Tables

Table 1. Definitions

Homocysteine An intermediate sulfur-containing amino acid formed during the complex metabolism of the amino acid methionine^[3]
Hyperhomocysteinemia Plasma homocysteine level greater than 12 µmol/L^[12] as confirmed by the methionine loading test^[25]
Methionine A sulfur-containing essential amino acid, supplied in the diet primarily by meat and dairy products. It is necessary for cell growth. The recommended dietary allowance for methionine is 0.9 g/day for adults, but it is estimated that most adults consume approximately 2 g/day.^[3]
Lp(a) Lipoprotein(a) is a particle formed by the combination of the apo B-100 component of low-density lipoprotein (LDL) and apolipoprotein(a). It is proatherogenic and prothombotic. It competes with plasminogen for plasma binding sites, thus inhibiting fibrinolysis and contributing to foam cell formation, reducing endothelium-dependent vasodilation, and contributing to LDL oxidation.^[11]
Factor V Leiden An inherited blood clotting disorder^[13]
Homocysteinuria A rare autosomal recessive disease characterized by markedly elevated homocysteine in the blood, which passes into the urine. It is usually caused by a deficiency of cystathionine b-synthase. Clinical manifestations include mental retardation, skeletal abnormalities, and lens dislocation. The condition has a marked tendency to lead to arterial and venous thromboembolic episodes.^[3]

Table 2. Factors contributing to elevated homocysteine levels

Increased age^[3,10,12]
Being male or postmenopausal female^[3,10,12]
Medications: anticonvulsants, lipid-lowering agents in combination with thiazide diuretics, estrogen-containing oral contraceptives, nitrous oxide, methotrexate^[3,4,10-12]
Low estrogen levels^[3]
Tobacco smoke and exposure to carbon disulfide^[2,6]
Nutritional vitamin deficiencies of folic acid, vitamin B6, and vitamin B12^[3,4,11,12]
Excessive coffee consumption^[14,15]
Chronic diseases: severe psoriasis, some cancers, systemic lupus erythematosus, renal failure, hypothyroidism, heart or kidney (recipients) transplantation^[3,4,10-12]
Inherited errors of methionine metabolism: cystathionine b-synthase deficiency, methionine synthase deficiency, 5-methyltetrahydrofolate reductace deficiency^[2,3,11,12]

Table 3. Sources of Information About Homocysteine

For information concerning laboratory tests involving the cystathionine b-synthase pathway or cardiovascular risk factor assessment, direct questions to Dr. Michael Tsai by e-mail: tsaix001@maroon.tc.umn.edu
There is a web site, The Homocysteine File, that has a bulletin board on which Dr. Glen Tisman answers posted questions: http://www.homocysteine.com/.
The Lipid Nurse Task Force is a newsletter published four times a year. For information email: lntf@tmahq.com.

Homocysteine	An intermediate sulfur-containing amino acid formed during the complex metabolism of the amino acid methionine^[3]
Hyperhomocysteinemia	Plasma homocysteine level greater than 12 µmol/L^[12] as confirmed by the methionine loading test^[25]
Methionine	A sulfur-containing essential amino acid, supplied in the diet primarily by meat and dairy products. It is necessary for cell growth. The recommended dietary allowance for methionine is 0.9 g/day for adults, but it is estimated that most adults consume approximately 2 g/day.^[3]
Lp(a)	Lipoprotein(a) is a particle formed by the combination of the apo B-100 component of low-density lipoprotein (LDL) and apolipoprotein(a). It is proatherogenic and prothombotic. It competes with plasminogen for plasma binding sites, thus inhibiting fibrinolysis and contributing to foam cell formation, reducing endothelium-dependent vasodilation, and contributing to LDL oxidation.^[11]
Factor V Leiden	An inherited blood clotting disorder^[13]
Homocysteinuria	A rare autosomal recessive disease characterized by markedly elevated homocysteine in the blood, which passes into the urine. It is usually caused by a deficiency of cystathionine b-synthase. Clinical manifestations include mental retardation, skeletal abnormalities, and lens dislocation. The condition has a marked tendency to lead to arterial and venous thromboembolic episodes.^[3]