes directly involved in the homocysteine metabolism: (1) methionine synthase (MS), (2) methylenetetrahydrofolatereductase (MTHFR) and (3) cystathione β-synthase (CBS).

Therefore, deficiencies of folate and vitamin B12 and reduced activity of the involved metabolic enzymes will inhibit the breakdown of homocysteine, leading to an accumulation of the intracellular homocysteine, followed by rapid excretion to the circulation and eventually increased plasma levels (Silaste et al., 2001).

Via the trans-sulfuration pathway homocysteine is converted into cystathionine to form cysteine by cystathionine-ß-synthase, with vitamin B6 as a co-factor. Another pathway of homocysteine metabolism is the re-methylation pathway, which is connected with the folate metabolic pathway (Fig. 2). It involves the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine to form methionine, and eventually Sadenosylmethionine. The methyl transfer from 5-methyl tetrahydrofolate to homocysteine is catalyzed by methionine-synthase, and requires vitamin B12 as a cofactor. Important to notice is that S-adenosylmethionine is the universal methyl donor for methylation reactions. The resulting tetrahydrofolate transfers into the 5,10-methyltetrahydrofolate with the enzyme 5,10-methyltetrahydrofolate reductase (MTHFR) and then into the 5 methyltetrahydrofolate 5-MTHF, (Fodinger et al., 2000). The cellular availability of 5-MTHF may be of great importance in regulating cellular effects of homocysteine related to cell growth.

The methyl group of 5-MTHF is transported to vitamin B12 linked to the enzyme homocysteine–methyl-transferase to yield methylcobalamin-enzyme. This complex adds the methyl group to homocysteine to form methionine (Pietrzik & Brönstrup, 1998).

Therefore, deficiencies of folate and vitamin B12 and reduced activity of the involved metabolic enzymes will inhibit the breakdown of homocysteine, which will lead to an increase of the intracellular homocysteine concentration (Silaste et al., 2001).

#### Fig. 2. Homocysteine metabolism

Pathophysiology and Clinical Aspects of 20 Venous Thromboembolism in Neonates, Renal Disease and Cancer Patients

B vitamins function as coenzymes in the synthesis of purines and thymidylate during normal DNA synthesis. Diminished levels of these vitamins may result in misincorporation of uracil into DNA, leading to chromosome breaks and disruption of DNA repair and both, folate and vitamin B12 levels are involved in DNA methylation. Deficient folate and vitamin B12 levels can reduce the availability of Sadenosylmethionine, the universal methyl donor, for DNA methylation and may thereby

Some people have elevated homocysteine levels due to an unbalanced diet with suboptimal intake of B vitamins (B6, B12 and folate), which act as coenzymes in the metabolism of homocysteine (de Bree et al., 1997, Stanger et al., 2003). Several studies have found that high blood levels of B vitamins influence the integrity and function of DNA, and, correlate with a low concentration of homocysteine, while folate depletion has been found to change DNA

B vitamines are very important in the transformation of homocysteine in methionine and are cofactors to three important enzymes directly involved in the homocysteine metabolism: (1) methionine synthase (MS), (2) methylenetetrahydrofolatereductase (MTHFR) and (3)

Therefore, deficiencies of folate and vitamin B12 and reduced activity of the involved metabolic enzymes will inhibit the breakdown of homocysteine, leading to an accumulation of the intracellular homocysteine, followed by rapid excretion to the circulation and

Via the trans-sulfuration pathway homocysteine is converted into cystathionine to form cysteine by cystathionine-ß-synthase, with vitamin B6 as a co-factor. Another pathway of homocysteine metabolism is the re-methylation pathway, which is connected with the folate metabolic pathway (Fig. 2). It involves the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine to form methionine, and eventually Sadenosylmethionine. The methyl transfer from 5-methyl tetrahydrofolate to homocysteine is catalyzed by methionine-synthase, and requires vitamin B12 as a cofactor. Important to notice is that S-adenosylmethionine is the universal methyl donor for methylation reactions. The resulting tetrahydrofolate transfers into the 5,10-methyltetrahydrofolate with the enzyme 5,10-methyltetrahydrofolate reductase (MTHFR) and then into the 5 methyltetrahydrofolate 5-MTHF, (Fodinger et al., 2000). The cellular availability of 5-MTHF may be of great importance in regulating cellular effects of homocysteine related to cell

The methyl group of 5-MTHF is transported to vitamin B12 linked to the enzyme homocysteine–methyl-transferase to yield methylcobalamin-enzyme. This complex adds the

Therefore, deficiencies of folate and vitamin B12 and reduced activity of the involved metabolic enzymes will inhibit the breakdown of homocysteine, which will lead to an

methyl group to homocysteine to form methionine (Pietrzik & Brönstrup, 1998).

increase of the intracellular homocysteine concentration (Silaste et al., 2001).

**2.2 Role of B vitamins and enzymes** 

influence gene expression (Blount et al., 1997).

cystathione β-synthase (CBS).

growth.

methylation and DNA synthesis in both animal and human studies.

eventually increased plasma levels (Silaste et al., 2001).

S-adenosylhomocysteine is formed during S-adenosylmethionine-dependent methylation reactions, and the hydrolysis of S-adenosylhomocysteine results in homocysteine. Homocysteine may be remethylated to form methionine by a folate-dependent reaction that is catalyzed by methionine synthase, a vitamin B12-dependent enzyme. Alternately, homocysteine may be metabolized to cysteine in reactions catalyzed by two vitamin B6 dependent enzymes.

## **3. Causes of hyperhomocysteinemia**

### **3.1 Genetic deffects**

Elevation in plasma homocysteine are typically caused either by genetic defects in the enzymes involved in homocysteine metabolism or by nutritional deficiencies in vitamin cofactors. Homocysteinuria and severe hyperhomocystenemia are caused by rare inborn errors of metabolism resulting in marked elevations of plasma and urine homocysteine concentrations.

Most studies refer to changes in the cystathionine β-synthase gene or in the GCT gene (γ cystathionase), both coding the trans-sulfuration pathway (references). Further, mutations do occur in the genes coding for the enzymes involved. Cystathionine β-synthase (CBS) deficiency is the most common genetic cause of severe hyperhomocysteinemia. As first shown in a study by Carey and colleagues as early as 1968, the homozygous form of this disease — congenital homocystinuria — can be associated with hyperhomocysteinemia, and

Hyperhomocysteinemia: Relation to Cardiovascular Disease and Venous Thromboembolism 23

Table 2. Common diseases associated with high homocysteine levels (Hultberg et al., 1993;

From a public health viewpoint, it is important to identify modifiable factors that influence the plasma homocysteine concentrations. The next lifestyle factors may have an effect on

Smoking is associated with vascular disease and other complications related to homocysteine (Bolander-Gouaille, 2001). The number of cigarettes smoked a day was one of the strongest determinants of homocysteine levels (Nygard et al., 1995). In women, the increase of plasma homocysteine levels was about 1% per each cigarette smoked, and in men about 0.5%. The mechanisms by which smoking increases the homocysteine levels may be manifold, however there is some experimental evidence that nicotine directly affects the methylation reactions. Besides, in smokers catabolism of folate has been suggested(Godin &

High alcohol consumption is often associated with gastrointestinal disturbances, which may result in decrease absorption of vitamins (the most important is folic acid), thus contributing to elevated homocysteine levels. Alcohol has also been reported to inhibit methionine synthase (MS), to decrease hepatic uptake and increase excretion, mainly via urine (Barak et al., 1993). The decreased concentration of serum folic acid may occur in 80% of alcohol

**Aging Heart conditions Alzheimers disease Mental retardation** 

**Anaemias Migraines Angina Miscarriages Arthritis Osteoporosis Artheriosclerosis Parkinson's disease Auto-immune diseases Polycystic ovary disease Birth defects Pregnancy complications** 

**Cancers Psoriasis** 

**Diabetes Epilepsy** 

**3.3 Lifestyle factors** 

**3.3.1 Smoking** 

Crooks, 1986).

**3.3.2 High alcohol intake** 

**Cholesterol - high Rheumatoid arthritis Chronic fatigue Schizophrenia Coeliac disease Strokes** 

**Chrohn's disease Thyroid disorders Depression Ulcerative colitis** 

Pettersson et al., 1998; Bolander-Gouaille, 2001; Hultberg, 1993).

plasma homocysteine concentration (Bolander-Gouaille, 2001):

abusers and this can lead to serious clinical consequences.

in these homozygotes there is a frequent development of atherothrombotic complications during young adulthood, which often are fatal. Mudd and colleagues estimated that approximately 50 percent of untreated homocystinuria patients will have a thromboembolic event before the age of 30 and that the disease-related mortality is approximately 20 percent (Mudd et al., 1970).

Other abnormalities of the remethylation cycle that are associated with hyperhomocysteinemia include genetic methionine synthase deficiency and genetic disorders of vitamin B12 metabolism both impairing methionine synthase activity.

Genetic mutations in MTHFR are the most commonly known inherited risk factor for elevated homocysteine levels. To have any detrimental effect, mutations must be present in both copies of a person's MTHFR genes (Varga et al., 2005). (1) A point mutation in the coding region for the 5,10-MTHFR binding site (C677T), leading to the substitution of an alanine to a valine effectively increases homocysteine levels increase and decreases methionine levels or (2) A1298C another common point mutation of the MTHFR gene, both affect the enzyme activity catalyzing the vitamin B12–dependent remethylation of homocysteine to methionine. C677T homozygotes carry the double TT (thermolabile) allele of the enzyme MTHFR gene of which the enzyme activity is reduced to 35% of the normal (Schriver et al., 1995), and having an average homocysteine level of 19.7 µM. In CT heterocygotes this is 10.3 µM, while for CC unaffected this is 10.0 µM. Further, data show that people with C677T TT have 21% increased risk of ischemic heart disease; in those with CT the risk is increased by only 6% (Dinesh -K, 2004).

Aproximally 10% to 20% of Caucasians carry the TT allele, whereas the remaining 80% - 90% carry either the CT or CC alleles. Black subjects have a very low frequency of carring the TT allele. The C677T mutation does have different regional incidences in Europe where the German and Italian populations show different incidences of 24.5% and 43.8% respectively.
