**2.1 Plasma homocysteine**

As mentioned in the introduction homocysteine is a sulfur–containing intermediate in the normal metabolism of the essential amino acid methionine, occuring in almost all body cells and in general 5 to 10% of the daily synthesized homocysteine (1.2 mmol/day) is transferred into the blood through hepatocytes (Weisberg et al., 2003). Besides, proliferating cells secrete more homocysteine compared to non-proliferating cells. Although plasma concentrations of homocysteine vary widely, on the other hand the intracellular concentrations are preserved within a narrow range (Moat et al., 2004). In the plasma, about circa 90% homocysteine is protein-bound, while circa 10% is present as the cysteine mixed disulphide and less than 1% is present in the free reduced form. The total plasma level includes the summed amount of all the homocysteine forms in the circulation (Hankey & Eikelboom, 1999).

Normal and abnormal homocysteine levels are set by individual laboratories. Typically, considered normal is less than 13 μmol/L, between 13 and 60 μmol/L is considered moderately elevated, and higher than 60 to 100 μmol/L is severely elevated (Moll, 2004). The total plasma homocysteine concentrations during hyperhomocysteinemia are between 12 and 30 μmol/L, with gender differences being present. Higher values are measured in men, and apparently the presence of estrogen in women determines the plasma concentration after the menopause the blood levels of homocysteine of woman approximate those in men (Ridker et al., 1999), Table1.


Table 1. Blood reference ranges for homocysteine

Pathophysiology and Clinical Aspects of 18 Venous Thromboembolism in Neonates, Renal Disease and Cancer Patients

premature vascular disease, occlusive cardiovascular disease in early life and childhood, leading to incidental strokes or heart attacks in teenagers. It is caused by inherited metabolic defects of the homocysteine metabolism, and is therefore positively correlated with a very

These observations raised the question whether moderately elevated plasma homocysteine concentrations, often called moderate hyperhomocisteinemia, may also cause irritation of the blood vessels and are a risk factor for cardiovascular disesase (CVD) in general (McCully, 1960). McCully proposed that elevated homocysteine can cause atherosclerotic vascular disease (McCully, 1960). Early support for this concept came from a study published in 1976 by Wilcken and Wilcken, who reported that, following an oral dose of methionine, serum homocysteine levels tended to be higher in patients with premature

Mild or moderate hyperhomocysteinemia which occurs in the health population with a frequency of 5 to 7 % is often caused by the interaction of environmental factors with mild

Venous thrombosis was clearly described in patients with mild/moderate homocysteinuria and since then, several case-control and prospective studies showed the association with increased risk of VTE, (Mudd et al., 1985). Besides, also a large number of retrospective studies show that mildly elevated homocysteine levels (mild/moderate hyperhomocysteinemia caused by the interaction of envirovmental factors with mild genetic abnormalities of homocysteine metabolism) are associated with VTE. Only recently, an elevated homocysteine level has also been established as a risk factor for venous thrombosis (Moll, 2004). Moreover, in patients with venous thrombosis elevated homocysteine levels have attracted considerable interest because homocysteine is an easy to monitor thrombophilic marker, and thus can indicate the time and need for measures, to potentially

Because of the already high prevalence of (hyper/moderate/mild) homocysteinemia in the healthly population and people with disease, this review focusses the attention on (1) the relevance of the metabolic pathway of homocysteine, (2) the importance of dietary intake of folate, vitamines B6 and B12 and (3) the recommendations to modify life style factors in order to prevent, in general, a further homocysteinemia-induced increase of the VTE and

As mentioned in the introduction homocysteine is a sulfur–containing intermediate in the normal metabolism of the essential amino acid methionine, occuring in almost all body cells and in general 5 to 10% of the daily synthesized homocysteine (1.2 mmol/day) is transferred into the blood through hepatocytes (Weisberg et al., 2003). Besides, proliferating cells secrete more homocysteine compared to non-proliferating cells. Although plasma concentrations of homocysteine vary widely, on the other hand the intracellular concentrations are preserved within a narrow range (Moat et al., 2004). In the plasma, about circa 90% homocysteine is

high risk of venous thromboembolism (VTE), (Mudd et al., 1970).

coronary disease than in healthy controls (Wilcken & Wilcken, 1976).

genetic abnormalities of homocysteine metabolism.

reverse the venous thrombosis (Cattaneo, 2006).

cardiovascular disease complications.

**2. Homocysteine metabolism** 

**2.1 Plasma homocysteine** 

The homocysteine levels are measured through a routine blood test, where blood samples are collected in EDTA or citrate anticoagulant tubes and should be centrifuged and the plasma separated immediately. Ideally, the homocysteine is measured in overnight fasting subjects, since high-protein meals will influence the results. Another test, the methionineload test measures the homocysteine levels before and after the intake of 100 mg/kg of methionine and can be used to diagnose abnormal homocysteine metabolism in people with a high risk for cardiovascular disesase, but who have normal homocysteine concentration during fasting. This test can be used to make decisions about therapy.

Homocysteine exists in plasma in a free and a bound form. The determination measures the total homocysteine level is the sum of all forms. The commercial methods of determination include the transformation of all forms of homocysteine, by means or reduction, into total homocysteine, which than is quantified by different methods: gas chromatography, mass spectrometry, high pressure liquid chromatography and the most frequently commercial methods as florescence polarization immunoassay, chemiluminescence immunoassay, or enzyme-linked immunoassay, used on different analyzers.

Results obtained with different methods are often not very com-parable each other because of considerable inter-method and inter-laboratory variability. Reported approaches for the measurement of plasma tHcys include: ion-exchange chromatography, immunoassays (uorescence polarization immunoassay, FPIA, or chemiluminescence immunoassay, ICL,or enzyme-linked immunoassay, EIA), HPLC (with photometric, uorescence or electrochemical detection),capillary electrophoresis with photometricor laser uorescence detec- tion), GC–MS,and LC–ESI-MS/MS. Many of them have signicant disadvantages, including derivatization protocols, are expensive and time-consuming. Compared with the above mentioned, LC–ESI-MS/MS seems to be the most suitable method because of its inherent accuracy, high sensitivity, specicity and high through put for t Hcys analysis.

Hyperhomocysteinemia: Relation to Cardiovascular Disease and Venous Thromboembolism 21

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-

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

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

Fig. 2. Homocysteine metabolism

**3. Causes of hyperhomocysteinemia** 

dependent enzymes.

**3.1 Genetic deffects** 

concentrations.
