Glutathione-mediated detoxification mechanisms of human placenta.

Glutathione peroxidase and GSH S-transferases play significant physiological roles in the detoxification mechanisms. The present study indicates that only selenium-dependent GSH-peroxidase I is present in human placenta. GSH-peroxidase II (non-selenium GSH-peroxidase) is totally absent from this tissue. The placental GSH-peroxidase has a relative molecular mass of 85 000 and is a tetramer of equal-size subunits. It is cyanide sensitive and its properties are similar to those of human erythrocyte GSH-peroxidase. In contrast to human liver, which has seven GSH S-transferases, the placenta has only one species of GSH S-transferase. This enzyme (pI 4.5, relative molecular mass 45 000, subunit size 22 500) has a different amino acid composition from that of liver and erythrocyte GSH S-transferases. From immunological studies, the placental GSH S-transferase appears also to have a genetic origin distinct from that of liver, erythrocyte and kidney GSH S-transferases. The placental GSH S-transferase does not express GSH-peroxidase activity. The overall substrate specificities of placental GSH S-transferase are also narrow, compared with liver enzymes. The present study indicates that, although these two GSH-linked detoxification mechanisms are operative in placenta, they appear to be less efficient than the similar mechanisms operating in liver.