A protein inhibitor of phosphofructokinase from the mucosa of rat small intestine. A mechanism for the regulation of glycolysis that is independent of glucose

Inhibition of hexokinase activity by a fructose 2,6-bisphosphate-dependent cytosolic protein from liver

Mammalian and yeast hexokinases were found to be reversibly inhibited by fructose 2,6-bisphosphate, an effect requiring the presence of a cytosolic protein factor. Experimental evidence suggests that this factor (inhibitor) is a regulatory protein, the interactions of which with hexokinases are modulated by fructose 2,6-bisphosphate. The Vmax of hexokinase D was decreased, and no changes on other kinetic parameters were observed. The inhibitor was present in fresh liver cytosol filtered through Sephadex G-25 and was partially isolated by negative absorption on DEAE-cellulose followed by ammonium sulfate fractionation. The inhibitor was also present in brain and kidney, but not in muscle. A molecular mass of 200,000 was determined by gel filtration. The inhibition was dependent on the concentrations of both the inhibitory protein and fructose 2,6-bisphosphate. No delay in fructose 2,6-bisphosphate inhibition was observed. Several other hexose phosphates were tested and were not effective. In the presence of amounts of inhibitor sufficient to produce complete inhibition of hexokinase D, the concentration of fructose 2,6-bisphosphate required to produce 50% inhibition was about 0.5 microM. The inhibitor was unstable and was stabilized by the presence of fructose 2,6-bisphosphate.

Beta-adrenergic agents increase the phosphorylation of phosphofructokinase in isolated rat epididymal white adipose tissue

Pieces of rat epididymal adipose tissue were incubated in medium containing [32P]phosphate for 2 h to achieve steady-state labelling of intracellular phosphoproteins and then with or without hormones for a further 15 min. Phosphofructokinase was rapidly isolated from the tissue by use of either Blue Dextran-Sepharose chromatography or immunoprecipitation with antisera raised against phosphofructokinase purified from rat interscapular brown adipose tissue. Similar extents of incorporation of 32P into phosphofructokinase were measured by both techniques. Exposure of the tissue to adrenaline or the beta-agonist isoprenaline increased phosphorylation by about 5-fold (to about 1.4 mol of phosphate/mol of enzyme tetramer). No change in phosphorylation was detected with the alpha-agonist phenylephrine, but exposure to insulin resulted in an approx. 2-fold increase. The increased phosphorylation observed with isoprenaline was found to be associated with a decrease in the apparent Ka for fructose 2,6-bisphosphate similar to that observed on phosphorylation of phosphofructokinase purified from rat epididymal white adipose tissue with the catalytic subunit of cyclic AMP-dependent protein kinase. These results support the view [Sale & Denton (1985) Biochem. J. 232, 897-904] that an increase in cyclic AMP in adipose tissue may result in an increase in glycolysis through the phosphorylation of phosphofructokinase by cyclic AMP-dependent protein kinase.

Biotin deficiency and susceptibility to fatty liver and kidney syndrome in broiler chicks: reduced 6-phosphofructokinase (EC 2.7.1.11) activity but normal fructose 2,6-bisphosphate content in birds with hepatomegaly

In two separate experiments, using different strains, broiler chicks were reared on either a commercial-type chick mash (control) or a fatty liver and kidney syndrome (FLKS)-inducing diet. In Expt a, chicks were killed on day 29 and in Expt b, on day 32. Body-weights and liver weights were measured, and values from those given the control ration used to construct a hepatomegaly index by employing a variant of linear discriminant analysis. Application of the index to FLKS birds revealed a statistically significant bimodal distribution of liver size. The birds with enlarged livers (high index) also possessed metabolic abnormalities in that 6-phosphofructokinase (EC 2.7.1.11; PFK-1) activity (measured at low substrate concentration) was depressed despite the presence of normal, or even slightly elevated fructose 2,6-bisphosphate concentration. This indicates the presence of an uncharacterized regulatory mechanism for PFK-1 in FLKS-susceptible birds.