Glucose and caffeine regulation of liver glycogen phosphorylase activity in the freeze-tolerant wood frog Rana sylvatica.
We have examined the effect of glucose and caffeine inhibition on the activity of liver glycogen phosphorylase a from the freeze-tolerant frog Rana sylvatica. Kinetic studies indicate that this enzyme exhibits similar sensitivity to glucose inhibition (glucose dissociation constant = 12.5 mM) as the mammalian enzyme. Little inhibition (less than 25%) was observed at normal glucose concentrations (1-5 mM), while significant inhibition (60-95%) occurred at glucose concentrations (50-500 mM) present in freezing-exposed animals. These results favour the hypothesis that in the normal state glucose regulates phosphorylase activity primarily through the promotion of dephosphorylation of phosphorylase a, whereas during freezing regulation is achieved through phosphorylase a inactivation. The caffeine dissociation constant (0.93 mM) and the degree of synergism between caffeine and glucose (interaction factor, alpha = 0.14) were also similar to that observed for the mammalian enzyme. Hence, if a caffeine-like ligand exists in vivo, it must be in low enough amounts during freezing to allow sufficient phosphorylase a activity for high glucose production.
Risman
C A
CA
Department of Biology, York University, Ont., Canada.
David
E S
ES
Storey
K B
KB
Crerar
M M
MM
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D004357:Drug Synergism; D005260:Female; D005615:Freezing; D005947:Glucose / Q000494:pharmacology*; D006003:Glycogen / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D006005:Phosphorylases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D011898:Ranidae
D002110:Caffeine; D006003:Glycogen; D006005:Phosphorylases; D005947:Glucose