Hemin-mediated oxidative inactivation of 3-hydroxy-3-methylglutaryl CoA reductase

Summary

Addition of hemoglobin, methemoglobin, hemin or hematin in the assay mixture of rat liver 3-hydroxy-3-methylglutaryl CoA (HMGCoA) reductase inhibited the activity of the enzyme. The inhibition by hemin was rapid, without any apparent dependence on time of preincubation. At 20 μM hemin, a maximum of about 50% inhibition was obtained in the case of the microsomal enzyme while the solubilized enzyme showed almost 80%6 inhibition. Dithiothreitol at high concentrations or either of the two substrates of the enzyme (HMGCoA and NADPH) could afford partial protection when added before hemin. The K_m for both the substrates increased in the presence of hemin. The inhibition by hemin appeared to be irreversible, the presence of KCN or NaN₃ being the only means of preventing the inhibition. Molecular oxygen was required for the inhibition. Oxygen radicals and H₂O₂, however, did not seem to be involved. This offered a clue that an oxidation reaction of the reductase protein may be the likely mechanism of its inactivation. The enzyme protein did not, however, get degraded under the conditions of inhibition.