Alterations in hepatic metabolism of sulfur amino acids in non-obese type-2 diabetic Goto-Kakizaki rats

Elevated plasma homocysteine has been identified as a risk factor for cardiovascular disease and non-alcoholic liver disease, which are major complications of diabetes. Hence, hepatic homocysteine metabolism has become a major focus of diabetes research. However, little information is available regarding plasma homocysteine levels in non-obese diabetic animals. Therefore, we investigated the hepatic metabolism of sulfur-amino acids in non-obese type-2 diabetic Goto-Kakizaki rats. The experiments were performed using 9-week-old Goto-Kakizaki rats and age-matched Wistar rats. The major finding of this study is that homocysteine levels in the liver and plasma are maintained by a balance between the up-regulation of betaine homocysteine methyltransferase and the inhibition of cystathionine β-synthase in non-obese type-2 diabetic rats. Hepatic levels of cysteine and its metabolites, such as hypotaurine, taurine, and glutathione, were increased despite inhibition of the transsulfuration of homocysteine to cysteine. The elevated hepatic taurine and glutathione levels may be attributed to the up-regulation of cysteine dioxygenase expression and increased cysteine availability for glutathione synthesis. Inhibition of hepatic methionine adenosyltransferase activity in Goto-Kakizaki rats was associated with a decrease in hepatic S-adenosylmethionine, which serves as an allosteric activator of cystathionine β-synthase. The non-obese type-2 diabetic condition results in profound changes in hepatic sulfur-amino acid metabolism and raises the possibility that sulfur-amino acid metabolism may be regulated by obesity- as well as diabetes-associated factors. Further study to elucidate the pathological significance of sulfur-amino acid metabolism in chronic liver disease in type-2 diabetic animals is underway in this laboratory.