Gamma-glutamyl transpeptidase (γ-GTP), an amino acid transport enzyme, has been demonstrated in a number of fetal and adult tissues of rodent and man. While the reexpression of the enzyme has been described in epithelia following carcinogen treatment or aging, little is known of the mechanism of its disappearance in some neonatal tissues. A description is presented of the rate and pattern of loss of histologically demonstrable γ-GTP activity from fetal and neonatal liver of the rat. The number of hepatocytes with histologically demonstrable γ-GTP activity declines rapidly from the 18th day of gestation. By the 6th day postpartum the activity is demonstrable in clusters of hepatocytes. By the 7th day postpartum there are essentially no hepatocytes with demonstrable activity, although the enzyme remains expressed in bile duct epithelium.
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