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 nitri c oxide synthase (NOS III) activity of human placentae from first trimester , term and pre-eclamptic pregnancies was investigated. In homogenates of fi rst-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 furt her stimulation was detectable over an extended concentration range from 1 muM to 50 muM BH4. NOS III present in microsomal and gel-filtered cytosol f ractions exhibited similar BH4-activation patterns, with an identical EC50 value of 50 nM. Remarkably, tissue concentrations of BH, showed a marked de crease in term placentae (57 +/- 23 nM, mean +/- s.d., n = 26) relative to first-trimester placentae (189 +/- 79 nM, mean +/- s.d., n = 17), suggestin g that alterations in cellular BH4 concentrations may play a more significa nt 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 physiologic al concentrations of BH, (20 nM to 1 muM) elicited no increase whatsoever i n basal NOS III activity, and only high BH4 concentrations (50 muM) caused notable stimulation (BH4 resistant group). In contrast, in three of 10 plac ental homogenates both physiological and 50 muM BH4 concentrations stimulat ed NOS III to levels similar to that of normal placentae (BH4 responsive gr oup). 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 place ntae. Taken together, the observations suggest that BH4 controls NOS III ac tivity 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 ma lfunction of placental NOS III rather than its actual tissue level in the p athogenesis of pre-eclampsia is discussed. (C) 2000 Harcourt Publishers Ltd .