The effect of glucose and insulin on the activity of methylene tetrahydrofolate reductase and cystathionine-β-synthase: studies in hepatocytes

Hyperhomocysteinemia is a well established risk factor for cardiovascular disease, and multiple factors likely lead to abnormal regulation of plasma homocysteine in patients with diabetes. To examine a possible role for insulin and glucose in homocysteine metabolism, we examined the activity of two important enzymes of homocysteine metabolism in hepatocytes. In various tissues of six mice, methylene tetrahydrofolate reductase (MTHFR) activity was present in all tissues tested and the highest concentration (per gram) was in the brain. In contrast, cystathionine β-synthase (CBS) activity appeared to be present only in the liver and to a small extent in the kidney. Using HEP G2 cells in culture, MTHFR activity was 3.3±0.8 nmol/h when the glucose concentration in the medium was 100 mg/dl and fell to 2.3±0.3 nmol/h when glucose was increased to 300 mg/dl. MTHFR activity was 3.4±0.3 nmol/h when cells were exposed to an insulin concentration of 5 mU/ml and fell to 2.8±0.3 nmol/h when insulin concentration was increased to 200 mU/ml (P<0.01). In contrast CBS activity increased from 0.017 to 0.13 U/ml by increasing the glucose concentration in the medium (P<0.01), but decreased from 0.04 to 0.02 (P<0.01) when the insulin concentration was increased from 5 to 200 mU/ml, respectively. We conclude that CBS and MTHFR have different tissue distributions, with CBS being present predominantly in liver and kidney, and MTHFR found in many tissues. In addition, both insulin and glucose affect the activity of the two enzymes when added to hepatocytes in vitro. If such effects occur in humans with hyperglycemia and hyperinsulinemia, then alterations in homocysteine metabolism may contribute to the accelerated macrovascular disease associated with insulin resistance or type 2 diabetes.