Expression of the heme oxygenase-carbon monoxide signalling system in human placenta.

Heme oxygenase (HO) catalyses the formation of endogenous carbon monoxide and bilirubin from heme. CO, a potent vasodilator, and bilirubin, an antioxidant may have local actions in the fetal-placental vasculature of the placenta. We sought evidence of expression of the two known isoforms of HO in normal human term placenta using reverse transcription polymerase chain reaction (RT-PCR), western blotting and immunohistochemistry. RT-PCR demonstrated the presence of messenger ribonucleic acid for HO-1 and HO-2. Immunoreactive proteins of appropriate size for each HO enzyme were identified in placental cell membrane preparations. Immunohistochemistry showed a wide distribution of HO immunoreactivity, including the syncytiotrophoblast layer of placental villi, the endothelium and smooth muscle cells of umbilical-placental blood vessels, and in all layers of the fetal membranes. These data demonstrate the expression of the two known isoforms of HO in human placenta and suggest that endogenous CO and bilirubin may have important roles in the control of placental vascular function.