Pathways and regulation of homocysteine metabolism in mammals

Two intersecting pathways, the methionine cycle and the transsulfuration sequence, compose the mechanisms for homocysteine metabolism in mammals. The methionine cycle occurs in all tissues and provides for the remethylation of homocysteine, which conserves methionine. In addition, the cycle is essential for the recycling of methyltetrahydrofolate. The synthesis of cystathionine is the first reaction in the irreversible pathway for the catabolism of homocysteine by means of the sequential conversion to cysteine and sulfate. This pathway has a limited distribution and is found primarily in the liver, kidney, small intestine and pancreas. Regulation of homocysteine metabolism is achieved by changes in the quantity of homocysteine distributed between the two competing pathways. Two mechanisms are basic to the regulatory process. Changes in tissue content of the relevant enzymes are the response to sustained perturbations. The inherent kinetic properties of the enzymes provide an immediate response to alterations in the tissue concentrations of substrates and other metabolic effecters. S-adenosylmethionine, S-adenosylhomocysteine, and methyltetrahydrofolate are of particular importance in that context.