In vitro effect of mercury on enzyme activities and its accumulation in the first-trimester human placenta.

The effect of incubating young placental explants with HgCl2 on the activities of aryl hydrocarbon hydroxylase (AHH) (a phase I enzyme), quinone reductase (QR), catecholamine-O-methyltransferase (COMT) (both phase II enzymes), and glucose-6-phosphate dehydrogenase (G-6-PD) is described. Mercury (Hg) at low doses significantly elevated placental phase I and phase II enzyme activities, but decreased the activity of G-6-PD. The increase in activities, which was time- and dose-dependent, was higher in explants incubated for 24 hr than in those incubated for 6 hr. The decrease in placental G-6-PD activity was drastic at low Hg dose levels but at higher levels the inhibitory effect was milder for both incubation periods. Placental explants accumulated Hg in amounts proportional to its concentration in the incubation medium and this accumulation was greater in explants incubated for 24 hr. The data suggest that contamination with low Hg levels from the environment during pregnancy may affect placental enzymatic activity. The accumulation of Hg during short incubation indicates a strong placental cell affinity for Hg, which could affect its other metabolic functions. The system used in sensitive, as it shows alteration in enzyme activity even with relatively low concentrations of the metal and the response is dose-related.