Homocysteine Buildup: The (Potential) Dark Side of Tyrosine and L-DOPA Supplementation for ADHD
Throughout the last six posts on this blog, all of which were concerned with tyrosine supplementation strategies for ADHD, we alluded to the fact that introducing high levels of tyrosine into the body can precipitate a number of other biochemical processes besides the conversion to dopamine and norepinephrine in the brain of the ADHD patient. For reference, I have included the diagram we've been following for the past six blog posts on ADHD and supplementing with tyrosine (you can click on the diagram below and get a larger picture in most browsers)
As we can see, there are a number of enzymes, processes and intermediate steps involved in just this one pathway of tyrosine. Please note that other nutrients, such as ascorbic acid (a.k.a. vitamin C, which has a number of connections to ADHD) and S-Adenosyl methionine (also known as SAM or SAMe, which has also been discussed in greater detail in relation to ADHD elsewhere) are required in this process.
Dopa Decarboxylase: (vitamin B6, zinc. Also note that excessive levels of some other amino acids, such as leucine, isoleucine, valine, and, especially, tryptophan can compromise this step of tyrosine metabolism. Furthermore, buildup of one of the products of tryptophan metabolism, serotonin, can inhibit or begin to shut down the activity of this Dopa Decarboxylase enzyme and compromise our tyrosine-to-dopamine conversion pathway. This spells bad news if we want to attempt to regulate these dopamine levels in an ADHD brain)
Phenylethanolamine N-methyltransferase: (S-Adenosyl-methionine or SAMe)
As mentioned in the last post on ADHD and tyrosine, the compound S-Adenosyl Methionine or SAMe, is a very important nutrient in a number of biochemical processes, in that it is able to "donate" (give-up) a CH3 methyl group. This is a relatively rare property among nutrients, and we are just beginning to scratch the surface with regards to the role of this nutrient in treating neurological and psychological disorders such as depression, ADHD and the like.
However, when SAMe does donate it's CH3 methyl group, as in the case above, we are left with homocysteine (please note that there are a few additional steps to go from SAMe to homocysteine, it is not a 1-step conversion process. For simplicity, however, we will not go into these in any further detail. Nevertheless, homocysteine is a major end product of SAMe-related CH3 donor reactions, such as the one given above).
In other words, higher tyrosine (or L-DOPA) levels can make their way to this step of the metabolic process and begin to deplete SAMe levels and lead to high levels of homocysteine. High levels of homocysteine are known as hyperhomocysteinemia, is commonly seen in Parkinson's patients, who often take large amounts of L-DOPA (the second step of tyrosine metabolism in our first diagram in this blog post). Numerous studies have shown that long-term treatment with L-DOPA leads to elevated homocysteine levels in the blood of Parkinson's patients.
Elevated homocysteine levels have been linked from everything from cancer to diabetes to autoimmune disorders to stroke (however, please note that these results are far from unanimous, there are a number of studies showing the contrary for each of the diseases listed. Furthermore, there is still some debate as to whether the high levels of homocysteine are a causal factor for these disorders or just another side effect or symptom of these disorders. Nevertheless, the near-ubiquitous presence of high homocysteine levels in so many diseases across the board should at least suggest that homocysteine-lowering efforts are of great potential benefit, at least in this blogger's opinion).
With regards to ADHD, the actual role of homocysteine is, admittedly, much more murky. While the mechanisms and overall physiology of an ADHD brain vs. a Parkinson's brain show acute differences (In ADHD, chemical imbalances between the "inside" and "outside" regions of a neuron exist, which can be chemically modified via medications which target the proteins which shuttle this neuro-transmitting agents in and out of the cells. In Parkinson's, however, the imbalances are caused by cell death and neuronal degeneration, requiring overall higher levels of neurotransmitters like dopamine need to be supplied via chemical precursor agents like L-DOPA), the fact that the two disorders both share similar treatment methods should (in this blogger's opinion) at least sound a warning bell that some of the negative effects for one might also be prevalent in the other.
Surprisingly, there are very few studies (at least to the best of this blogger's knowledge) on homocysteine levels in the ADHD population, so it is difficult to get a good read on the subject. Nevertheless, given some of the points made earlier on tyrosine or L-DOPA supplementation or treatment and homocysteine buildup, we should at least examine some of the ways to reduce high homocysteine levels. Fortunately (at least in most cases), homocysteine-lowering efforts often respond very well to vitamin and mineral based treatments via supplementation or food fortification. At the center of this are the some of the well-known B vitamins.
B vitamin-based nutritional "weapons" which can combat potentially high homocysteine levels arising from tyrosine or L-DOPA supplementation:
The last step, the conversion of norepinephrine to epinephrine was discussed in the last posting on ADHD and tyrosine. The curved arrow simply refers to the fact that the norepinephrine to epinephrine conversion requires another nutrient-based compound SAMe. The norepinephrine essentially "steals" a methyl (CH3) group from SAMe, leaving SAMe to transform into another compound S-Adenosylhomocysteine (which then proceeds to our "dreaded" homocysteine). To put it another way, in order to make the norepinephrine to epinephrine conversion, the valuable nutrient SAMe must be "sacrificed" to a more potentially harmful compound homocysteine.
If this SAMe to homocysteine conversion process is not kept in check, we run the potential risk of homocysteine buildup. However, based on the diagram above (look at the far right side of the diagram for this part), there are 2 different ways to "dump off" high levels of homocysteine by converting it to other more benign compounds. However, each of these two "paths" requires at least one type of B vitamin.
Path #1: conversion of homocysteine to the amino acid cysteine: This is actually a multi-step process, but for the sake of brevity and simplicity, I have left out some of the middle steps. The two major points of note here as follows:
Path #2: the conversion of homocysteine to the amino acid methionine: While path #1 is dependent on one type of B vitamin (B6), this pathway is dependent on 2 different B's: a form of vitamin B12 and a derivative of folic acid (vitamin B9). Without going into too much detail, this process requires a methyl (CH3) "donor" (which, in this case, is the modified form of folic acid here. This is very similar to like way the nutrient SAMe acts in helping the conversion from norepinephrine to epinephrine as mentioned earlier).
Perhaps not surprisingly, taking B12 (also known as cobalamin) and a form of folic acid (folate) together has shown to be advantageous in a number of cases. Combinations of folate and cobalamin have also shown to be useful in reducing homocysteine levels in those treated with L-DOPA.
A quick summary on using B vitamins to reduce potential homocysteine buildup from tyrosine (or L-DOPA) supplementation:
This is our second-to-last post on ADHD and tyrosine. The last one on tyrosine supplementation strategies for ADHD will give a recap of all the key enzymes, nutrients, and chemical intermediates we've covered throughout the past seven postings. It will also provide a summary of what the main studies on exactly how effective tyrosine supplements really are based on clinical studies. Finally, we will briefly mention how tyrosine may be able to augment the effects of common ADHD stimulant medications.