para-Aminobenzoic acid; p-Aminobenzoic acid; PABA; Vitamin Bx; Bacterial vitamin H1
|Molar mass||137.14 g·mol−1|
|Melting point||187 to 189 °C (369 to 372 °F; 460 to 462 K)|
|Boiling point||340 °C (644 °F; 613 K)|
|1 g/170 mL (25 °C)
1 g/90 mL (90 °C)
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
4-Aminobenzoic acid (also known as para-aminobenzoic acid or PABA because the number 4 carbon in the benzene ring is also known as the para position) is an organic compound with the formula H2NC6H4CO2H. PABA, a white-grey crystalline substance, is only slightly soluble in water. It consists of a benzene ring substituted with an amino group and a carboxyl group.
Production and occurrence
In industry, PABA is prepared mainly by two routes: reduction of 4-nitrobenzoic acid and the Hoffman degradation of the monoamide derived from terephthalic acid. (Toluene + HNO3/H2SO4) = nitro-toluene, then oxidation of the nitrotoluene (with sodium dichromate, sulfuric acid, and heat), and reduction of the nitro group.
Food sources of PABA include liver, brewer's yeast (and unfiltered beer), kidney, molasses, mushrooms, and whole grains.
PABA is an intermediate in the synthesis of folate by bacteria, plants, and fungi. Many bacteria, including those found in the human intestinal tract such as E. coli, generate PABA from chorismate by the combined action of the enzymes 4-amino-4-deoxychorismate synthase and 4-amino-4-deoxychorismate lyase. Plants produce PABA in their chloroplasts, and store it as a glucose ester (pABA-Glc) in their tissues. Humans lack the enzymes to convert PABA to folate, so require folate from dietary sources such as green leafy vegetables. Although some intestinal bacteria can synthesize folate from PABA and some E. coli can synthesize folate, this requires six enzymatic activities in folate synthesis which are not all done in the same bacteria. In humans, PABA is considered nonessential and, although it has been referred to historically as "vitamin Bx", is no longer recognized as a vitamin.
Sulfonamide drugs are structurally similar to PABA, and their antibacterial activity is due to their ability to interfere with the conversion of PABA to folate by the enzyme dihydropteroate synthetase. Thus, bacterial growth is limited through folate deficiency.
The potassium salt is used as a drug against fibrotic skin disorders, such as Peyronie's disease, under the trade name Potaba. PABA is also occasionally used in pill form by sufferers of irritable bowel syndrome to treat its associated gastrointestinal symptoms, and in nutritional epidemiological studies to assess the completeness of 24-hour urine collection for the determination of urinary sodium, potassium, or nitrogen levels.
Despite the lack of any recognized syndromes of PABA deficiency in humans, many claims of benefit are made by commercial suppliers of PABA as a nutritional supplement. Benefit is claimed for fatigue, irritability, depression, weeping eczema (moist eczema), scleroderma (premature hardening of skin), patchy pigment loss in skin (vitiligo), and premature grey hair.
Commercial and industrial use
In the past, PABA was widely used in sunscreens as a UV filter. It is a UVB absorber, meaning it can absorb wavelengths between 290 and 320 nm.  Patented in 1943, PABA was one of the first active ingredients to be used in sunscreen. The first in vivo studies on mice showed that PABA reduced UV damage. In addition, it was shown to protect against skin tumors in rodents. Animal and in vitro studies in the early 1980s suggested PABA might increase the risk of cellular UV damage. On the basis of these studies, as well as problems with allergies and clothing discoloration, PABA fell out of favor as a sunscreen. However, water-insoluble PABA derivatives such as padimate O are currently used in some products.
PABA is largely nontoxic; the median lethal dose of PABA in dogs (oral) is 2 g/kg. Allergic reactions to PABA can occur. It is formed in the metabolism of certain ester local anesthetics, and many allergic reactions to local anesthetics are the result of reactions to PABA.
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- Folate Synthesis (Abstract)
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- In vivo Folate Production
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- Toxicity, Local Anesthetics: eMedicine Emergency Medicine