The effect of glucose and insulin on the activity of enzymes in homocysteine metabolism

Purpose: Hyperhomocysteinemia (HH(e)) is established as a risk factor for cardiovascular disease and occurs with increased frequency in patients with type 2 diabetes. Plasma homocysteine (H(e)) concentrations fall following acute hyperinsulinemia in normal subjects but not in patients with type 2 diabetes. To determine whether hyperinsulinemia and hyperglycemia affect the activity of two important enzymes that metabolize H(e), methylene tetrahydrofolate reductase (MTHFR) (the enzyme in the remethylation of homocysteine to methionine) and cystathionine-β-synthase (CBS)(the enzyme responsible for transsulfuration of homocysteine to cystathionine), we studied the effect of different concentrations of glucose and insulin on the activity of these enzymes in vitro. Methods: The activity of MTHFR and CBS were measured in HEP G2 cells in culture with three concentrations each of glucose (100, 200 and 300 mg/dl) and insulin (5, 50 and 200 μU/ml) in the culture medium, over four days. Results: The results of this study are summarized in the table. Glucose 100 Glucose 200 Glucose 300 Insulin 5 Insilin 50 Insulin 200 mg/dl mg/dl mg/dl μU/ml μU/ml μU/ml MTHFR 3.3 ±0.8 2.8 ±0.5 2.3±0.3# 3.4±0.27 3.0±0.15## 2.8±0.29** nmol/hr CBS 0.017±0.003 0.032±0.003* 0.03±0.004* 0.04±0.003 0.03±0.003** 0.02±0.004** U/mg protein *p<0.01 vs Glucose 100 mg/dl; ** p<0.01 vs Insulin 5μU/ml; #p<0.05 vs Glucose 100 mg/dl; ## p<0.02 vs Insulin 5μU/ml Conclusions: We conclude that both insulin and glucose affect the activity of the two key enzymes of homocysteine metabolism in vitro. Since patients with type 2 diabetes have both hyperglycemia and hyperinsulinemia, the resultant decrease in MTHFR activity will lead to fasting HH(e). Hyperinsulinemia may also lead to post-meal HH(e) due to a decrease in the activity of CBS. It is possible that these changes may contribute to the accelerated macrovascular disease that is common in patients with type 2 diabetes.