Species and tissue distribution of the regulatory protein of glucokinase

Abstract

Rat liver is known to contain a regulatory protein that inhibits glucokinase (hexokinase IV or D) competitively versus glucose. This inhibition is greatly reinforced by the presence of fructose 6-phosphate and antagonized by fructose 1-phosphate and by KCl. This protein was now measured in various rat tissues and in the livers of various species by the inhibition it exerts on rat liver glucokinase. Rat, mouse, rabbit, guinea-pig and pig liver, all of which contain glucokinase, also contained between 60 and 200 units/g of tissue of a regulatory protein displaying the properties mentioned above. By contrast, this protein could not be detected in cat, goat, chicken or trout liver, or in rat brain, heart, skeletal muscle, kidney and spleen, all tissues from which glucokinase is missing. Fructose 1-phosphate stimulated glucokinase in extracts of human liver, indicating the presence of regulatory protein. In addition, antibodies raised against rat regulatory protein allowed the detection of an approximately 60 kDa polypeptide in rat, guinea pig, rabbit and human liver. The livers of the toad Bufo marinus, of Xenopus laevis and of the turtle Pseudemys scripta elegans contained a regulatory protein similar to that of the rat, with, however, the major difference that it was not sensitive to fructose 6-phosphate or fructose 1-phosphate. In rat liver, the regulatory protein was detectable 4 days before birth. Its concentration increased afterwards to reach the adult level at day 30 of extrauterine life, whereas glucokinase only appeared after day 15. In the liver of the adult rat, starvation and streptozotocin-diabetes caused a 50-60% decrease in the concentration of regulatory protein after 7 days, whereas glucokinase activity fell to about 20% of its initial level. When 4-day-starved rats were refed, or when diabetic rats were treated with insulin, the concentration of regulatory protein slowly increased to reach about 85% of the control level after 3 days, whereas the glucokinase activity was normalized after the same delay. The fact that there appears to be no situation in which glucokinase is expressed without regulatory protein is in agreement with the notion that the regulatory protein forms a functional entity with this enzyme.

Structure and alternative splicing of the ketohexokinase gene

“…Islet β-cell glucokinase (hexokinase IV, GCK) is known to be central to the regulation of blood glucose levels. GCK activity is inhibited by its regulatory protein (GCKR) in the liver and possibly in the pancreatic islet [7,8]. This inhibition of GCK by GCKR is relieved by F1P, suggesting that intracellular F1P levels could indirectly affect GCK activity and thereby perhaps the glucose sensor in the islet β cell.…”

Evolution of glucose utilization: Glucokinase and glucokinase regulator protein

“…Levels of fructose-6-phosphate, which change in parallel with blood glucose levels, regulate the binding of glucokinase with GCKR, allowing glucokinase to be active only when blood glucose levels are elevated (Agius and Peak, 1997;van Schaftingen et al, 1997). GCKR is only abundantly expressed in the liver, and not appreciably in the other sites of glucokinase expression (Detheux et al, 1993;Vandercammen and Van Schaftingen, 1993). The tissue-specific expression of GCKR in the liver therefore permits glucokinase to be post-transcriptionally regulated only in the liver, the one tissue where activity must be regulated to prevent function when blood glucose levels are low.…”

Cloning and Expression of a Xenopus Liver cDNA Encoding a Fructose‐Phosphate‐Insensitive Regulatory Protein of Glucokinase

“…A fructose-6-phosphate-sensitive and fructose-l-phosphate-sensitive regulatory protein appears to be present in the livers of all mammalian species that express hepatic glucokinase, including man (Vandercammen and Van Schaftingen, 1993). The livers of the amphibians Bufo marinus and Xenopus laevis, and of the turtle Pseudemys scripta elegans con- tain a protein that inhibits glucokinase competitively with respect to glucose but is insensitive to fructose 6-phosphate and fructose 1-phosphate (Vandercammen and Van Schaftingen, 1993).…”

“…tain a protein that inhibits glucokinase competitively with respect to glucose but is insensitive to fructose 6-phosphate and fructose 1-phosphate (Vandercammen and Van Schaftingen, 1993). It was of interest to know if this form of regulatory protein of glucokinase was homologous to the mammalian form of regulatory protein.…”

“…The presence of the regulatory protein in rat liver explains the activating effect of fructose (Van Schaftingen and Vandercammen, 1989;Davies et al, 1990) and of a potassium-rich medium on the phosphorylation of glucose in isolated hepatocytes. The cDNA encoding the rat liver regulatory protein has been recently cloned (Detheux et al, 1993).A fructose-6-phosphate-sensitive and fructose-l-phosphate-sensitive regulatory protein appears to be present in the livers of all mammalian species that express hepatic glucokinase, including man (Vandercammen and Van Schaftingen, 1993). The livers of the amphibians Bufo marinus and Xenopus laevis, and of the turtle Pseudemys scripta elegans con- tain a protein that inhibits glucokinase competitively with respect to glucose but is insensitive to fructose 6-phosphate and fructose 1-phosphate (Vandercammen and Van Schaftingen, 1993).…”