Regulation of nitric oxide synthase activity by tetrahydrobiopterin in human placentae from normal and pre-eclamptic pregnancies.

The possible regulatory role of tetrahydrobiopterin (BH(4)) in Type III nitric oxide synthase (NOS III) activity of human placentae from first trimester, term and pre-eclamptic pregnancies was investigated. In homogenates of first-trimester or term placentae, BH(4)stimulated NOS III activity up to 2.5-fold in a concentration dependent manner from 20 n m to 1 microm BH(4), and half-maximal stimulation (EC(50)) was observed at 100-110 n m. No significant further stimulation was detectable over an extended concentration range from 1 microm to 50 microm BH(4). NOS III present in microsomal and gel-filtered cytosol fractions exhibited similar BH(4)-activation patterns, with an identical EC(50)value of 50 n m. Remarkably, tissue concentrations of BH(4)showed a marked decrease in term placentae (57+/-23 n m, mean+/-s.d., n=26) relative to first-trimester placentae (189+/-79 n m, mean+/-s.d., n=17), suggesting that alterations in cellular BH(4)concentrations may play a more significant role in the regulation of NOS III activity in late pregnancy. Placental homogenates from 10 pre-eclamptic pregnancies exhibited two distinct types of response to BH(4). In seven placental homogenates, addition of physiological concentrations of BH(4)(20 n m to 1 microm) elicited no increase whatsoever in basal NOS III activity, and only high BH(4)concentrations (50 microm) caused notable stimulation (BH(4)resistant group). In contrast, in three of 10 placental homogenates both physiological and 50 microm BH(4)concentrations stimulated NOS III to levels similar to that of normal placentae (BH(4)responsive group). There were no appreciable differences in the clinical presentation of pre-eclampsia between the two groups. Importantly, BH(4)concentrations in pre-eclamptic placentae were comparable with those of normal, control placentae. Taken together, the observations suggest that BH(4)controls NOS III activity in the human placenta, and a defect in BH(4)regulation of NOS III may contribute to the development of pre-eclampsia. A model implicating the malfunction of placental NOS III rather than its actual tissue level in the pathogenesis of pre-eclampsia is discussed.