Integrated effects of multiple modulators on human liver glycogen phosphorylase a.
Hepatic glucose production is increased in people with type 2 diabetes. Glucose released from storage in liver glycogen by phosphorylase accounts for approximately 50% of the glucose produced after an overnight fast. Therefore, understanding how glycogenolysis in the liver is regulated is of great importance. Toward this goal, we have determined the kinetic characteristics of recombinant human liver glycogen phosphorylase a (HLGPa) (active form) and compared them with those of the purified rat enzyme (RLGPa). The Michaelis-Menten constant (K(m)) of HLGPa for P(i), 5 mM, was about fivefold greater than the K(m) of RLGPa. Two P(i) (substrate) concentrations were used (1 and 5 mM) to cover the physiological range for P(i). Other effectors were added at estimated intracellular concentrations. When added individually, AMP stimulated, whereas ADP, ATP and glucose inhibited, activity. These results were similar to those of the RLGPa. However, glucose inhibition was about twofold more potent with the human enzyme. UDP-glucose, glucose 6-phosphate, and fructose 1-phosphate were only minor inhibitors of both enzymes. We reported previously that when all known effectors were present in combination at physiological concentrations, the net effect was no change in RLGPa activity. However, the same combination reduced HLGPa activity, and the inhibition was glucose dependent. We conclude that a combination of the known effectors of phosphorylase a activity, when present at estimated intracellular concentrations, is inhibitory. Of these effectors, only glucose changes greatly in vivo. Thus it may be the major regulator of HLGPa activity.
Ercan-Fang
Nacide
N
Metabolic Research Laboratory and Section of Endocrinology, Metabolism and Nutrition, Minneapolis Veterans Affairs Medical Center and Department of Medicine, University of Minnesota, Minneapolis, Minnesota 55417, USA. ercan001@tc.umn.edu
Gannon
Mary C
MC
Rath
Virginia L
VL
Treadway
Judith L
JL
Taylor
Miriam R
MR
Nuttall
Frank Q
FQ
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000249:Adenosine Monophosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005636:Fructosephosphates / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D025001:Glycogen Phosphorylase, Liver Form / Q000037:antagonists & inhibitors / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000737:chemistry / Q000201:enzymology*; D010710:Phosphates / Q000737:chemistry; D011817:Rabbits; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D013045:Species Specificity; D014532:Uridine Diphosphate Glucose / Q000494:pharmacology
D004791:Enzyme Inhibitors; D005636:Fructosephosphates; D010710:Phosphates; D011994:Recombinant Proteins; C032284:fructose-1-phosphate; D000249:Adenosine Monophosphate; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D025001:Glycogen Phosphorylase, Liver Form; D005947:Glucose; D014532:Uridine Diphosphate Glucose