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

The possible regulatory role of tetrahydrobiopterin (BH,) 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, BH4 stimulated NOS III activity up to 2.5-fold in a concentration dependent manner from 20 nna to 1 muM BH4, and half-maximal stimulation (EC50) was observed at 100-110 nM. No significant further stimulation was detectable over an extended concentration range from 1 muM to 50 muM BH4. NOS III present in microsomal and gel-filtered cytosol fractions exhibited similar BH4-activation patterns, with an identical EC50 value of 50 nM. Remarkably, tissue concentrations of BH, showed a marked decrease in term placentae (57 +/- 23 nM, mean +/- s.d., n = 26) relative to first-trimester placentae (189 +/- 79 nM, mean +/- s.d., n = 17), suggesting that alterations in cellular BH4 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 BH4. In seven placental homogenates, addition of physiological concentrations of BH, (20 nM to 1 muM) elicited no increase whatsoever in basal NOS III activity, and only high BH4 concentrations (50 muM) caused notable stimulation (BH4 resistant group). In contrast, in three of 10 placental homogenates both physiological and 50 muM BH4 concentrations stimulated NOS III to levels similar to that of normal placentae (BH4 responsive group). There were no appreciable differences in the clinical presentation of pre-eclampsia between the two groups. Importantly, BH4, concentrations in pre-eclamptic placentae were comparable with those of normal, control placentae. Taken together, the observations suggest that BH4 controls NOS III activity in the human placenta, and a defect in BH4 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. (C) 2000 Harcourt Publishers Ltd.