Vitamin B9

Vitamin B9 (folate or folic acid) is a water soluble vitamin that is vital for the synthesis of several amino acids, the purines (adenosineadenosine and guanine) and the pyrimidine thymine (three of the four nucleotide bases). Folate is vital and hence critical for the synthesis of nucleic acids.)

The antimicrobial group sulphonamides inhibit dihydropteroate synthetase, the enzyme which forms folic acid from PABA. This reaction that does not occur in mammalian cells hence does not affect human DNA synthesis and is specific for microbes.

The chemotherapeutic methotrexate inhibits dihydrofolate reductase which converts folate into its active form tetrohydrofolaic acid (THF).

Interestingly, it was formerly known as vitamin M.

Related pathology

Folate deficiency is the most common vitamin deficiency in the western world. It is especially important in pregnant women and common amongst alcoholics. 

Deficiency leads to megaloblastic anaemia due to the reduced synthesis of DNA in the highly THF-dependent stem cells of erythropoiesis.

Defeicency arises in two distinct populations:

1. increased demand (pregnancy and lactation)
2. decreased absorption (alcoholism and chronic small bowel pathology)

Earlier in embryogenesis, formation of the neural tube is dependent on adequate levels of folate. Deficiency in this critical time is a well known cause of neural tube defects and as such there may be a need for pregnant women (and women planning to fall pregnant) to take folate supplementation.