Constraints on prostaglandin biosynthesis in tissues

Area of Science:

• Biochemistry
• Enzymology
• Cellular Biology

Background:

• Prostaglandin formation is regulated by prostaglandin H synthase (PGHS).
• PGHS activity can be limited by substrate availability (fatty acids, hydroperoxides) and enzyme inactivation.
• Understanding these regulatory mechanisms is crucial for controlling prostaglandin biosynthesis.

Purpose of the Study:

• To investigate the factors limiting prostaglandin H synthase activity.
• To determine the role of enzyme inactivation and hydroperoxide availability in regulating prostaglandin synthesis.
• To compare the capacity for prostaglandin synthesis and degradation in different tissues.

Main Methods:

• Enzyme assays measuring prostaglandin formation from arachidonate.
• Investigating the effect of substrate complexation with cytosolic proteins on PGHS activity.
• Utilizing glutathione peroxidase to modulate hydroperoxide levels and assess its impact on PGHS activity.
• Comparing tissue-specific ratios of glutathione peroxidase to prostaglandin H synthase.

Main Results:

• Each molecule of PGHS synthesized approximately 1300 pmol of prostaglandin before inactivation.
• Complexing fatty acid substrates with cytosolic proteins decreased reaction velocity but not the extent of synthesis.
• Glutathione peroxidase significantly reduced the extent of prostaglandin synthesis per mole of PGHS.
• Tissue ratios of glutathione peroxidase to PGHS varied widely, from over 1800 in rat liver to less than 30 in leukocytes.

Conclusions:

• Prostaglandin H synthase inactivation and glutathione peroxidase-mediated suppression are key regulators of prostaglandin biosynthesis.
• These mechanisms likely play a significant role in limiting prostaglandin production within cells.
• Tissue-specific differences in regulatory enzyme ratios contribute to varying prostaglandin synthetic capacities.