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The primary function of the human placenta is to ensure an optimal environment for fetal growth and development. Nutrients, endogenous substances and xenobiotics enter the placenta via the maternal circulation. The placenta possesses the ability to metabolize a number of xenobiotics and endogenous compounds by processes (cytochrome P450 enzymes and/or glutathione conjugation pathways) similar to those in the liver. Although the metabolites produced are usually inactive, reactive or toxic metabolites may also be generated. Therefore, changes in placental enzymatic function could alter fetal exposure to these reactive/toxic metabolites. Animal and in vivo human studies suggest diabetes alters the liver's xenobiotic metabolizing enzymes, however whether diabetes alters placental enzymatic activity is unknown. To determine the relative contribution of each area to the metabolizing capacity of the placenta and the relationship to placental changes observed in diabetes, four regions were studied to assess the activities of CYP1A1, CYP19A1 (aromatase) and glutathione S-transferase (GST). The areas selected for study were chorionic plate, maternal surface, placental margin and whole tissue. No differences in either CYP1A1 or glutathione S-transferase activities were noted. With respect to CYP19A1, the placental margin exhibited significantly less activity when compared to other regions (p {dollar}<{dollar} 0.05). Thus it appears that whole tissue samples of the human placenta are adequate for placental cytochrome P450 and glutathione S-transferase metabolism studies. To evaluate whether diabetes affects placental xenobiotic and steroid metabolizing activity, the catalytic activities of 7-ethoxyresorufin O-deethylation (EROD) (CYP1A1), chlorzoxazone 6-hydroxylation (CYP2E1), dextromethorphan N-demethylation (CYP3A4), dextromethorphan O-demethylation (CYP2D6), (7-{dollar}\\sp3{dollar}H) androst-4-ene-3,17-dione aromatization (CYP19A1, aromatase) and 1-chloro-2,4-dinitrobenzene (CDNB) conjugation with glutathione (GST) from placentas of diet (class A1) and insulin-dependent (class A2) gestational diabetics and overt diabetics were compared to matched controls. EROD (CYP1A1) and aromatase (CYP19A1) activities were measured in all placental tissues. No differences were observed among overt or gestational diabetics with their respective matched controls. CDNB conjugation (GST) ranged from 0.275 and 1.65 units/min/mg protein. In contrast to that observed with CYP1A1 and aromatase, significantly less GST activity was noted in overt diabetics as compared to their matched controls and gestational diabetics. The activities of chlorzoxazone 6-hydroxylation (CYP2E1), dextromethorphan O-demethylation (CYP2D6), and dextromethorphan N-demethylation (CYP3A4) were below the level of detection in all placentas. GST protein was detectable in all placentas, but no cytochrome P450 proteins (CYP1A, CYP2E1 or CYP3A) could be detected in any of the tissues. To conclude, it appears that pregnant women with overt diabetes have reduced GST activity in the placenta which could potentially result in the exposure of the fetus to harmful electrophiles. However, the full clinical significance of this finding remains to be determined.