Inhibition of insulin metabolism by hydroxychloroquine and its enantiomersin cytosolic fraction of liver homogenates from healthy and diabetic rats

To elucidate the mechanism by which hydroxychloroquine (HCQ) affects glucos e metabolism, the effect of this drug and its enantiomers on insulin metabo lism was studied using the cytosolic fraction of liver homogenates from hea lthy and diabetic rats. Eadie-Hofstee plots were monophasic suggesting that only a one-component enzyme system is involved in insulin degradation in t he fraction used. Reaction velocity (V) vs substrate concentration plots we re consistent with a V-max model. HCQ caused a significant reduction in V-m ax and V-max/K-m values in both healthy (V-max, 3.63 +/- 0.46 vs 1.97 +/- 0 .13, ng/min/mg; protein P < 0.001; and V-max/K-m, 0.265 +/- 0.015 vs 0.112 +/- 0.004, ml/min/g protein) and diabetic rats (V-max, 0.718 +/- 0.06 vs 0. 360 +/- 0.024, ng/min/mg protein; and V-max/K-m, 0.05 +/- 0.002 vs 0.023 +/ - 0.001, ml/min/g protein). Significant reduction in the V was observed in the presence of racemic (rac)-, R-, or S-HCQ. Ranking of the inhibitory pot ency was HCQ > S = R except at highest examined concentration (20 mg/mL) wh ich was HCQ > S > R. In conclusion, the effect of HCQ on insulin degradatio n appears to be, in part, through inhibition of cytosolic insulin metaboliz ing enzyme. The effect is not stereoselective except at high concentrations . The R- and S-HCQ may have synergistic effects on inhibition of insulin de gradation.