Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Vitamins
Vitamin-like Compounds
Para-aminobenzoic Acid

The history of the discovery and study of $p$-aminobenzoic acid as an essential growth factor for microorganisms is closely intertwined with The Development of Chemotherapy, particularly the advent of sulfonamide drugs in clinical practice. This growth-stimulating factor was isolated in pure form from Yeast extracts and identified as $p$-aminobenzoic acid with the following Structure:

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As noted earlier, the vitamin properties of $p$-aminobenzoic acid are apparently related to its incorporation into the Folic acid molecule. $p$-Aminobenzoic Acid is a crystalline substance that is sparingly soluble in Water, but readily soluble in alcohol and ether. Chemically stable, it is not destroyed by autoclaving and withstands boiling in both acidic and alkaline environments. In addition to microorganisms (although some of them, such as Mycobacterium tuberculosis, are capable of synthesizing it themselves), animals also require $p$-aminobenzoic acid. It has been proven essential for the normal pigmentation of Hair, fur, feathers, and Skin. Furthermore, this vitamin has been shown to activate tyrosinase, a key enzyme in The Biosynthesis of skin Melanins responsible for normal skin coloration.

Structural analogues of $p$-aminobenzoic acid, particularly sulfonamides with antibacterial properties, are widely used in medicine. It is hypothesized that, due to structural similarity*, sulfonamide drugs can competitively replace $p$-aminobenzoic acid in microbial enzyme systems, thereby halting their GROWTH AND REPRODUCTION. Although the coenzyme Functions of this acid have not been fully elucidated, as a structural component of folic acid Coenzymes, $p$-aminobenzoic acid thereby participates in numerous metabolic processes.

Dietary sources of $p$-aminobenzoic acid for humans include Liver, Kidneys, meat, and yeast; smaller amounts are found in milk, chicken eggs, potatoes, bread, spinach, and carrots.