Demonstration of a metabolically active glucose-6-phosphate pool in the lumen of liver microsomal vesicles.
Glucose-6-phosphate transport was investigated in rat or human liver microsomal vesicles using rapid filtration and light-scattering methods. Upon addition of glucose-6-phosphate, rat liver microsomes accumulated the radioactive tracer, reaching a steady-state level of uptake. In this phase, the majority of the accumulated tracer was glucose, but a significant intraluminal glucose-6-phosphate pool could also be observed. The extent of the intravesicular glucose pool was proportional with glucose-6-phosphatase activity. The relative size of the intravesicular glucose-6-phosphate pool (irrespective of the concentration of the extravesicular concentration of added glucose-6-phosphate) expressed as the apparent intravesicular space of the hexose phosphate was inversely dependent on glucose-6-phosphatase activity. The increase of hydrolysis by elevating the extravesicular glucose-6-phosphate concentration or temperature resulted in lower apparent intravesicular glucose-6-phosphate spaces and, thus, in a higher transmembrane gradient of glucose-6-phosphate concentrations. In contrast, inhibition of glucose-6-phosphate hydrolysis by vanadate, inactivation of glucose-6-phosphatase by acidic pH, or genetically determined low or absent glucose-6-phosphatase activity in human hepatic microsomes of patients suffering from glycogen storage disease type 1a led to relatively high intravesicular glucose-6-phosphate levels. Glucose-6-phosphate transport investigated by light-scattering technique resulted in similar traces in control and vanadate-treated rat microsomes as well as in microsomes from human patients with glycogen storage disease type 1a. It is concluded that liver microsomes take up glucose-6-phosphate, constituting a pool directly accessible to intraluminal glucose-6-phosphatase activity. In addition, normal glucose-6-phosphate uptake can take place in the absence of the glucose-6-phosphatase enzyme protein, confirming the existence of separate transport proteins.
Bánhegyi
G
G
Institute of General Pathology, University of Siena, 53100 Siena, Italy.
Marcolongo
P
P
Fulceri
R
R
Hinds
C
C
Burchell
A
A
Benedetti
A
A
D000818:Animals; D001692:Biological Transport / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D005952:Glucose-6-Phosphatase / Q000378:metabolism; D019298:Glucose-6-Phosphate / Q000378:metabolism*; D006801:Humans; D000072179:Interleukin-1 Receptor-Like 1 Protein; D007700:Kinetics; D008027:Light; D008297:Male; D008565:Membrane Proteins; D008862:Microsomes, Liver / Q000378:metabolism*; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D011956:Receptors, Cell Surface; D012542:Scattering, Radiation
C079241:IL1RL1 protein, human; D000072179:Interleukin-1 Receptor-Like 1 Protein; D008565:Membrane Proteins; D011506:Proteins; D011956:Receptors, Cell Surface; D019298:Glucose-6-Phosphate; D005952:Glucose-6-Phosphatase; D005947:Glucose