Transmethylation and Transsulfuration Enzymes in Rat Brains: Their Subcellular Distribution

Abstract

Our laboratory has been involved in a continuing investigation of the sulfur-containing amino acids, particularly with regard to their function and metabolism in brain (Figure 1). These amino acids are associated with inborn errors of metabolism (such as homocystinuria) that may be associated with brain dysfunction. Methionine and its metabolites are involved not only with protein production, but also with the synthesis of the methyl donors, S-adenosylmethionine, and 5-methyltetrahydrofolic acid (Banerjee and Snyder, 1973; Laduron, 1973; Laduron et al., 1974). In addition, cystathionine, cysteine, cysteine sulflnic acid, cysteic acid, hypotaurine, and taurine have all been shown to have either biochemical, electrophysiological, or behavioral effects that suggest that they may function as neurotransmitters or modulators at the synapse (Curtis and Watkins, 1960, 1965; Werman et al., 1966; Key and White, 1970; Olney et al., 1971; Davison and Kaczmarek, 1971). It should be emphasized that the evidence that these compounds are neurotransmitters is still controversial and is much less convincing than the evidence that glycine (Aprison et al., 1968; Werman, 1972) and γ-aminobutyric acid (GABA) (Curtis, 1973) are neurotransmitters. Glycine, synthesized from serine by the enzyme serine hydroxymethyltransferase, is associated with the folate cycle, which is an integral part of the regeneration of homocysteine to methionine.