Ca2+ or Mg2+ nucleotide phosphohydrolases in myometrium: two ecto-enzymes

Area of Science:

• Biochemistry
• Cell Biology
• Pharmacology

Background:

• Myometrial cells exhibit significant ecto-enzyme activity.
• This activity is associated with Ca2+ or Mg2+ nucleotide phosphohydrolase.
• The enzymes are located on the cell surface and in plasma membranes.

Purpose of the Study:

• To characterize the Ca2+ or Mg2+ nucleotide phosphohydrolase activity on myometrial cells.
• To differentiate between labile and stable components of this activity.
• To investigate the enzymes' response to oxytocin and potential roles.

Main Methods:

• Isolation of myometrial plasma membranes.
• Assay of nucleotide phosphohydrolase activity using various substrates and inhibitors.
• Determination of kinetic parameters (Km) and inhibition profiles.
• Testing the effect of oxytocin on enzyme activity.

Main Results:

• Two distinct ecto-enzymes were identified: a stable and a labile component.
• The stable enzyme accepts XTP/XDP, while the labile enzyme cleaves XTP only.
• Labile enzyme activity is sensitive to product inhibition by inorganic orthophosphate.
• Neither enzyme was inhibited by oxytocin up to 10 microM.

Conclusions:

• Myometrial cells possess at least two distinct ecto-enzymes with differing substrate specificities and sensitivities.
• Product inhibition significantly impacts the labile enzyme's activity.
• These ecto-enzymes are not directly regulated by oxytocin, unlike the Ca2+ pump.
• The labile enzyme may play a role in regulating purinergic receptors due to its activity and inhibition characteristics.