Endothelium-derived relaxing factor is a nitrosyl iron complex with thiol ligands.

A hypothesis is put forward on the nature of the endothelium-derived relaxing factor (EDRF) which is released from vascular endothelial cells by acetylcholine, bradykinin and other agonists. It is suggested that EDRF is a nitrosyl iron complex with low-molecular thiol ligands, most probably with cysteine. Its active principle is nitric oxide (NO). This free radical is stabilized by inclusion into the iron complex, which promotes NO transfer within the cell and between cells. Subsequent release of NO from these complexes results from thiol group oxidation.