Molecular Human Reproduction, Vol 3, 293-298, Copyright © 1997 by Oxford University Press
M Sahin-Toth, Z Kukor and M Toth
Type III nitric oxide synthase (NOS III) is responsible for > 90% of
nitric oxide (NO) synthesizing activity in first trimester placentae.
Enzyme activity is distributed between cytosolic (30%) and membrane- bound
forms (70%), with highest specific activity observed in microsomal
fractions. In the present study, the effect of tetrahydrobiopterin (BH4) on
subunit structure and activity of microsomal and cytosolic NOS III was
compared. As revealed by immunoblot analysis, incubation of microsomal
membranes with 50 microM final concentration BH4 for 10 min at 37 degrees C
resulted in a striking conversion of monomeric NOS III into a protein
having the characteristics (electrophoretic mobility, resistance to sodium
dodecyl sulphate) of the homodimeric form. In contrast, BH4 induced
significantly less marked changes in the NOS III dimer content of cytosolic
fractions. Enzyme activity in microsomes is stimulated approximately 6-fold
upon addition of 50 microM BH4, while only a 2- fold activation is
detectable in cytosolic fractions. Taken together, the observations suggest
that BH4 activates NOS III in the primordial human placenta by promoting
its subunit assembly in the membrane, while cytosolic NOS III is relatively
insensitive to BH4. Compartment- specific action of BH4 represents a novel
mechanism which is implicated in the regulation of placental NOS activity.
JOURNAL ARTICLE
Tetrahydrobiopterin preferentially stimulates activity and promotes subunit aggregation of membrane-bound calcium-dependent nitric oxide synthase in human placenta
Department of Physiology, Semmelweis University of Medicine, Budapest, Hungary.
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