title: In situ kinetic parameters of glucose-6-phosphatase in the rat liver lobulus.
abstract: Glucose-6-phosphatase activity has been determined in periportal and pericentral zones of the rat liver lobule using a quantitative histochemical method. The study was performed on unfixed cryostat sections of livers from fasted and fed female and male rats. Highest activity was found in periportal zones, and starvation caused a 2-3-fold increase of glucose-6-phosphatase activity in periportal and pericentral zones of both sexes. Unexpectedly, KM values were also significantly different in periportal and pericentral zones and were found to increase linearly with Vmax values, irrespective of sex and feeding condition. Because the cryofixation procedure was shown to permeabilize the biomembranes in the tissue sections, it can be concluded that the rise in KM and Vmax values has to be attributed to the catalytic unit of the glucose-6-phosphatase system. It is suggested that the enzyme exists in a high affinity configuration at low enzyme concentrations but that at high enzyme concentrations a hysteretic mechanism, as proposed by Berteloot et al. (Berteloot, A., Vidal, H., and Van de Werve, G. (1991) J. Biol. Chem. 266, 5497-5507), transforms the enzyme from a high to a low affinity configuration. The present study indicates that the concept of functional heterogeneity of liver parenchyma may be more complex than thus far assumed.
authors.affiliation: Laboratory of Cell Biology and Histology, University of Amsterdam, The Netherlands.
authors.forename: G N
authors.initials: GN
authors.lastname: Jonges
authors.forename: C J
authors.initials: CJ
authors.lastname: Van Noorden
authors.forename: W H
authors.initials: WH
authors.lastname: Lamers
mesh_terms: D000818:Animals; D005260:Female; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D007150:Immunohistochemistry; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013217:Starvation
chemical_list: D005952:Glucose-6-Phosphatase