Localization of ammonia‐metabolizing enzymes in human liver: Ontogenesis of heterogeneity

Immunohistochemical analysis of human liver (8 to 94 years) shows a compartmentation of ammonia‐metabolizing enzymes across the acinus. The highest concentration of carbamoylphosphate synthetase (ammonia) is found in the parenchymal cells around the terminal portal venules. Glutamine synthetase is found in a small pericentral compartment two to three cells thick. In contrast to observations in rat liver, in human liver a well‐recognizable intermediate zone can be distinguished in which neither enzyme can be detected. This intermediate zone is not yet established at the age of 8 years but can be recognized in livers from 25 years onward. Carbamoylphosphate synthetase can already be detected in the liver of human fetuses at 5 weeks of development. The enzyme distribution reveals a random heterogeneity among the hepatocytes, suggesting that not all hepatocytes start to accumulate carbamoylphosphate synthetase at the same time. From 9 weeks of development onward, the enzyme becomes homogeneously distributed throughout the liver parenchyma until at least 2 days after birth. Glutamine synthetase cannot be detected during this period. In addition, the definitive architecture of the acinus is not yet completed at birth. These results therefore support the idea that in human liver, metabolic zonation with respect to NH 3 metabolism exists as it does in rat liver. Furthermore, the data show that this functional compartmentation becomes established concomitant with the development of the acinar architecture. The fact that enzymic zonation is not yet established in the perinatal period may have important consequences for the regulation of ammonia fixation and pH homeostasis in preterm infants and neonates.