STEREOCHEMISTRY OF OXYGENATION OF LINOLEIC-ACID CATALYZED BY PROSTAGLANDIN-ENDOPEROXIDE H-SYNTHASE-2

Linoleic acid was incubated with prostaglandin-endoperoxide H synthase -2 (PGHS-2) from ovine placenta. A product consisting of regio- and st ereoisomeric hydroxyoctadecadienoic (HOD) acids was obtained, Analysis by straight-phase high-performance liquid chromatography followed by chiral-phase high-performance liquid chromatography demonstrated that linoleic acid was preferentially oxygenated at C-9 to produce the foll owing mixture of HODs: 9(R)-HOD (52%), 9(S)-HOD (11%), 13(R)-HOD (2%), and 13(S)-HOD (35%). As a comparison, linoleic acid was incubated wit h microsomal prostaglandin-endoperoxide H synthase-1 (PGHS-1) from ovi ne vesicular gland. This resulted in a product having the following co mposition: 9(R)-HOD (75%), 9(S)-HOD (9%), 13(R)-HOD (1%), and 13(S)-HO D (17%). The stereochemistry of the hydrogen which was removed from C- 11 during the conversion of linoleic acid into hydroxy acids in the pr esence of PG;IIS-l or PGHS-2 was determined by incubation of [(11R)-H- 2]- and [(11S)-H-2]linoleic acids followed by mass spectrometric analy sis of the isotope contents of the individual hydroxy acid isomers, Bo th enzymes were found to catalyze oxygenations which involved stereosp ecific removal of the (11S) hydrogen and retention of the (11R) hydrog en. The major hydroxy acids, i.e., 9(R)-HOD and 13(S)-HOD, were formed from linoleic acid in reactions which involved antarafacial hydrogen abstraction and oxygen insertion, It is concluded that the initial ste ps of the PGHS-2- and PGHS-1-catalyzed oxygenations proceed with ident ical stereochemistry and involve stereospecific removal of the pro-S h ydrogen from the omega 8-methylene group of the substrate. (C) 1998 Ac ademic Press.