4-Aminobenzoic acid

4-Aminobenzoic acid
IUPAC name 4-Aminobenzoic acid
Other names para-Aminobenzoic acid; p-Aminobenzoic acid; PABA; Vitamin Bx; Bacterial vitamin H1
Identifiers
CAS number	150-13-0 Y
PubChem	978
ChemSpider	953 Y
UNII	TL2TJE8QTX Y
DrugBank	DB02362
KEGG	D02456 Y
ChEBI	CHEBI:30753 Y
ChEMBL	CHEMBL542 Y
Jmol-3D images	Image 1
SMILES O=C(O)c1ccc(N)cc1
InChI InChI=1S/C7H7NO2/c8-6-3-1-5(2-4-6)7(9)10/h1-4H,8H2,(H,9,10) Y Key: ALYNCZNDIQEVRV-UHFFFAOYSA-N Y InChI=1/C7H7NO2/c8-6-3-1-5(2-4-6)7(9)10/h1-4H,8H2,(H,9,10) Key: ALYNCZNDIQEVRV-UHFFFAOYAH
Properties
Molecular formula	C7H7NO2
Molar mass	137.14 g mol−1
Appearance	White-grey crystals
Density	1.374 g/mL
Melting point	187–189 °C
Boiling point	340°C
Solubility in water	1 g/170 mL (25 °C) 1 g/90 mL (90 °C)
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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 is a white-grey crystalline substance that 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.^[1]

Food sources of PABA include liver, brewer's yeast (and unfiltered beer), kidney, molasses, mushrooms, and whole grains.^[2]

Biology

Biochemistry

Tetrahydrofolate synthesis pathway

PABA is an intermediate in the bacterial synthesis of folate. PABA has been referred to as Vitamin B_x.^[3] Some bacteria in the human intestinal tract such as E. coli generate PABA from chorismate.^[4] Humans lack the enzymes to convert PABA to folate, and therefore require folate from dietary sources such as green leafy vegetables. Although some intestinal bacteria can synthesize folate from PABA^[5] and some E. coli can synthesize folate,^[6] this requires six enzymatic activities in folate synthesis which are not all done in the same bacteria. In humans PABA is considered nonessential and is not recognized as a vitamin.^[3]

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 without effect on human cells.^{[citation needed]}

Medical use

The potassium salt is used as a drug against fibrotic skin disorders, such as Peyronie's disease, under the trade name Potaba.^[7] 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.

Nutritional supplement

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.^[8] It is also sometimes claimed that oral supplements of PABA can make the skin less sensitive to sun damage.^{[citation needed]}

Commercial and industrial use

PABA finds use mainly in the biomedical sector. Other uses include its conversion to specialty azo dyes and crosslinking agents.

In the past, PABA was widely used in sunscreens as a UV filter. It is a UVB absorber, meaning that it can absorb wavelengths between 290 and 320 nm. ^[9] Patented in 1943, PABA was one of the first active ingredients to be used in sunscreen.^[10] The first in vivo studies on mice showed that PABA reduced UV damage. In addition, PABA was shown to protect against skin tumors in rodents.^[11] Animal and in vitro studies in the early 1980s suggested PABA might increase the risk of cellular UV damage.^[12] 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.

Safety considerations

PABA is largely nontoxic; the median lethal dose of PABA in dogs (oral) is 2 g/kg.^[1] Allergic reactions to PABA can occur. PABA is formed in the metabolism of certain ester local anesthetics, and many allergic reactions to local anesthetics are the result of reactions to PABA.^[13]

References

External links

• PABA Cancer link