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Abstract
Objectives: To characterize the activity and subcellular distribution of nitric oxide synthase (NOS) in primordial human placenta.
Methods: Enzyme activity was assayed in chorion frondosum homoge-nate and subcellular fractions by measuring the conversion of radiolabeled L-arginine to L-citrulline in the presence or absence of EGTA, NAME (a NOS inhibitor), and NADPH.
Main Outcome Measures: Analysis of NOS activity in first-trimester human placental tissue, in terms of citrulline production that is generated in 1:1 ratio with nitric oxide, a potent vasodilator.
Results: Both Ca2-dependent and -independent NOS activities were detected in whole homogenate, as well as in subcellular fractions. Specific activity of NOS was highest in the microsomal fraction, where the enzyme exhibited an absolute requirement for NADPH. Unexpectedly, both Ca2+-activated and Ca2+-independent activities were insensitive to the calmodulin antagonist chlorpromazine, while aminoguanidine, a selective inhibitor of the inducible NOS. effectively suppressed Ca+-dependent generation of cit-rulline.
Conclusion: Some of the properties of the NO-generating enzyme in the primordial placenta, described here for the first time, differ in several respects from those reported for the term placenta. A coordinated local interaction of NO with the immune system of the developing placenta and the prostaglandin-generating mechanism is suggested, ensuring proper implantation, which may be compromised in hypertensive pregnancies associated with impaired NO synthesis.