External Hemin as an Inhibitor of Mitochondrial Large-Conductance Calcium-Activated Potassium Channel Activity.

The mitochondrial large-conductance calcium-activated potassium channel (mitoBK) is located in the inner mitochondrial membrane and seems to play a crucial role in cytoprotection. The mitoBK channel is regulated by many modulators, including activators, such as calcium ions and inhibitors, such as heme and its oxidized form hemin. Heme/hemin binds to the heme-binding motif (CXXCH) located between two RCK domains present in the mitochondrial matrix. In the present study, we used the patch-clamp technique in the outside-out configuration to record the activity of mitoBK channels. This allowed for the application of channel modulators to the intermembrane-space side of the mitoBK. We found that hemin applied in this configuration inhibits the activity of mitoBK. In addition, we proved that the observed hemin effect is specific and it is not due to its interaction with the inner mitochondrial membrane. Our data suggest the existence of a new potential heme/hemin binding site in the structure of the mitoBK channel located on the mitochondrial intermembrane space side, which could constitute a new way for the regulation of mitoBK channel activity.