Updated publication reference for PubMed record(s): 25170432. Prostaglandin H synthases (PGHSs) are N-glycosylated membrane proteins that catalyse the committed step in prostaglandin synthesis. Unlike PGHS-2, the production of recombinant PGHS-1 in non-mammalian expression systems is complicated. The majority of the heterologous enzyme is inactive due to misfolding. N-glycosylation is proposed to be obligatory for the correct folding of mammalian PGHSs. In this study, human PGHS-1 and -2 (hPGHS-1 and -2) were expressed in the yeast Pichia pastoris, and the N-glycosylation patterns of the purified recombinant proteins were characterised using nano-LC/MS/MS. Recombinant hPGHS-2 was catalytically active, whereas hPGHS-1 was inactive. Unexpectedly, the accumulation of non-glycosylated hPGHS-1 was not observed in the crude lysate of the yeast cells. In addition, the purified hPGHS isoforms exhibited similar N-glycosylation site occupancy. The results indicate that there are more complex grounds for the inactivity of the recombinant hPGHS-1 produced in yeast.