Homocysteine

Homocysteine
William H. Wong, Ph.D, April 12, 1999

Introduction

Cardiovascular disease is still the leading cause of mortality in the developed countries. Although there were tremendous gains in our understanding of coronary heart disease, the traditionally recognized risk factors, such as cholesterol levels, blood pressure, alcohol and smoking, do not fully account for all the incidences of coronary heart disease. In 1976, Dr. Kilmer McCully first made the connection between elevated homocysteine level and cardiovascular disease. He found that children with homocystinuria had a severe buildup of placque in their arteries. Since then, more and more clinicians are looking into using homocysteine levels to assess an individual’s risk of developing cardiac disease.

Mounting evidence is now pointing towards homocysteine as an independent risk factor in coronary heart disease. Elevated levels of this amino acid, an intermediary in the cysteine metabolic pathway, may increase cardiovascular risk, through pathways yet to be fully established.

Homocysteine Metabolism

Homocysteine is the intermediary amino acid in the Methionine to Cysteine metabolic pathway. Methionine is metabolized to homocysteine. Homocysteine is then irreversibly catabolized to cysteine, in the pyridoxal phosphate (vitamin B6) dependent trans-sulfuration pathway, through the actions of cystathionine synthase and cysthathionine lyase. A large portion of homocysteine is however re-methylated to methionine through the cobalamin (vitamin B12) and methyl-tetrahydrofolate dependent pathway, catalyzed by the enzyme methionine synthase. Impaired reactions in either pathways, through defects or deficiencies in the critical enzymes or cofactors, result in the accumulation of homocysteine in the blood.

The relationship of homocysteine to cardiovascular disease is not entirely understood. Studies have pointed to some of the following as critical factors:

1. Prolonged elevated levels of homocysteine have been shown to be toxic to the vascular endothelium,
2. Homocysteine is a potent inducer of atherosclerosis, by inducing oxidation of LDL and increasing fibrin incorporation of Lp(a), and
3. Homocysteine interferes with thrombin activation of protein C and promotes thrombosis.

Causes of Elevated Homocysteine

Genetics:

• Cystathionine-b-synthase deficiency
• Methionine synthase defect
• Defect in vitamin B12 transport or coenzyme synthesis

Nutrition:

• Cobalmin (vitamin B12) deficiency
• Pyridoxal phosphate (vitamin B6) deficiency
• Folic acid deficiency

Medications and Supplements:

• S-adenosyl-methionine
• Methotrexate
• Carbamazepine
• Phenytoin
• 6-azauridine triacetate
• Theophylline
• Nitrous oxide
• Estrogen oral contraceptives

Disease states:

• Chronic renal failure
• Acute lymphoblastic leukemia
• Hyperthyroidism
• Malignancies
• Psoriasis

Reference Guidelines

Homocysteine values are usually lower in females than in males. Average homocysteine levels also increase with age. However, the adult reference range typically stated in the literature is 5 – 15 umol/L, without regard to age or gender.

A Tuft University study of the elderly showed that the higher the homocysteine level the greater the chance of carotid artery obstruction, a warning sign of increased risk for both stroke and coronary heart disease. In the study, the odds for carotid blockage were more than twice as high in the top quartile of the group with the highest homocysteine levels as in the lowest quartile.

In addition, there was an observation that risk began to rise at homocysteine levels as low as 11 umol/L, which were now considered high normal. This finding may suggest that the normal reference values would need to be further refined when more studies are published.

At the present time, the reference values for fasting homocysteine (plasma or serum) are 5.0 to 15.0 umol/L.

For Methionine Load Test, the reference values for homocysteine 4 hours after an oral methionine load of 100 mg per kilogram body weight is 5.0 to 20.0 umol/L.

Differential Diagnosis

If the cause of the hyperhomocysteinemia is important, the following table may be helpful:

HCY
MMA
MET Blood
MET Urine
CYS

Primary Homocystinurua
HH

H
H
L

Cobalamin defect
H
H
N/L
L
H

5-methyltetrahydro folate reductase defect HH
N
N/L
L
H

• HCY: Homocysteine
• MMA: Methylmalonic acid
• MET: Methionine
• CYS: Cysteine
• H: elevated levels
• N: normal levels
• L: decreased levels

Methodology

The method used at Diagnostic Laboratory Services is the Abbott IMX Homocysteine, which is a fluorescence polarization immunoassay.

Comparisons of the results with those obtained by gas chromatography-mass spectrometry (GC-MS) or High-performance Liquid Chromatography (HPLC) show excellent correlation. Due to the use of mouse antibodies in the reagent, patients who have received mouse monoclonal antibodies for therapy or diagnosis may contain Human anti-mouse antibody (HAMA) in their serum. This may interfere with this assay.

In some literature, an alternate unit, mg/mL, is used. To convert results in umol/L to results in ug/mL:

Conc. in ug/mL = Conc. in umol/L X 0.1352

References

1. Lussier-Cacan, S et al. Am J Clin Nutr 64: 587-593 (1996).
2. Fickerstrand, T et al. Clin Chem 39: 263-271 (1993).
3. Deloughery, TG et al. Circulation 94: 3074-3078 (1996).
4. Boushey, CJ et al. JAMA 274: 1049-1057 (1995).
5. Clarke, R et al. N Engl J Med 324: 1149-1155 (1991).
6. Selhub, J et al. N Engl J Med 332: 286-291 (1995).

Ordering Information

Testing Site: DLS Special Chemistry.
Methodology: Fluorescence PolarizationImmunoassay (FPIA)

Specimen Requirements:
Preferred: EDTA or Heparin Plasma, 1.0 mL (min. 0.4 mL),6 hours fasting
Acceptable: Serum, 1.0 mL (min 0.4 mL), 6 hours fasting

Unacceptable Specimens:
Non-fasting specimens
Gross hemolysis and lipemia.

Storage and Specimen Stability:
Stable refrigerated below 4°C for up to 2 weeks.
Frozen (below -20°C) for long-term storage.

Reference Ranges:
Honocysteine
5.0 – 15.0 umol/mL
(0.7 – 2.0 ug/mL)

Procedure Codes: 82131 Amino acid, single, quantitative