Homocysteine: Friend or Foe?

We are all well aware that high levels of homocysteine are associated with a wide range of diseases, as shown in Table 1. This would tend to imply that homocysteine is a toxic molecule, and the less the better.

Table 1.

Diseases and Conditions Associated With Hyperhomocysteinemia

Alzheimer’s disease

Birth defects

Blood clots

Cancer

Coronary artery disease

Dementia

Endothelial damage

Miscarriage

Myocardial infarction

Parkinson’s disease

Pre-eclampsia

Stroke

I have watched with interest as the recommended maximum “safe” levels have dropped several times through the past 2 decades as research has progressed. Several months ago, one of the thought leaders in nutritional medicine recommended, at a lecture I attended, that homocysteine should be less than 6 mg/L. I found this intriguing and likely very difficult to attain. Then as I was studying the biochemistry of glutathione for my editorial in February 2014,1 I couldn’t help but notice the homocysteine/methylation cycle, right in the middle of a lot of metabolism. It got me to wondering if, like cholesterol (see my editorial from June 2014),2 perhaps we were inappropriately vilifying a molecule important for health.

Initially, it looked to me like homocysteine might be an important storage/transfer molecule and the factors that cause its elevation are the actual problem, not the homocysteine itself. So this led me to ask 4 questions:

1. Does homocysteine play an important role in metabolism?

2. Is homocysteine itself toxic?

3. What causes homocysteine to increase?

4. What is the optimal level of homocysteine in the blood?

Following is what I found in the research. As a PubMed search of homocysteine produces more than 13 000 hits, there is obviously much more than I can cover here.

Metabolic Role

Does homocysteine play an important role in metabolism? Homocysteine is not obtained from the diet. Rather, it is synthesized from methionine via a multistep process. The first clue that homocysteine plays an important role in the body is that this synthesis requires energy. Homocysteine is a nonprotein α-amino acid homologue of cysteine, which, as can be seen in Figure 1, differs from cysteine by an additional methylene bridge (-CH₂-). It is biosynthesized from methionine by the removal of a terminal methyl group.

Figure 1.

Methionine Cycle3

Stating the question differently: Is hypohomocysteinemia associated with metabolic dysfunction or disease?

Turns out the answer is yes: Low homocysteine levels do indeed have disease correlations. For example, low homocysteine has been shown to have a strong association with peripheral neuropathy. A surprising 41% of patients with idiopathic peripheral neuropathy have hypohomocysteinemia.4 As can be seen in Figure 2, homocysteine can be considered a storage molecule for sulfur and a transfer molecule for methyl metabolism. For example, homocysteine is clearly a storage molecule for cysteine, the rate limiting amino acid for glutathione production.

Figure 2.

Homocysteine as Storage for Sulfur and Transfer for Methyl Molecules

Abbreviations: MS = methionine synthesis;

MTHFR = methylenetetrahydrofolate reductase.

Homocysteine enables single carbon units to be shuttled from the reduced folate pool to the principal methyl donor in the cell. Hypohomocysteinemia—defined as less than 6.0 mmol/L—is not common, occurring in only 0.5% to 1.0% of the population.4

Low homocysteine may also be indicative of excessive conversion to cystathione for use in the transsulfuration pathways (see Figure 2) for production of glutathione, taurine, and sulfate. Low homocysteine would suggest impaired ability for de novo production of glutathione and thus increased susceptibility to oxidative stress.

Is Homocysteine Itself Toxic?

We can thank Kilmer McCully, md, (Winner of the Linus Pauling Award from Institute for Functional Medicine in 1999) for first discovering that elevated levels of homocysteine are associated with cardiovascular disease. Since then, as shown in Table 1, many more disease associations have been found. The question arises: Is homocysteine itself toxic or is it an indirect measure of the actual causative factors? It looks like the answer is a bit of both.

In cell cultures, homocysteine induces programmed cell death in human vascular endothelial cells by interfering in protein syntheses.5 Because of its similarity to methionine, homocysteine can enter the protein biosynthetic apparatus. However, it cannot complete the protein biosynthetic pathway, resulting in abnormal proteins and adducts that are toxic to cells.6 These abnormal proteins can also initiate an immune response as they are now foreign to normal body tissues. As can be seen in Figure 3, in addition to impairing protein synthesis, homocysteine is also directly toxic to the endoplasmic reticulum, activates glutamate receptors, and damages DNA.

Figure 3.

Homocysteine Metabolism8

Equally important, homocysteine elevation is clearly a marker of dysfunctional metabolism.7 The many nutritional, hormonal, and genetic factors that raise homocysteine levels are also associated with common pathological conditions, such as cancer, autoimmune diseases, endothelial dysfunction, and neurodegenerative disease.

There are 3 pathways for reducing homocysteine levels: 2 are for homocysteine remethylation to methionine and the third through conversion into cystathione for transsulfuration, as can be seen in Figure 4. The first is dependent on the folate coenzyme 5-methyltetrahydrofolate that can donate a methyl group to homocysteine in a reaction catalyzed by the vitamin B₁₂–dependent enzyme methionine synthase. The supply of 5-methyltetrahydrofolate depends on vitamin B₆ and the catalytic activity of MTHFR where single nucleotide polymorphisms of MTHFR (C677T) are common, thus impairing folate-dependent remethylation of homocysteine. The second route for homocysteine remethylation is independent of the 1-carbon pool, using betaine, derived from diet or the oxidation of choline, as a methyl group source in a reaction catalyzed by betaine-homocysteine methyltransferase (BHMT). Although folate-dependent remethylation is found in all tissues, the betaine pathway is only found in the liver and kidneys.

Figure 4.

Pathways for Reducing Homocysteine Levels

Somewhat surprising, most of the research looking at the correlation between hyperhomocysteinemia and a wide range of diseases—cardiovascular disease (eg, atherosclerosis and thrombosis), autoimmune disease (eg, type 1 and type 2 diabetes), gastrointestinal disorders (eg, constipation, inflammatory bowel disease, Crohn’s disease, and colorectal cancer), bone density, and neurodegenerative disease (eg, Alzheimer’s disease, depression, and Parkinson’s disease)—actually find significantly stronger associations with aberrant methylation and folate metabolism abnormalities.5 This seems to substantiate the perspective that although homocysteine is directly toxic, more important is that hyperhomocysteinemia is a marker of metabolic dysfunction, primarily methylation disruption.

What Causes Homocysteine to Increase?

Now this is where it gets clinically interesting, as in many ways serum levels of homocysteine can be seen as an indirect measure of other factors known to be critical to health.

What Is the Optimal Blood Level of Homocysteine?

In the final analysis, homocysteine is somewhat like glucose: It plays an important metabolic role, but too much is toxic and too little results in metabolic problems. This seems true of so many metabolites in the body. They play key roles in physiological function—but when outside of a typically narrow range, problems arise. I think it important that we refrain from the current habit of so many clinicians and patients of defining any molecules in the body as bad. Although there are certainly a few that appear toxic at any level, such as lead, there are others, even arsenic, that at some level are beneficial. The problem is not the molecule, but what happens when the molecule is damaged—such as oxidized cholesterol—or at such a high level in the body—such as blood sugar higher than 200—that it alters physiology. These are typically not the result of ingestion of the molecule, but rather all the physiology around them that is dysfunctional. For homocysteine, my best estimate is that the ideal range is 5.0 to 7.0 mmol/L. What do you think?

If homocysteine is low, supplementation with methionine, N-acetylcysteine, and taurine is indicated. If homocysteine is high, the many factors listed in Table 3 need to be addressed as well as supplementation with the activated forms of B_6, folate (not folic acid) and B₁₂. If unresponsive, betaine should also be used.

Table 3.

Genes Directly and Indirectly Involved in Homocysteine Metabolism11

Symbol	Gene	Name Function
MTHFR	Methylenetetrahydrofolate reductase	Conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate
CBS	Cystathionine-β-synthase	Condensation of homocysteine and serine to form cystathionine
MTR	Methyltetrahydrofolatehomocysteine methyltransferase	Remethylation of homocysteine to methionine
MTRR	Methionine synthase reductase	Reductive regeneration of cob(I)alamin cofactor required for the maintenance of MTR in a functional state
RFC1	Reduced-folate carrier	5-methyltetrahydrofolate internalization in cell
GCP2/FOLH1	Glutamate carboxypeptidase II	Polyglutamate converted to monoglutamate folate by action of the enzyme folylpoly-γ-glutamate-carboxypeptidase (FGCPI), an enzyme expressed by GCP2
ENOS	Endothelial nitric oxide synthase	Conversion of l-arginine to l-citrulline and nitric oxide synthase (NO)
TC2	Transcobalamine II	Transport of vitamin B12
SHMT1	Serine hydroxymethyltransferase I	Reversible conversion of serine and tetrahydrofolate to glycine and 5,10-methylenetetrahydrofolate
TYMS	Thymidylate synthase	5,10-methylenetetrahydrofolate and deoxyuridylate to form dihydrofolate and thymidylate
CTH	Cystathionine-γ-lyase	Hydrolysis of cystathionine to cysteine and α-ketoglutarate
MTHFD	Methylenetetrahydrofolate dehydrogenase	Conversion of 5,10-methylenetetrahydrofolate to 5,10-methenyltetrahydrofolate
MTHFS	Methenyltetrahydrofolate synthetase	Conversion of 5-formyltetrahydrofolate to 5,10-methenyltetrahydrofolate
APOE	Apolipoproteine E	Mediates the binding, internalization, and catabolism of lipoprotein particles
VEGF	Vascular endothelial growth factor	Growth factor active in angiogenesis, vasculogenesis and endothelial cell growth
PON1	Paraoxonase I	Hydrolyzes the toxic organophosphorus. It also mediates an enzymatic protection of LDL against oxidative modification
BHMT	Betaine-homocysteine methyltransferase	In liver and kidney, it catalyses the conversion of betaine to dimethylglycine (DMG)
MAT1A	Methionine adenosyltransferase IA	Methionine to SAM by transfer of the adenosyl-moiety of ATP to the sulfur atom of methionine
AHCY	S-adenosy-l-homocysteine hydrolase	Hydrolysis of S-adenosy-l-homocysteine to adenosine and homocysteine
CBL	Cystathionine-β-lyase	Conversion of cystathionine to homocysteine
F5	Coagulation factor V	Cofactor for the factor Xa-catalyzed activation of prothrombin to the clotting enzyme thrombin
PAI1	Prothrombin activator inhibitor I	Inhibition of fibrinolysis by inhibiting the plasminogenactivator and t-PA

Although supplementation with folate, B₆ and B₁₂ does normalize homocysteine levels, the clinical outcomes have been disappointedly mixed. This seems to further support the idea of elevated homocysteine as an indirect measure of disturbed metabolism.

I find very revealing that the Mediterranean diet, with so much research showing that it reduces almost every chronic disease, is associated with lower homocysteine levels. The same is true of a nutrient and fiber rich diet: 19% decrease in homocysteine after only 2 weeks.12

In This Issue

I write this editorial with great sadness; Sandi Cutler, who played a foundational role in the success of Bastyr University and the advancement of natural medicine, suddenly passed away July 1. I have the honor of his eulogy, which we will publish in the next issue of IMCJ. He was my closest friend, and I am devastated by his untimely loss.

One of the most compelling books on environmental medicine I have seen is Slow Death By Rubber Duck, written by Rick Smith and Bruce Lourie. Managing Editor, Craig Gustafson, interviews Mr Smith. This is a must-read story and an entertaining resource for educating your patients about environmental toxins.

Because we focus on being a resource for clinicians, we would normally not publish an academic article on education. However, I was struck by the very interesting presentation by Michael Tims, phd, on how challenging it is to change a student’s worldviews. As more and more conventionally trained health care professionals enter integrative medicine, they are seriously challenged to think differently about their patients. Moving from a disease-centric to patient-focused approach requires transformative thinking and experiences. Unlearning is indeed every bit as important as learning.

We have 2 articles on mitochondria, which complement well my editorial in the April issue of IMCJ.13 An excellent commentary article by Alex Vasquez, dc, nd, do, facn, where he brings up the compelling idea of leaky mitochondrial membranes. If you have not heard Alex speak or read his work, you are in for a real treat. His grasp of health and nutrition is remarkable. I know of no one else to achieve degrees in 3 different health care professions. I have been harassing him for several years now on an article comparing the schools … Garth L. Nicolson, phd, provides great guidance on the use of nutritional supplements to promote mitochondrial energy production and protect from oxidative stress. Very encouraging to see the growing number of positive clinical trials.

I appreciate how well John Weeks keeps us abreast of developing public policy in integrative medicine. I find of particular interest the proposed change of name of the National Center for Complementary and Alternative Medicine to the National Center for Research in Complementary and Integrative Health. Far more important than the name is what is being researched. We must study whole systems of healing, not isolated therapies. Otherwise, the model will not change and the health care crisis will continue to worsen.

A very different BackTalk from Bill Benda, md. A good reminder of what is important in the world.

Joseph Pizzorno, nd, Editor in Chief

drpizzorno@innovisionhm.com

http://twitter.com/drpizzorno

Table 2.

Causes of Elevated Homocysteine9,10

Enzyme deficiencies

Cystathione-β-synthase

Methionine synthase

5-Methyltetrahydrofolate reductase

Genetic polymorphisms (worst)

MTHFR 677CT (TT)

MTHFR 1298A>C (AA, CC)

MTR 2756A>G (AA)

MTRR 66A>G (AG)

CBS 884ins68 (WW, WI)

(See Table 3 for a complete list of genes involved in homocysteine metabolism. Polymorphisms have been detected in many of them.)

Vitamin deficiencies

Folate

Vitamin B6

Vitamin B12

Demographics

Increasing age

Male

Tobacco use

Solid organ transplant recipients

Chronic diseases

Renal dysfunction

Systemic lupus erythematosus

Malignant neoplasm

Psoriasis

Osteoporosis

Acute-phase response to systemic disease

Prescription drugs

Colestipol

Methotrexate

Nitrous oxide

Niacin

Thiazide diuretics