Catabolism and Excretion of Folate

There is very little urinary loss of folate, some 5 to 10 nmol of microbiologically active material per day. Not only is most folate in plasma bound to proteins (either folate binding protein for unsubstituted folate or albumin for methyl-tetrahydrofolate), and thus protected from glomerular filtration, but also the renal brush border has a high concentration of folate binding protein that acts to reabsorb any that is filtered.

Folate polyglutamate in cells that is not enzyme bound undergoes hydrolysis of the y-glutamyl side chain, catalyzed by lysosomal conjugase, yielding folate monoglutamate, which thenleaves the cell freely. Both carboxypeptidase G and ferritin catalyze hydrolysis of the C-9 to N-10 bond of folate monoglutamate to yield p-aminobenzoylglutamate (much of which is acetylated before excretion) and pterin, which is excreted either unchanged or as isoxan-thopterin and other biologically inactive compounds. As a result of increased synthesis of ferritin in pregnancy, the catabolism of folate and excretion of p-aminobenzoylglutamate increase significantly, suggesting that the folate requirement in pregnancy may be considerably higher that would be expected on the basis of fetal requirements (Suh et al., 2001).

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