The Role of Methionine in the Intracellular Accumulation and Function of Folates

It is suggested that mammalian cells have evolved to respond to methionine deficiency since in such circumstances vital methylation reactions are put at risk, due to decreased levels of S-adenosyl-methionine. Enzymatic changes occurring as a result of decreased methionine, S-adenosylmethionine and S-adenosylhomocysteine, optimize the remethylation of homocysteine to methionine by decreasing homocysteine catabolism and channelling cellular folates into 5-methyltetrahydropteroylglutamate (5-CH3-H4PteGlu). The latter, in addition to optimising the remethylation cycle, directs the folate cofactors away from purine and pyrimidine biosynthesis and decreases the rate of proliferation of rapidly dividing cells thus reducing competition for methionine incorporation into proteins. Decreased cellular homocysteine, as a result of decreased methionine, would also restrict cell division by decreased conversion of plasma 5-CH3-H4PteGlu into intracellular polyglutamates.