Regulation of homocysteine metabolism.

摘要

We sought to investigate the role of the liver in homocysteine metabolism using various in vivo and in vitro techniques. To this end we posed several specific questions: (1) What are the effects of dietary modification (protein and amino acids) on homocysteine metabolism in the rat? (2) How is plasma homocysteine regulated by methylation demand imposed by various substrates? (3) Does the hormone glucagon have a role in the regulation of plasma homocysteine?; In Chapter 3 we describe the development of a primary rat hepatocyte model whereby factors that acutely affect homocysteine metabolism and secretion could be examined. We found that the dependence of homocysteine secretion from incubated hepatocytes fitted well to a rectangular hyperbola, with half maximal homocysteine secretion achieved at methionine concentrations of approximately 0.44 mM.; In our second series of experiments, (Chapter 4) we sought to investigate the effects of methylation demand imposed by physiological substrates on homocysteine metabolism. We found that rats fed a guanidinoacetate-(GAA) supplemented diet exhibited a ∼50% increased plasma homocysteine level, whereas rats fed a creative-supplemented diet had a ∼25% lower plasma homocysteine level. In our second methylation demand study, we investigated the roles of phosphatidylethanolamine N-methyltransferase (PEMT) and CTP:phosphocholine cytidylyltransferase (CT) in homocysteine metabolism using Pemt -/- and CT-alpha-/- "knockout" mouse models. We observed that mice lacking PEMT have plasma homocysteine levels that are ∼50% of those in wild type mice.; Given the broad regulatory effects of glucagon on amino acid metabolism, we investigated the effects of the hormone glucagon on homocysteine metabolism in our final series of experiments (Chapter 5). Glucagon treatment resulted in a 30% decrease in total plasma homocysteine and increased hepatic activities of glycine N-methyltransferase, cystathionine beta-synthase and cystathionine gamma-lyase. Hepatic concentrations of S-adenosylmethionine and S-adenosylhomocysteine, allosteric activators of cystathionine beta-synthase, were also increased following glucagon treatment. These results indicate that glucagon can regulate plasma homocysteine through its effects on the hepatic transsulfuration pathway. (Abstract shortened by UMI.)