A conformational model for the human liver microsomal glucose-6-phosphatase system: evidence from rapid kinetics and defects in glycogen storage disease type 1.
Rapid kinetics of glucose-6-phosphate (G6P) uptake and hydrolysis as well as of orthophosphate uptake were investigated in microsomes prepared from normal and glycogen storage disease type 1a (GSD 1a) human livers using a fast sampling, rapid filtration apparatus and were compared to those of rat liver microsomes. As shown before with rat microsomes, the production of [U-14C]glucose from 0.2 mmol/L [U-14C]G6P by untreated normal human microsomes was characterized by a burst in activity during the first seconds of incubation, followed by a slower linear rate. The initial velocity of the burst was equal to the rate of glucose production in detergent-treated microsomes. In untreated and detergent-treated GSD 1a microsomes, no glucose-6-phosphatase activity was observed. When untreated normal human or rat microsomes were incubated in the presence of 0.2 mmol/L [U-14C]G6P, an accumulation of [U-14C]glucose was observed, whereas no radioactive compound (G6P and/or glucose) was taken up by GSD 1a microsomes. Orthophosphate uptake was, however, detectable in both GSD 1a and normal untreated vesicles. These results do not support a rate-limiting transport of G6P in untreated normal human microsomes and further show that in this case of GSD 1a, no distinct G6P transport activity is present.
St-Denis
J F
JF
Department of Nutrition, Hôpital Sainte-Justine, Quebec, Canada.
Comte
B
B
Nguyen
D K
DK
Seidman
E
E
Paradis
K
K
Lévy
E
E
van de Werve
G
G
D000293:Adolescent; D000818:Animals; D005260:Female; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D005953:Glycogen Storage Disease Type I / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D008954:Models, Biological; D010758:Phosphorus / Q000378:metabolism; D051381:Rats; D012016:Reference Values; D013997:Time Factors
D010758:Phosphorus; D005952:Glucose-6-Phosphatase