14C-labeled arachidonic acid bioconversion in guinea pig placenta during the last third of gestation

Abstract

In the presence study we investigated the arachidonic acid metabolism in guinea pig placenta during the last third of gestation. Homogenates were incubated with 14C-labeled subtrate, and eicosanoid formation was determined using rp HPLC. Arachidonic acid was substantially converted to cyclooxygenase products i.e.-keto-PGF 1α, TxB 2, PGF 2α, PGE 2, PGD 2 and 12-HHT. Lipoxygenase activity was also found but of a much lower degree and represented by the mono-hydroxy acid 12-HETE and 15-HETE. The total conversion of arachiodonic acid exhibited a progressive rise from day 50 to term, due principally to the increasing part of TxB 2, PGE 2 and 12-HHT throughout this gestational perid and in addition, near term, of 6-keto-PGF 1α and PGF 2α. The results suggest that there is an increasing concentration and/or activity of cyclooxygenase system enzymes with placenta development in guinea pig, which may contribute to the augmented intrauterine availability of prostanoids under parturition. Additional experiments were performed to compare the metabolism of exogenously added 14C-arachidonic acid and endogenously present 12C-arachidonic acid during placental homogenate incubation by means of isotopes dilution GC-MS. Although the 14C- and 12-C prostanoid patterns were comparable, the 14C/12C ratios of the prostanoids formed during incubation were significantly different. These data indicate that exogenous arachidonic acid and endogenous arachidonic acid in placental homogenate do not follow up extractly the same metabolic pathway so that assumption of biochemical identity between exogenous radio-tracer and studied endogenous substrate is not quite true.