Insulin-degrading enzyme (IDE) in the blood may play a role in insulin clearance, thus decreased IDE activity could contribute to hyperinsulinemia and possibly type 2 diabetes mellitus (T2DM). We hypothesized that decreased IDE in plasma may be associated with obesity and/or T2DM. We recruited non-obese (BMI<30, no significant disease), obese (BMI>30) and diabetic (T2DM; ICD-9 code) patients and obtained fasting blood samples. Microvesicular (containing exosomes) and soluble fractions were isolated from plasma by ultracentrifugation Insulin degrading activity was assayed by trichloroacetic acid precipitation of 125I-iodoinsulin (TCA assay), while IDE protein was detected by Western blotting. Differences were analyzed by ANOVA with a Bonferroni posttest. There was no IDE present in the soluble fraction as confirmed by both the TCA assay and Western blot. IDE activity was present in the microvesicular fraction, and the Western blot intensity correlated significantly with activity (p=.01). However, there were no significant differences in IDE activity or protein levels among the 3 groups. We then conducted a post hoc analysis byseparating the non-obese and obese patients into two groups: a healthy group (HbA1c<6) and a pre-diabetic group (HbA1c of 6.0–6.4). We also separated the diabetic patients into two groups: a diabetic group and an insulin-treated group. Although there was no statistical difference in IDE activity among the healthy group, pre-diabetic and diabetic groups, the latter two groups showed a trend toward decreased IDE activity. Interestingly, in patients receiving insulin treatment, the effect of diabetes was reversed, with, increased microvesicular degrading activity compared to the pre-diabetic group (p<0.05) and the diabetic group (p<0.05). The increased IDE activity in the insulin-treated diabetics roughly correlated with the patient's insulin dose, but did not reach statistical significance (r2=.38; p=0.14). We saw no statistically significant correlations of degrading activity with a number of clinical parameters including: fasting glucose; triglycerides, LDL, HDL, age, eGFR, and HbA1c by linear regression. This shows that the microvesicular IDE is not affected by glucose or lipid control. We conclude: A) IDE is present in the blood, but does not significantly contribute to insulin clearance because the microvesicular fraction showed no insulin clearance unless they were first frozen and thawed. This freezing and thawing process most likely allowed the microvesicular membranes to rupture releasing the enzyme. B) enzymatically active IDE is associated with a fraction consistent with exosomes and may be decreased in pre-diabetes and diabetes; and C) insulin treatment increases microvesicular IDE. IDE in the exosomes may serve as a marker for the progression of the pre-diabetic and diabetic disease states independent of glucose control. One could speculate that inflammation and/or insulin resistance result in a decrease of vesicular IDE activity and that insulin treatment reverses this through its anti-inflammatory properties, or by overcoming insulin resistance and increasing insulin signaling.
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