NITRIC-OXIDE SYNTHASE IN FIRST-TRIMESTER HUMAN PLACENTA - CHARACTERIZATION AND SUBCELLULAR-DISTRIBUTION

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 homogenate and subcel lular fractions by measuring the conversion of radiolabeled L-arginine to L-citrulline in the presence or absence of EGTA, NAME (a NOS inhib itor), and NADPH. Main Outcome Measures: Analysis of NOS activity in f irst-trimester human placental tissue, in terms of citrulline producti on that is generated in 1:1 ratio with nitric oxide, a potent vasodila tor. Results: Both Ca2+-dependent and -independent NOS activities were detected in whole homogenate, as well as in subcellular fractions. Sp ecific activity of NOS was highest in the microsomal fraction, where t he enzyme exhibited an absolute requirement for NADPH. Unexpectedly, b oth Ca2+-activated and Ca2+-independent activities were insensitive to the calmodulin antagonist chlorpromazine, while aminoguanidine, a sel ective inhibitor of the inducible NOS, effectively suppressed Ca2+-dep endent generation of citrulline. Conclusion: Some of the properties of the NO-generating enzyme in the primordial placenta. described here f or the first time, differ in several respects from those reported for the term placenta. A coordinated local interaction of NO with the immu ne system of the developing placenta and the prostaglandin-generating mechanism is suggested, ensuring proper implantation, which may be com promised in hypertensive pregnancies associated with impaired NO synth esis.