Multiple forms of hepatic glucokinase

Alternative splicing of glucokinase mRNA in rat liver

The sequences of two near full-length cDNAs encoding rat liver glucokinase are reported. One of the cDNAs is essentially identical to the cDNA cloned by Andreone, Printz, Pilkis, Magnuson & Granner. [(1989) J. Biol. Chem. 264, 363-369]. The other cDNA contains a 151 bp insertion and a downstream 52 bp deletion. The inserted block of bases has been shown to originate from an optional cassette exon, termed 2A, between the previously described exons 1 and 2. The conceptual translation product from the variant mRNA is identical to the original glucokinase protein for the first 15 amino acids. Next there is a novel polypeptide sequence of 87 residues, comprising 50 residues encoded by the cassette exon and 37 residues specified by an altered reading frame in exon 2. Due to the 52 bp deletion, 17 amino acids of the reference sequence are then missing, after which the sequence reverts to the original. Northern blot analysis with oligonucleotide probes has shown that alternatively spliced mRNA represents about 5% of total glucokinase mRNA. Alternative splicing of glucokinase mRNA in liver may explain earlier findings of minor isoforms of hepatic glucokinase.

The isolation and preliminary characterization of N-acetyl-D-glucosamine kinase from rat kidney and liver

1. Procedures for the extensive purification in high yield of N-acetyl-D-glucosamine kinase from rat liver and kidney are described. The separation of this enzyme from hepatic glucokinase depended primarily on their differing behaviour on an affinity column of Sepharose--N-(6-aminohexanoyl)-2-amino-2-deoxy-D-glucopyranose. 2. This N-acetyl-D-glucosamine kinase also catalyses the phosphorylation of N-acetyl-D-mannosamine and, at a lower rate, several other sugar analogues, including D-glucose. 3. A comparison of the behaviour of the enzyme during gel filtration and electrophoresis in sodium dodecyl sulphate/polyacrylamide gels suggests that N-acetyl-D-glucosamine kinase is a symmetrical dimer of mol.wt. 80000.

Affinity labelling of rat-muscle hexokinase type II by a glucose-derived alkylating agent

The glucose-derived alkylating agent N-bromoacetylglucosamine (GlcNBrAc) is shown to cause a time-dependent irreversible inactivation of rat muscle hexokinase type II. The kinetics of inactivation are in accord with the reversible formation of an enzyme-inhibitor complex prior to modification, indicating that the reagent is active-site-directed. A Ki of 0.57 mM obtained for this reversible complexing is in agreement with a Ki of 0.65 mM obtained for the inhibition caused by N-propionylglucosamine, an isosteric analogue of GlcNBrAc and a competitive inhibitor with respect to glucose. Glucose itself protects competitively against inactivation. A KG of 0.26 mM obtained for the formation of enzyme-glucose complex from these studies is in agreement with the kinetically-determined Km of 0.2 mM. The substrate-unrelated but chemically similar alkylating agents bromoacetic acid and N-bromoacetylgalactosamine inactivate the enzyme at 20% of the rate caused by GlcNBrAc. The inactivation rate increases rapidly over the pH range 7--9. Analysis of this pH dependence shows that a single residue of pKa 8.9 is reacting with GlcNBrAc with a kmax (pH corrected, pseudo-first-order rate constant) of 1.5 x 10(-3) S-1. These values are typical of the reaction of model thiols with alkylating agents and suggests the reacting residue is probably a cysteine. Use of radioactively labelled GlcNBrAc indicates that uptake of 1 mol of reagent per mol protein causes complete activity loss. Finally the behaviour of this enzyme with active-site-directed alkylating agents is compared with published results of similar experiments carried out with yeast hexokinase and bovine brain hexokinase type I.