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.
Department of Nutrition, Hôpital Sainte-Justine, Quebec, Canada.
J F
JF
St-Denis
B
B
Comte
D K
DK
Nguyen
E
E
Seidman
K
K
Paradis
E
E
Lévy
G
G
van de Werve
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