Increase in S-Adenosylhomocysteine Content and Its Effect on the S-Adenosylhomocysteine Hydrolase Activity under Transient High Plasma Homocysteine Levels in Rats

Abstract

SummaryThe objective of this study was to examine how transient high plasma homocysteine (Hcy) levels affect the metabolism of Hcy, the activity and expression of S -adenosylhomocysteine (SAH) hydrolase which catalyzes both SAH hydrolysis and SAH synthesis. Wistar ST rats (males) were cannulated in the right jugular vein for intravenous infusion of physiological saline or DL -Hcy solutions (15 and 30 mg/mL) for 1 h at 1.1 mL/h/rat. The content of S -adenosylmethionine (SAM), SAH-synthetic activity of SAH hydrolase and the expression of SAH hydrolase mRNA in liver extracts showed no significant difference in the Hcy infused groups as compared to the Control group. On the other hand, the contents of hepatic SAH in the Hcy infused groups were dose-dependent and significantly higher than that of the Control group. Thus, this study showed that hepatic SAH increased without any increase in the SAH-synthetic activity and the expression of SAH hydrolase mRNA under transient high plasma Hcy levels after intravenous infusion of Hcy. Key Words plasma homocysteine, S -adenosylhomocysteine, S -adenosylhomocysteine hydrolase, methionine metabolism Homocysteine (Hcy) is an intermediate of the methionine metabolic pathway, and it occupies a branch point of two metabolic pathways, the remethylation pathway to methionine and the transsulfuration one to cysteine. These two pathways are coordinated by S -adenosylmethionine (SAM), which acts as an allosteric inhibitor of the methylentetrahydrofolate reductase (EC 1.1.1.68) and as an activator of cystathionine ␤ -synthase (EC 4.2.1.22; CBS) ( 1 ). SAM is converted to S -adenosylhomocysteine (SAH) by some transmethylation reactions, and then SAH is hydrolyzed by SAH hydrolase (EC 3.3.1.1), a reversible enzyme which catalyzes SAHhydrolytic reaction to Hcy and adenosine, and SAHsynthetic reaction from Hcy and adenosine. In general, equilibrium of the SAH hydrolase reaction favors the direction of SAH synthesis, while the SAH-hydrolytic reaction predominates under physiological conditions. Predominant SAH hydrolysis is maintained by rapid removal of Hcy. However, if it is difficult for sufficient Hcy to be removed from this cycle due to CBS deficiency or inhibition of methionine synthase, it means that the methionine cycle can not maintain its balance. The balance of the methionine cycle is maintained intricately by methionine metabolites. Therefore disturbed methionine metabolism induces several diseases, e.g. hyperhomocysteinemia.An increase of plasma Hcy is caused by nutritional deficiencies or genetic mutations. Vitamin B 6 (B 6 ) deficiency ( 2 , 3 ) and folic acid deficiency ( 4 ) were reported as the factors affecting Hcy accumulation. Furthermore, the relationship between elevated plasma Hcy and accumulated SAH was observed in several studies using rats ( 5 , 6 ). Elevated plasma Hcy and accumulated SAH are likely to be caused by disturbed methionine metabolism. We reported in our previous study that the elevation of plasma Hcy and accumulation of hepatic SAH were observed in B 6...

A regulatory circuit involving miR-143 and DNMT3a mediates vascular smooth muscle cell proliferation induced by homocysteine

“…In addition, the VSMCs transfected pre-miR-143 and miR-143 inhibitor further confirmed the suppressive effect of miR-143 on VSMC proliferation, which had the same effect as folate. In the 'methionine cycle', Hcy can be converted to methionine by acquiring a methyl group from N-5-methyltetrahydrofolate by methionine synthase, and be subsequently activated to S-adenosyl methionine (SAM) (31). Folate supply increases the production of N-5-methyltetrahydrofolate, promoting the transformation of Hcy to SAM (32).…”

Sulfur amino acid metabolism in Zucker diabetic fatty rats

“…The SAHH enzyme assay was performed using a modification of a method by Isa et al [17]. The reaction mixtures (final volume 500 mL) consisted of 70 mM Tris buffer containing 35 mM KCl, 35 mM MgCl 2 (pH 8.0), 1.4 mM dithiothreitol, 10 mM homocysteine, 1 mM adenosine, and 0.8 mg protein/mL.…”

Homocysteine as a Risk Factor for Atherosclerosis: Is Its Conversion toS-Adenosyl-L-Homocysteine the Key to Deregulated Lipid Metabolism?

“…An alternative way for Hcy metabolism is the reversal of the reaction catalyzed by S -adenosyl- L -homocysteine hydrolase. This occurs upon accumulation of the hydrolytic products of the reaction, in particular Hcy, and leads to AdoHcy synthesis and accumulation [19, 67–69]. Thus, elevated Hcy levels via accumulation of AdoHcy lead to the disruption of the methylation cycle and, potentially, to methylation deficiency.…”