Pyridoxic Acid

Pyridoxic acid (PA), also known as 4-pyridoxic acid, is a primary catabolic product of vitamin B6 (pyridoxine) metabolism and is excreted in the urine. 

It reflects recent dietary intake and varies based on factors like gender and riboflavin deficiency. 

Formed by enzymes such as aldehyde oxidase I and microbial enzymes, pyridoxic acid can be further broken down by gut microflora. 

It's a crucial marker for vitamin B6 status, especially when assessing deficiencies or the effectiveness of supplementation programs. 

In certain conditions like hypophosphatasia, where alkaline phosphatase activity is low, pyridoxal-5’-phosphate (PLP) levels are high while PA levels remain normal, indicating a disruption in vitamin B6 metabolism.

What is Pyridoxic Acid? [2., 8.] 

Pyridoxic acid, also known as 4-pyridoxic acid, is a catabolic product of vitamin B6 (pyridoxine) metabolism. 

Pyridoxic acid is a 2-methylpyridine derivative. 

It is the primary catabolic product of vitamin B6 and is excreted in the urine. Pyridoxic acid reflects immediate or very recent dietary intake. [3.] 

Urinary levels of 4-pyridoxic acid are lower in females than males and are further reduced in individuals with riboflavin deficiency. It is formed by the action of aldehyde oxidase I and microbial enzymes, specifically pyridoxal 4-dehydrogenase, an NAD-dependent aldehyde dehydrogenase. [2., 12.] 

Gut microflora can further break it down via 4-pyridoxic acid dehydrogenase, which catalyzes its oxidation to 3-hydroxy-2-methylpyridine-4,5-dicarboxylate using NAD as a cofactor. [2.] 

Vitamin B6, Pyridoxic Acid, and Alkaline Phosphatase [1., 4.] 

Vitamin B6, primarily in its active form pyridoxal 5'-phosphate (PLP), is crucial for numerous enzymatic reactions in the body, including amino acid metabolism. 

Alkaline phosphatase (ALP) is an enzyme that dephosphorylates PLP to pyridoxal (PL), which can then be further metabolized to pyridoxic acid (PA) for excretion. 

In conditions like hypophosphatasia, where ALP activity is deficient, PLP accumulates in the blood, while PA levels remain relatively normal, highlighting the role of ALP in vitamin B6 metabolism.

Clinical Significance of Pyridoxic Acid Testing

Pyridoxic acid is assessed as a marker of vitamin B6 deficiency. 

Pyridoxic acid levels in urine correlate with plasma pyridoxal phosphate (PLP) and red blood cell (RBC) PLP levels, which are the active forms of vitamin B6. This correlation makes pyridoxic acid a reliable marker for assessing vitamin B6 status. [9.] 

When tested in blood, it is often tested alongside PLP (the active form of vitamin B6) for a more complete assessment of vitamin B6 status. 

Vitamin B6 is a group of six compounds: pyridoxal, pyridoxol, pyridoxamine, and their 5'-phosphate esters. Pyridoxal 5-phosphate (PLP) is the biologically active form and serves as a cofactor in many enzymatic reactions.

Eukaryotes cannot synthesize vitamin B6 and must obtain it from their diet. 

Pyridoxal 5′-phosphate (PLP), the most clinically significant form, is commonly measured in plasma and acts as a coenzyme for over 100 enzymes involved in amino acid, neurotransmitter, nucleic acid, heme, and lipid metabolism. It also plays a role in energy homeostasis. 

Clinical Significance of Low Vitamin B6 Levels

Low plasma PLP levels are inversely associated with several chronic and acute diseases, including rheumatoid arthritis, cardiovascular disease, and cancer. This deficiency can occur without dietary insufficiency, possibly due to its mobilization to inflammation sites or increased catabolism during inflammation. [1.] 

PLP is crucial for heme biosynthesis and B6 deficiency can lead to anemia and other inherited metabolic disorders. [1.] 

Low PLP levels are linked with an increased risk of colorectal cancer, with a 49% decrease in risk for every 100-pmol/mL increase in blood PLP levels. [1.] 

Various genetic conditions, malabsorption issues, and certain medications can lead to B6 deficiency. 

Evaluating vitamin B6 status is crucial for diagnosing deficiencies, particularly in individuals with progressive nerve compression disorders like carpal and tarsal tunnel syndromes. 

Deficiency in vitamin B6 can contribute to conditions such as burning mouth syndrome and exacerbate nerve compression disorders. 

Symptoms of Vitamin B6 Deficiency

Symptoms of deficiency include skin scaling, severe gingivitis, irritability, weakness, depression, dizziness, peripheral neuropathy, and seizures. 

In children, it may cause diarrhea, anemia, and seizures.

Clinical Significance of Elevated Vitamin B6 Levels [1., 4.] 

Elevated PLP with normal pyridoxic acid levels are seen in hypophosphatasia (HPP), a genetic disorder characterized by low serum alkaline phosphatase activity and high plasma PLP levels, due to impaired dephosphorylation by tissue nonspecific isoenzyme of alkaline phosphatase (TNSALP). 

Despite high PLP levels, HPP patients generally do not exhibit B6-related symptoms.

Monitoring vitamin B6 status is also important for assessing the success of supplementation programs. 

Laboratory Testing for Pyridoxic Acid

Test Information, Sample Collection and Preparation

Pyridoxic acid is typically measured in urine samples, as it is primarily excreted through the kidneys. However, pyridoxic acid may also be tested in the blood, often alongside PLP, the active form of vitamin B6.

Blood samples require a venipuncture, while urine samples may be collected from home.

It is important to consult with the ordering provider regarding specific sample collection instructions. Generally, fasting and avoiding multivitamin supplements are required prior to sample collection. 

Interpretation of Pyridoxic Acid Levels

Optimal Levels of Pyridoxic Acid

Optimal levels of pyridoxic acid are reported as < 111.9

nmol/mg Creatinine. [10.]

Clinical Significance of Elevated Pyridoxic Acid

Elevated levels of pyridoxic acid may indicate excessive supplementation, renal insufficiency, or certain metabolic disorders. [6.]

One large study of over 15,000 participants found that higher 4-pyridoxic acid levels were linearly associated with increased all-cause and cancer mortality risk. [11.]

Clinical Significance of Low Pyridoxic Acid

Low levels of pyridoxic acid in urine can indicate a deficiency in vitamin B6. This may be due to inadequate intake, or issues with absorption. 

Low pyridoxic acid levels can be seen in chronic conditions that affect vitamin B6 metabolism, such as chronic alcoholism and certain genetic disorders. [9.] 

Pyridoxic acid levels can also be affected by riboflavin (vitamin B2) deficiency, as riboflavin is a cofactor for the enzyme that converts pyridoxal to pyridoxic acid. [9.]

Pyridoxic Acid Ratios and Indices

Pyridoxic Acid/Pyridoxine Ratio 

Increased vitamin B6 catabolism to 4-PA, reflected by a higher 4-PA/pyridoxine ratio, is associated with higher cardiovascular risk in patients with type 2 diabetes. 

This ratio is a strong marker for assessing cardiovascular risk, suggesting that enhanced vitamin B6 catabolism may be linked to vascular damage. [7.]

Pyridoxal-5’-Phosphate/Pyridoxic Acid Ratio [1.,13.] 

Patients with hypophosphatasia, an inborn error characterized by deficient activity of tissue nonspecific alkaline phosphatase (AP), exhibit markedly increased circulating levels of pyridoxal-5'-phosphate (PLP). 

One study demonstrated that the mean PLP concentration in patients with hypophosphatasia was significantly higher (1174 nM) compared to control subjects (57±26 nM). [13.] 

Despite these elevated PLP levels, the urinary excretion of 4-pyridoxic acid (PA), a degradation product of PLP, remained unremarkable in affected children consuming normal dietary amounts of vitamin B6. [13.]

The study indicates that tissue nonspecific AP plays a crucial role in vitamin B6 metabolism by regulating extracellular PLP levels but not intracellular levels. 

Consequently, measuring only circulating PLP concentrations may not accurately assess vitamin B6 nutrition in conditions with altered AP activity, such as hypophosphatasia. 

This suggests that hypophosphatasia results in elevated PLP due to a failure in PLP hydrolysis by tissue nonspecific AP, while PA levels remain normal, highlighting the enzyme's role as an ectoenzyme.

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