TY - JOUR
T1 - Integrated effects of multiple modulators on human liver glycogen phosphorylase a
AU - Ercan-Fang, Nacide
AU - Gannon, Mary C
AU - Rath, Virginia L.
AU - Treadway, Judith L.
AU - Taylor, Miriam R.
AU - Nuttall, frank q
PY - 2002
Y1 - 2002
N2 - Hepatic glucose production is increased in people with type 2 diabetes. Glucose released from storage in liver glycogen by phosphorylase accounts for ∼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 (Km) of HLGPa for Pi, 5 mM, was about fivefold greater than the Km of RLGPa. Two Pi (substrate) concentrations were used (1 and 5 mM) to cover the physiological range for Pi. 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.
AB - Hepatic glucose production is increased in people with type 2 diabetes. Glucose released from storage in liver glycogen by phosphorylase accounts for ∼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 (Km) of HLGPa for Pi, 5 mM, was about fivefold greater than the Km of RLGPa. Two Pi (substrate) concentrations were used (1 and 5 mM) to cover the physiological range for Pi. 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.
KW - Adenosine 5′-diphosphate
KW - Adenosine 5′-monophosphate
KW - Adenosine 5′-triphosphate
KW - Enzyme regulation
KW - Fructose 1-phosphate
KW - Glucose
KW - Glucose 1-phosphate
KW - Glycogen metabolism
KW - Uridine 5′-diphosphate-glucose
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U2 - 10.1152/ajpendo.00425.2001
DO - 10.1152/ajpendo.00425.2001
M3 - Article
C2 - 12067839
AN - SCOPUS:0036293652
SN - 0193-1849
VL - 283
SP - E29-E37
JO - American Journal of Physiology - Endocrinology and Metabolism
JF - American Journal of Physiology - Endocrinology and Metabolism
IS - 1 46-1
ER -