Heme oxygenase activity in term human placenta.

Carbon monoxide (CO) is a novel gaseous chemical messenger, formed during heme oxygenase (HO)-catalysed oxidation of heme. CO is proposed to play a key role(s) in cell function in many organ systems, including vasodilator action in the cardiovascular system. Recently, it has been demonstrated that there is expression of HO protein in the human placenta and this appears to have a regulatory role in placental perfusion. The objective of the present study was to determine HO enzymatic activity in vitro in five different regions of term human placenta. HO activity was determined in the microsomal fraction of tissue homogenate by measuring the rate of formation of CO from heme, using a gas-chromatographic method. HO activity, expressed as nmol CO formed/g tissue wet weight/h, was higher (P< 0.05) in the chorionic plate, chorionic villi, basal plate and chorio-decidua compared with the amnion. The finding that HO enzymatic activity is present in different regions of term human placenta supports the concept that the heme-CO (HO) pathway plays a complementary role with the L -arginine-nitric oxide (nitric oxide synthase) pathway in the regulation of placental haemodynamics.