**1. Introduction**

Pathophysiology and Clinical Aspects of 16 Venous Thromboembolism in Neonates, Renal Disease and Cancer Patients

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Homocysteine is a sulphur-containing amino acid, which structurally is closely related to the essential amino acids methionine and cysteine. The cellular methylation cycle performs the metabolism of methionine and since homocysteine is an intermediate within this cycle, the body in this way is provided with all organic homocysteine. The term homocysteine is used to define the combined pool of homocysteine, homocystine, and mixed disulfide compounds (Fig. 1) even involving homocysteine thiolactone a cyclic form which is often found in the plasma of patients with hyperhomocisteinemia.

Fig. 1. Structural formulae of homocysteine

Homocysteine was first isolated by Butz and du Vigneaud in 1932. However, the relation of homocysteine to human disease was first suggested in 1962, in the classical paper of Carson and Neil, reporting an elevated homocysteine level in the urine of mentally retarded children. Nowadays, it has long been known that homocystinuria—also known as severe hyperhomocysteinemia, a genetic disorder in which blood levels of homocysteine are about 20-fold higher than the normal concentration—is associated with greatly increased risk for

Hyperhomocysteinemia: Relation to Cardiovascular Disease and Venous Thromboembolism 19

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

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

Sex Age Lower limit Upper limit Unit Moderatelly elevated

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

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

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.

Female 12–19 years 3.3 7.2 μmol/L 13-60 μmol/L >60 years 5 12 μmol/L

Male 12–19 years 4.3 9.9 μmol/L > 11.4 μmol/L

>60 years 6 15.3 μmol/L

during fasting. This test can be used to make decisions about therapy.

enzyme-linked immunoassay, used on different analyzers.

all the homocysteine forms in the circulation (Hankey & Eikelboom, 1999).

those in men (Ridker et al., 1999), Table1.

Table 1. Blood reference ranges for homocysteine

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

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

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 genetic abnormalities of homocysteine metabolism.

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 reverse the venous thrombosis (Cattaneo, 2006).

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 cardiovascular disease complications.
