No abstract

Mild hyperhomocysteinemia is a risk factor for many diseases, including cardiovascular disease. We determined the effects of insulin resistance and of type 2 diabetes on homocysteine (Hcy) metabolism using Zucker diabetic fatty rats (ZDF/Gmi fa/fa and ZDF/Gmi fa/?). Plasma total Hcy was reduced in ZDF fa/fa rats by 24% in the prediabetic insulin-resistant stage, while in the frank diabetic stage there was a 59% reduction. Hepatic activities of several enzymes that play a role in the removal of Hcy: cystathionine ␤-synthase (CBS), cystathionine ␥-lyase, and betaine:Hcy methyltransferase (BHMT) were increased as was methionine adenosyltransferase. CBS and BHMT mRNA levels and the hepatic level of S-adenosylmethionine were also increased in the ZDF fa/fa rats. Studies with primary hepatocytes showed that Hcy export and the transsulfuration flux in cells from ZDF fa/fa rats were particularly sensitive to betaine. Interestingly, liver betaine concentration was found to be significantly lower in the ZDf fa/fa rats at both 5 and 11 weeks. These results emphasize the importance of betaine metabolism in determining plasma Hcy levels in type 2 diabetes. Diabetes 54: 3245-3251, 2005

Elevation of plasma homocysteine levels has been recognized as an independent risk factor for the development of cardiovascular disease, a major complication of diabetes. Plasma homocysteine reflects a balance between its synthesis via S-adenosyl-L-methionine-dependent methylation reactions and its removal through the transmethylation and the transsulfuration pathways. Betaine-homocysteine methyltransferase (BHMT, EC 2.1.1.5) is one of the enzymes involved in the remethylation pathway. BHMT, a major zinc metalloenzyme in the liver, catalyzes the transfer of methyl groups from betaine to homocysteine to form dimethylglycine and methionine. We have previously shown that plasma homocysteine levels and the transsulfuration pathway are affected by diabetes. In the present study, we found increased BHMT activity and mRNA levels in livers from streptozotocin-diabetic rats. In the rat hepatoma cell line (H4IIE cells), glucocorticoids (triamcinolone) increased the level and rate of BHMT mRNA synthesis. In the same cell line, insulin decreased the abundance of BHMT mRNA and the rate of de novo mRNA transcription of the gene. Thus the decreased plasma homocysteine in various models of diabetes could be due to enhanced homocysteine removal brought about by a combination of increased transsulfuration of homocysteine to cysteine and increased remethylation of homocysteine to methionine by BHMT.homocysteine; diabetes; insulin; glucocorticoids; remethylation AN ELEVATION OF PLASMA HOMOCYSTEINE is recognized as an independent risk factor for the development of Alzheimer's disease, premature arteriosclerosis, thrombosis, and connective tissue disorders, including skeletal abnormalities, osteoporosis, and fractures (3,19,20,22,38,41). Cardiovascular disease is a major complication of diabetes, and plasma homocysteine levels are often perturbed in patients with diabetes (13-15). Although diabetic patients with nephropathy tend to have elevated plasma homocysteine levels, those with no kidney dysfunction have decreased levels. (15). Plasma homocysteine reflects a balance between its synthesis via S-adenosyl-Lmethionine (SAM)-dependent methylation reactions and its catabolism through the transmethylation and transsulfuration pathways. Methionine synthase (MS) and betaine-homocysteine methyltransferase (BHMT) are the major enzymes involved in the remethylation pathway. BHMT (EC 2.1.1.5) is a zinc metalloenzyme that catalyzes the transfer of methyl groups from betaine to homocysteine to produce dimethylglycine and methionine (21). Betaine, which arises from the oxidation of choline and is also a minor dietary constituent, serves as a folate-independent source of methyl groups for homocysteine remethylation via BHMT. This enzyme is found mainly in the liver and kidney of mammals. A developmentally regulated BHMT is also expressed in the lens of rhesus monkeys and humans (25). BHMT is fairly abundant in the liver and represents 0.5-1.6% of the total soluble protein in the mammalian liver (8).The importance of the BHMT reaction to homocys...

The removal of the 1-carbon of threonine can occur via threonine dehydrogenase or threonine aldolase, this carbon ending up in glycine to be liberated by the mitochondrial glycine cleavage system and producing CO(2). Alternatively, in the threonine dehydratase pathway, the 1-carbon ends up in alpha-ketobutyrate, which is oxidized in the mitochondria to CO(2). Rat hepatocytes, incubated in Krebs-Henseleit medium, were incubated with 0.5 mM L-[1-(14)C]threonine, and (14)CO(2) production was measured. Added glycine (0.3 mM) marginally suppressed threonine oxidation. Cysteamine (0.5 mM), a potent inhibitor of the glycine cleavage system, reduced threonine oxidation to 65% of controls. However, alpha-cyanocinnamate (0.5 mM), a competitive inhibitor of mitochondrial alpha-keto acid uptake, reduced threonine oxidation to 35% of controls. These data provided strong evidence that approximately 65% of threonine oxidation occurs through the glycine-independent threonine dehydratase pathway. Glucagon (10(-7) M) increased threonine oxidation and stimulated threonine uptake by these cells. In summary, the majority of threonine oxidation occurs through the threonine dehydratase pathway in rat hepatocytes, and threonine oxidation is increased by glucagon, which also increases threonine's transport.

Renal serine production in rats was quantitated by simultaneously measuring renal blood flow and the renal arteriovenous difference for this amino acid. The rate of synthesis was 0.24 +/- 0.02 mumol.min-1.100 g-1 in rats fed a diet containing 12% casein. This rate was not altered by the inclusion of an additional 1% serine in the diet for 7 days or by acute infusion of serine, although both protocols increased blood serine by 50%. When rats were fed a diet in which protein was entirely replaced by crystalline amino acids the rate of renal serine production was also 0.25 +/- 0.05 mumol.min-1.100g-1. Omission of serine or both serine and glycine from this diet did not alter the rate of renal serine synthesis. Renal serine production does not respond to the serine content of the diet.