There are a number of functional, metabolic differences between periportal and perivenous hepatocytes in the mammalian liver resulting from zonal differences in the activity of several enzymes, and possibly from morphological differences as well (1 ). Examples of key enzymes where the periportal/perivenous activity ratio is >1 are glucose-σ-phosphatase (2 ), phosphoenolpyruvate carboxykinase (3 ), alanine aminotransferase (4 ), ornithine carbamoyltransferase (5 ), and carbamoylphosphate synthetase (6 ), whereas enzymes such as glucokinase (7 ), P-450-dependent hydroxyl-ation reactions (8 ), and glutamine synthetase (9 ) show the opposite zona-tion. Recent studies on lipogenic enzymes (acetyl-CoA carboxylase, citrate lyase, and fatty acid synthase) show that, although there is a periportal predominance in both the fed and fasted male rat, the ca. threefold enzyme induction observed after a fasting-refeeding transition takes place almost entirely in the perivenous zone (10 ).
