MCTR Biosynthesis and Structures. The MCTR biosynthetic pathway is initiated by lipoxygenation of DHA at carbon position 14 leading to 14S-hydro(peroxy)-4Z,7Z,10Z,12E,16Z,19Z-docosahexaenoic acid, this is converted by lipoxygenase activity to 13S,14S-epoxy −4Z,7Z,9E,11E,16Z,19Z- docosahexaenoic acid. Conversion of this allylic epoxide is to MCTR1 is catalyzed by either glutathione s-transferase mu4 (GSTM4) and by leukotriene C4 synthase (LTC4S). MCTR1 is converted by gammaglutamyl transferase (GGT) to MCTR2 that in turn is substrate for conversion by dipeptidase (DPEP) to MCTR3.