Amino Acid Conjugation
Amino Acid Conjugation
Amino acid conjugation is a major route for a number of small aromoatic acids, and involves the formation of an amide bond between the xenobiotic acyl-CoA and the amino acid. Glycine is the amino acid most frequently used for conjugation, although glutamine and taurine conjugates are also seen in humans.
Amino acid conjugation is a biotransformation pathway in humans for xenobiotic (foreign substances) carboxylic acids as well as those created by gut bacteria and various other substances . Some of the xenobiotics that contain a carboxyl group (-COOH) are drugs (examples are simvastatin, valproic acid, fenfluramine and acetylsalicylic acid), herbicides, insecticides, and food preservatives. Additionally, there are many xenobiotics that can be metabolized into carboxylic acids, and thereby allow conjugation with amino acids. Amino acid conjugation of carboxylic acids is a special form of acetylation and leads to amide bond formation.
It is known that amino acid conjugation of exogenous carboxylic acids occurs in a two-step process. It involves two mitochondrial enzymatic processes located principally in the liver and kidney. (conjugation of bile acids is extramitochondrial, involving enzymes located in the endoplasmic reticulum and peroxisomes)
The Two Steps:
1. Formation of a reactive xenobiotic-CoA thioester intermediate, catalyzed by an ATP-dependent acid:CoA synthetase (ACSM).
2. Linkage of the activated acyl group via an acyl-CoA:amino acid N-acyltransferase (GLYAT) to the amino group of the acceptor amino acid.
Knowledge of amino acid conjugation in terms of enzyme multiplicity, protein structure, and xenobiotic substrate selectivity is not well advanced.
Amino Acid Conjugation Requires Amino Acid Transferase Enzymes And Cosubstrates
Amino acid conjugation requires the amino acid transferase enzymes, and the cosubstrates are amino acids and ATP.
The conjugation reactions of amino acid acyltransferases use the cofactor acetyl-coenzyme A (acetyl-CoA), which is synthesized from vitamin B5, using enzymes that themselves depend on multiple B vitamins.
Several amino acids are needed for amino acid conjugation. Glycine, taurine, glutamine, arginine, and ornithine are used to combine with and neutralize toxins. Of these, glycine is the most commonly utilized in phase II amino acid detoxification. Patients suffering from hepatitis, alcoholic liver disorders, carcinomas, chronic arthritis, hypothyroidism, toxemia of pregnancy, and excessive chemical exposure are commonly found to have a poorly functioning amino acid conjugation system. For example, using the benzoate clearance test (a measure of the rate at which the body detoxifies benzoate by conjugating it with glycine to form hippuric acid, which is excreted by the kidneys), the rate of clearance in those with liver disease is 50% of that in healthy adults.
Even in normal adults, a wide variation exists in the activity of the glycine conjugation pathway. This is due to genetic variation, as well as the availability of glycine in the liver. Glycine, and the other amino acids used for conjugation, become deficient on a low-protein diet. Of course chronic exposure to toxins results in depletion.
Adequate dietary protein is necessary for availability of these 5 amino acids.
The enzymes of amino acid conjugation reside in mitochondria except for those used for bile acid conjugation.
Glycine Conjugation Is The Most Common Amino Acid Conjugation
Studies have shown, the major role of glycine conjugation is to dispose of the end products of dietary polyphenols formed by the gut microbiome. Glycine conjugation facilitates the metabolism of toxic aromatic acids, capable of disrupting mitochondrial integrity. Glycine conjugation has been shown to prevent accumulation of benzoic acid in the mitochondrial matrix by forming hippurate, a less lipophilic conjugate that can be more readily transported out of the mitochondria. The glycine conjugation capacity of individuals can vary significantly. The rate of glycine conjugation can be influenced by several factors, including the availability of ATP, Coenzyme A, and glycine; genetic variation in the ATP dependent acid: CoA ligase (ACSM2B) (encoding HXMA) and glycine N-acyltransferase (GLYAT) (encoding GLYAT) genes as well as variable expression of HXMA and GLYAT. It has been established that the observed variation is not due to genetic variation in GLYAT as the GLYAT gene which is highly conserved among humans.
Bile Acid Conjugation
Bile acids are conjugated with amino acids, glycine and taurine in the liver, catalyzed by two enzymes, bile acid CoA ligase (BAL) and bile acid CoA:amino acid N-acyltransferase (BAT). The bile acid pool in humans contains 2-3 times more glycine-conjugated bile acids than taurine-conjugated bile acids. Bile acid conjugation may also occur with sulfate, and glucuronic acid. Sulfation and glucuronidation increase in cholestasis and cirrhosis. Taurine conjugates increase in cholestasis. Glycine conjugates increase in intestinal malabsorption because insufficient amounts of taurine is absorbed by the intestine. Taurine is a conditionally essential amino acid and I find some people need taurine as well as glycine in supplemental form. I usually give the glycine at night as it often helps people sleep better also. Be careful with supplementing glycine as it can lead to excess oxalate production in people with oxalate producing issues due to dietary, genetic or other causes. In cases of interupted bile recycling in the enterophepatic recycling process by bile acid sequestrants it has been noted that taurine production rises rapidly, and glycine also rises while there is a lowering of cysteine and glutathione, so sometimes it might look like these amino acids could help when bile slows down, but they might not be necessary or helpful. In this case cysteine has been shown to be more helpful in animal research.
Deconjugation Is Possible
Glycine conjugates are subject to possible cleavage by hyrolases/esterases in vivo.
Testing For Amino Acid Conjugation Competancy
Since glycine is the main amino acid used in amino acid conjugation in humans, the challenge test for amino acid conjugation is a test for glycine conjugation. Sodium benzoate, or Aspirin is used to test glycine conjugation. Aspirin is the most common test unless the individual has a salicylate sensitivity. After ingestion of one of these two substances, the metabolites are then measured in the urine.
Inducing Amino Acid Conjugation
- Adequate dietary protein is necessary for availability of amino acids for conjugation.
- Specific amino acids needed are:
Glycine is the most commonly used amino acid.
- B Vitamins
- B5 (pantothenic acid)- often not in B vitamin supplements, so may need to take it seperately if needed
Drugs Commonly Conjugated With Glycine
- m-trifuoromethylbenzoic acid, a metabolite of fenfluramine
- benzoic acid (benzoate)
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