Methionine synthetase in mammalian brain: Function, development and distribution

Abstract

Recycling of methionine via homocysteine occurs in vivo in rat and rabbit brain. Methyltetrahydrofolate - homocysteiine methyltransferase (methionine synthetase, EC 2.1.1.13) is the only enzyme in the brain capable of remethylating homocysteine to methionine. The enzyme is uniformly distributed in various regions of the rabbit brain (except for the choroid plexus). The specific activity of the enzyme in glial cells is twice that of neurons. The specific activity and concentration of the enzyme in rabbit brain is highest at one day of age and then declines until it stabilizes in the adult at 50 percent of the original activity. Moderate folate deficiency did not affect the activity of the enzyme but brief treatment with nitrous oxide resulted in a 50 percent reduction in the enzyme level. We conclude that methionine recycling in mammalian brain occurs throughout the brain during the entire life span. Methionine and its active form S-adenosylmethionine (SAM) is one of Nature's most important molecules, serving as a methyl group donor and a precursor in the biosynthesis of polyamines (1,2). In the past, several authors (2,3,4) have suggested that, in brain, the biosynthesis of methionine occurs in both rabbit and rat brains in vivo via methionine synthetase. We will discuss the function, development and distribution of this enzyme in brain.