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

The mitochondrial large-conductance calcium-activated potassium channel (mitoBK Ca ) is located in the inner mitochondrial membrane and seems to play a crucial role in cytoprotection. The mitoBK Ca 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 Ca channels. This allowed for the application of channel modulators to the intermembrane-space side of the mitoBK Ca . We found that hemin applied in this configuration inhibits the activity of mitoBK Ca . 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 Ca channel located on the mitochondrial intermembrane space side, which could constitute a new way for the regulation of mitoBK Ca channel activity.