Effects of partial hepatectomy, phenobarbital and 3-methylcholanthrene on kinetic parameters of glucose-6-phosphate and phosphogluconate dehydrogenase in situ in periportal, intermediate and pericentral zones of rat liver lobules

Abstract

Glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) are heterogeneously distributed in liver lobules of female rats. The maximum activity of both enzymes is approximately twice higher in intermediate and pericentral zones than in periportal zones. Enzyme activities and their distribution patterns were manipulated by partial hepatectomy and treatment with phenobarbital (PB) or 3-methylcholanthrene (3-MC). V max values of G6PDH for glucose-6-phosphate decreased mainly in intermediate and pericentral zones after partial hepatectomy, whereas they increased after PB treatment. V max values of PGDH for phosphogluconate decreased after partial hepatectomy in both zones, whereas other treatments did not have any effect. The affinity of G6PDH for glucose-6-phosphate was similar in all zones and it was decreased 2–3 fold by PB and 3-MC treatment. The affinity of PGDH for phosphogluconate was 1.4-2.3 times lower in intermediate and pericentral zones than in periportal zones of all livers tested and was not affected by treatment. From these data it can be concluded that not only the maximum activity of enzymes may differ in periportal, intermediate and pericentral zones of the liver lobule but also the affinity of enzymes for their substrates. The implication of these findings is that metabolic flux rates as they occur in vivo in these different metabolic compartments may be significantly different from predictions on the basis of maximum enzyme activities as detected immunohistochemically, microchemically or cytophotometrically.