2-Hydroxyphenylacetic Acid

Organic acids encompass a diverse group of compounds with acidic properties, commonly produced as metabolic byproducts. They play pivotal roles in metabolic pathways like the tricarboxylic acid cycle and are markers for many detoxification pathways.  

Inherited metabolic conditions known as organic acid disorders result in the excessive excretion of these acids due to enzymatic defects, often presenting symptoms in newborns and requiring early intervention for improved outcomes. 

Moreover, organic acids can be produced by the microbiome, offering additional insights into gut health and metabolic functions.  

2-hydroxyphenylacetic acid is an organic acid linked to conditions such as phenylketonuria, highlighting their importance in clinical diagnostics and therapeutic interventions.

What Are Organic Acids?  [3., 4.]

Organic acids are organic compounds with acidic properties.  They include a variety of functional groups like carboxyl, phenol, enol, and thiol, with carboxylic acids having the strongest acidity.

Organic acids are considered weak acids, with those containing phenol, enol, alcohol, or thiol groups being even weaker.  

Their structures vary in terms of carbon chain types—aromatic, aliphatic, alicyclic, heterocyclic—saturation, substitutions, and the number of functional groups. 

These acids play critical roles in metabolic and catabolic pathways, notably in the tricarboxylic acid cycle inside mitochondria, which is central to energy production in eukaryotes.  They are also pivotal in determining the sensory properties of fruits and vegetables.

Organic Acid Disorders  [2., 8.]

Organic acid disorders are inherited metabolic conditions that affect the enzymes or transport proteins essential for the breakdown of amino acids, lipids, or carbohydrates. They are marked by the excessive excretion of non-amino organic acids in urine, primarily due to defects in specific enzymes involved in amino acid breakdown that cause buildup of organic acids in tissues.

Conditions can manifest as inborn metabolic disorders of organic acids and amino acids, urea cycle anomalies, and mitochondrial respiratory chain deficiencies.

These disorders are typically passed down through autosomal recessive inheritance.  They often present in newborns with symptoms like vomiting and lethargy, progressing to more severe neurological symptoms. 

Early diagnosis and intervention are critical and can improve outcomes. Diagnostic methods include urine organic acid analysis via gas chromatography-mass spectrometry (GC/MS). 

Current treatments focus on managing symptoms and preventing complications, although definitive therapies are still under research.  Treatment focuses may include dietary management, detoxifying harmful metabolites, and in severe cases, organ transplantation. 

Continuous monitoring and management are essential for managing symptoms and preventing complications.

Organic Acids and the Microbiome  [5.]

Increasingly, research highlights new relationships between the microbiome and human health.  Many organisms that comprise the microbiome produce organic acids that can then be tested for additional diagnostic capability.  

Certain organic acids in urine like hippuric acid, benzoic acid, and indoleacetic acid are metabolites produced by gut bacteria from the breakdown of amino acids, dietary polyphenols, and other substances. 

These acids provide insights into gut health and metabolic functions.  For example, elevated levels of certain acids may indicate gut dysbiosis or specific metabolic imbalances, such as phenylketonuria. 

Some organic acids known to be produced by the microbiome include: 

Benzoic Acid (BA): 

Produced from phenylalanine and polyphenol metabolism by intestinal bacteria. High levels in urine can indicate glycine deficiency or liver dysfunction.

Hippuric Acid (HA):

Formed in the liver by conjugation of benzoic acid with glycine. Elevated levels may indicate exposure to environmental toxins like toluene.

Phenylacetic Acid (PAA) and Phenylpropionic Acid (PPA): 

These acids result from phenylalanine metabolism by gut bacteria. High urinary levels can suggest dysbiosis or disorders like phenylketonuria. PAA is also associated with depression markers.

4-Hydroxybenzoic Acid (4-HBA) and 4-Hydroxyphenylacetic Acid (4-HPAA): 

Derivatives of tyrosine metabolism. 4-HBA is linked to catechin (green tea) metabolism, and 4-HPAA is useful in diagnosing small bowel diseases related to bacterial overgrowth.

3-Hydroxyphenylpropionic Acid (3-HPPA): 

A metabolite from dietary polyphenols like proanthocyanidins, indicative of robust bacterial metabolism in the intestines.

3,4-Dihydroxyphenyl Propionic Acid (3,4-DHPPA): 

Produced from dietary quinolones by clostridial species, with high levels suggesting an overgrowth.

3-Indoleacetic Acid (IAA): A breakdown product of tryptophan by gut bacteria such as Bifidobacterium and Bacteroides. Elevated levels are seen in conditions like phenylketonuria or dietary changes.

These organic acids are important markers in clinical diagnostics, helping to monitor metabolic disturbances, gut microbiota balance, and exposure to environmental toxins.

Their presence and concentration are influenced by diet, gut microbiota composition, and overall metabolic health, making them valuable indicators in clinical settings for assessing both metabolic and gastrointestinal conditions.

Understanding 2-Hydroxyphenylacetic Acid  [5., 7., 8.]

2-Hydroxyphenylacetic acid, also referred to as ortho-hydroxyphenylacetic acid or 2-hydroxybenzeneacetic acid, is a compound belonging to the group of 2(hydroxyphenyl)acetic acids, characterized by a phenyl ring with a hydroxyl group (-OH) attached to the aromatic ring, along with a carboxylic acid group (-COOH) attached to the adjacent carbon atom. 

It is weakly acidic and slightly soluble in water. This compound can be found in various food sources like natal plum, lemon verbena, half-highbush blueberry, and parsley, making it a potential biomarker for consumption of these foods. 

Its presence in bodily fluids such as blood, feces, and urine is associated with phenylketonuria, a metabolic disorder.

2-Hydroxyphenylacetic Acid and Phenylketonuria [11.]

2-Hydroxyphenylacetic Acid is produced from the metabolism of phenylalanine, an essential amino acid obtained from dietary sources.  Phenylketonuria (PKU) is a genetic disorder marked by a deficiency in phenylalanine hydroxylase (PAH), which causes phenylalanine to accumulate in the blood and brain, resulting in severe neurological complications if left untreated. 

The enzyme phenylalanine hydroxylase normally converts phenylalanine into tyrosine.  Tyrosine, in turn, is a crucial precursor for various important molecules in the body, including neurotransmitters like dopamine and norepinephrine as well as hormones like thyroxine. 

In individuals with PKU, the decreased conversion of phenylalanine to tyrosine results in reduced tyrosine levels, leading to disruptions in these essential biochemical pathways.  

One notable metabolic consequence of PAH deficiency is the accumulation of 2-hydroxyphenylacetic acid, among other toxic by-products, due to alternative pathways of Phe metabolism. 

The discovery of PKU by Dr. Asbjørn Følling in the early 20th century marked a pivotal moment in medical history, illustrating the connection between metabolic disorders and neurological dysfunction. 

Subsequent research has focused on elucidating the biochemistry and genetics underlying PKU, leading to the development of therapeutic strategies, including dietary restriction and emerging therapies like BH4 supplementation and gene therapy. 

Newborn screening programs have revolutionized early detection, allowing for prompt dietary intervention to mitigate symptoms and prevent cognitive impairment. 

Organic Acid Testing in Functional Medicine

Organic Acid Testing in Functional Medicine

In functional medicine, organic acid testing is utilized to evaluate a patient's metabolic function through a simple urine test. This testing can identify metabolic imbalances that may affect a patient’s mood, energy, and overall health. 

Testing provides insights into nutrient deficiencies, dietary habits, toxic exposures, and gut microbiome activity. 

The results assist practitioners in customizing treatment plans to address specific metabolic dysfunctions and improve health outcomes. 

Additionally, it helps in assessing the impact of microbial metabolism and the efficiency of the Krebs Cycle, aiding in personalized healthcare.

Lab Testing for 2-Hydroxyphenylacetic Acid

Test Information, Sampling Methods and Preparation

Laboratory testing for organic acids including 2-Hydroxyphenylacetic Acid is typically done in urine, although it can also be tested in blood.  Testing may be ordered to diagnose an inborn metabolic disorder, or to assess metabolic function and gastrointestinal health in a functional medicine setting.  

Urine samples may be collected in a clinical setting; they can also be collected at home.  Some labs recommend or require a first morning void sample, to provide a concentrated sample.  

Interpreting 2-Hydroxyphenylacetic Acid Results

Optimal Range for 2-Hydroxyphenylacetic Acid Testing

Generally, falling within reference ranges for organic acids is recommended, although for many of these organic acids, a level towards the lower end of the reference range is considered optimal.  

It is essential to consult with the laboratory company used for their recommended reference range for 2-hydroxyphenylacetic acid.  

One company reports the following reference range for 2-hydroxyphenylacetic acid:  <2 mmol/mol creatinine  [7.]

Clinical Significance of Elevated Levels of 2-hydroxyphenylacetic Acid

Elevated levels of 2-hydroxyphenylacetic acid are commonly associated with several clinical conditions that involve metabolic stress and insulin resistance.  

One of the primary conditions linked with high 2-hydroxyphenylacetic acid levels is Type 2 Diabetes Mellitus (T2D), where it serves as an indicator of impaired glucose metabolism and insulin sensitivity.  In T2D, insulin resistance leads to altered lipid and carbohydrate metabolism, contributing to increased oxidative stress and subsequent production of 2-hydroxyphenylacetic acid. 

As a metabolite closely associated with oxidative stress, the presence of elevated 2-hydroxyphenylacetic acid levels can also signal an increased risk or progression of other metabolic disorders, such as obesity and non-alcoholic fatty liver disease (NAFLD), both of which share underlying mechanisms of metabolic dysregulation with insulin resistance.

Additionally, high levels of 2-hydroxyphenylacetic acid are noted in conditions characterized by severe metabolic disturbances including lactic acidosis and ketoacidosis.  

These conditions often occur in severe diabetic states or in response to extreme physical stress such as prolonged fasting or intense exercise, where the body relies heavily on lipolysis and amino acid catabolism for energy, leading to an accumulation of organic acids including 2-hydroxyphenylacetic acid. 

Monitoring 2-hydroxyphenylacetic acid levels can provide valuable insights into the metabolic status of patients and help in the early detection and management of metabolic complications related to insulin resistance and other stress-related metabolic states.

Clinical Significance of Low Levels of 2-hydroxyphenylacetic Acid

Low levels of 2-hydroxyphenylacetic acid are not considered clinically relevant. 

2-hydroxyphenylacetic Acid Related Biomarkers and Comparative Analysis

2-hydroxyphenylacetic Acid is typically tested along with other organic acids to gain deeper insights into metabolic pathways and physiological processes.

Organic acids that may be tested as part of a panel include: 

2-Hydroxybutyric Acid: this acid is a marker for insulin resistance and increased oxidative stress.

‍2-Hydroxyphenylacetic Acid: derived from phenylalanine metabolism, this acid is used as a biomarker in various metabolic assessments.

3-Hydroxybutyric Acid: a ketone body produced during fat metabolism, indicative of carbohydrate deprivation or ketogenic conditions.

3-Hydroxyisovaleric Acid: an organic acid that accumulates in leucine catabolism disorders, often elevated in maple syrup urine disease.

3-Indoleacetic Acid: a metabolite of tryptophan, it is significant in the study of serotonin pathways and plant growth regulation.

4-Hydroxybenzoic Acid: a derivative of tyrosine metabolism, it is linked to catechin (green tea) metabolism and may be produced by some intestinal bacteria.

4-Hydroxyphenyllactic Acid: a metabolite associated with disorders of phenylalanine tyrosine metabolism.

‍4-Hydroxyphenylacetic Acid: a breakdown product of tyrosine, used in diagnosing disorders involving the degradation of aromatic amino acids.

5-Hydroxyindoleacetic Acid: the main metabolite of serotonin, used as a marker in the diagnosis of carcinoid syndrome.

Adipic Acid: a dicarboxylic acid that can also be formed metabolically in humans through the oxidation of certain fatty acids.

‍a-Keto-b-Methylvaleric Acid: an intermediate in isoleucine metabolism, which can accumulate in certain metabolic disorders.

a-Ketoisocaproic Acid: an intermediate in the metabolism of leucine, elevated in maple syrup urine disease.

‍a-Ketoisovaleric Acid: a breakdown product of valine metabolism, also linked to maple syrup urine disease.

a-Ketoglutaric Acid: a key intermediate in the citric acid cycle, essential for energy production and nitrogen transport.

‍Benzoic Acid: produced from phenylalanine and polyphenol metabolism by intestinal bacteria. High levels in urine can indicate glycine deficiency or liver dysfunction.

Cis-Aconitic Acid: an intermediate in the tricarboxylic acid cycle, formed by the dehydration of citric acid.

‍Citric Acid: a central compound in the citric acid cycle, crucial for energy production in cells.

Ethylmalonic Acid: this acid accumulates in ethylmalonic encephalopathy and is involved in fatty acid metabolism.

Fumaric Acid: an intermediate in the tricarboxylic acid (TCA) cycle, participating in energy production through its conversion to malate and subsequent participation in the generation of ATP.

Homovanillic Acid: a major metabolite of dopamine, used as a marker to monitor dopamine levels.

Hippuric Acid: formed from the conjugation of benzoic acid and glycine; elevated levels can indicate exposure to certain environmental toxins.

Hydroxymethylglutarate: an intermediate in leucine metabolism, also associated with disorders of ketogenesis and ketolysis.

Isocitric Acid: an isomer of citric acid and an important part of the citric acid cycle, pivotal in cellular energy production.

Kynurenic Acid: a product of tryptophan metabolism, known for its role as a neuroprotective agent.

Lactic Acid: produced from pyruvate via anaerobic metabolism, an indicator of hypoxia and strenuous exercise.

Malic Acid: a dicarboxylic acid found in fruits, and involved  in the citric acid cycle.

Methylmalonic Acid: an indicator of Vitamin B12 deficiency, it accumulates when the vitamin is deficient.

‍Methylsuccinic Acid: a dicarboxylic acid often involved in alternative pathways of fatty acid metabolism.

Orotic Acid: involved in the metabolism of pyrimidines, abnormalities in its levels can indicate metabolic disorders.

Pyroglutamic Acid: an uncommon amino acid derivative that can accumulate in glutathione synthesis disorders.

Pyruvic Acid: a key intersection in several metabolic pathways; its levels are crucial for assessing cellular respiration and metabolic function.

Quinolinic Acid: a neuroactive metabolite of the kynurenine pathway, elevated levels are associated with neurodegenerative diseases.

Suberic Acid: a dicarboxylic acid that is a biomarker in adipic aciduria, often studied in relation to fatty acid oxidation disorders.

Succinic Acid: a four-carbon dicarboxylic acid that plays a central role in the Krebs cycle, crucial for energy production.

Tricarballylic Acid: an organic acid that can inhibit aconitase in the citric acid cycle and is sometimes associated with glyphosate exposure.

Vanillylmandelic Acid: a metabolite of epinephrine and norepinephrine, used as a marker for neuroblastoma and other catecholamine-secreting tumors.

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