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Dual inhibition of cyclooxygenase and lipoxygenase by human haptoglobin: its polymorphism and relation to hemoglobin

Sheikh Arshad Saeed1, Nabeel Ahmad, Sagheer Ahmed

  • 1Dr. Panjwani Center for Molecular Medicine and Drug Research, International Center for Chemical and Biological Sciences, University of Karachi, Karachi-75270, Pakistan. sarshadsaeed@yahoo.com

Biochemical and Biophysical Research Communications
|January 3, 2007
PubMed

Haptoglobin (Hp) binds hemoglobin (Hb) specifically and stoichiometrically. Since Hb stimulates prostaglandin (PG biosynthesis), we investigated if Hp effects arachidonic acid (AA) metabolism. The results showed that Hp (50-250 microg protein) inhibited the biosynthesis of PGs via cyclooxygenase (COX) and 12-HETE via lipoxygenase pathway in human platelets. Additional evidence was obtained by the loss of Hp inhibitory activity upon removal of Hp by affinity chromatography on hemoglobin sepharose and by inhibition of AA or bradykinin-induced bronchoconstriction in the guinea pig. Hb reduced the inhibitory effect of Hp in a concentration-related manner such that all its inhibitory activity was lost when completely bound by Hb. Of the three Hp phenotypes, Hp 1-1 showed maximum binding capacity to Hb indicating its greater protective role. These findings implicate Hp in the regulation of COX and lipoxygenase pathways and show Hp involvement in the body's endogenous defense system against inflammation. This indicates that mammals have dual defense system, i.e., a specific immune system and non-specific Hp defense system.

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