Regulation of phosphofructokinase at physiological concentration of enzyme studied by stopped-flow measurements

Determination of Phosphofructokinase and Enolase Activities in Cultured Mouse Neuroblastoma Cells: Application to the In Vitro Detection of Neurotoxic Effects

Simple methods for the determination of phosphofructokinase (PFK) and total enolase (END activities in cultured mouse neuroblastoma cells have been developed. The influences of changes in glucose metabolism, induced by chlorpromazine, cycloheximide, 2,4-dinitrophenol and iodoacetic acid, on PFK and ENL activities in vitro were compared as a practical application of the methods. Mouse neuroblastoma cell cultures (Neuro-2a) were exposed for 24 hours and cytotoxic effects were evaluated. All the determinations were carried out in the same 96-well tissue culture plates in which exposure took place. Chlorpromazine and cycloheximide produced opposite effects on both PFK and ENL activities. While chlorpromazine increased PFK activity by 50% and decreased ENL activity by 30%, cycloheximide inhibited PFK activity by 50% and increased ENL activity by 45%. The response to dinitrophenol was quite different; both PFK and ENL activities were increased, by 45% and 35% respectively. After exposure of the cells to iodoacetic acid, neither PFK nor ENL activities showed statistically significant differences from the levels in control cells. The different responses elicited by the four toxicants suggest that the two enzymes selected are useful for differentiating among diverse types of mechanism of action.

The changes in the energy metabolism of human muscle induced by training

The biochemical effects of training programmes have been studied with a kinetic model of central metabolism, using enzyme activities and metabolite concentrations measured at rest and after 30 s maximum-intensity exercise, collected before and after long and short periods of training, which differed only by the duration of the rest intervals. After short periods of training the glycolytic flux at rest was three times higher than it had been before training, whereas during exercise the flux and energy consumption remained the same as before training. Long periods of training had less effect on the glycolytic flux at rest, but increased it in response to exercise, increasing the contribution of oxidative phosphorylation.

Activation of in situ glycolytic flux by bisphosphorylated compounds: studies in porous rat adipocytes

By carefully permeabilizing eukaryotic cells such that intracellular enzymes are largely retained, an opportunity is created to explore the regulation of in situ flux. This is particularly important since the latter may not be accurately represented by kinetic measurements of isolated, solubilized enzymes from disrupted cells. In this study the action of fructose 2,6-diphosphate (F2,6DP) and other bisphosphorylated sugars which purportedly activate phosphofructokinase-1 (PFK-1; EC 2.7.1.11) were studied. Using porous adipocytcs and initiating flux with radiolabeled glucose 6-phosphate, the regulation of lactate production under both 0.1 and 1.0 mM ATP conditions by F2,6DP, glucose 1,6-diphosphate (G1,6DP), ribulose 1,5-diphosphate (R1,5DP), 2,3 diphosphoglycerate (2,3DPG), and mannose 6-phosphate (M6P) was examined. Studied at 1, 5, and 25 microM concentrations, F2,6DP and 2,3DPG significantly (and to the same extent) augmented glycolysis compared to control (at 0.1 mM ATP, the respective glycolytic rates--as % above control--at these three above-mentioned concentrations for F2,6DP were 60, 84, and 77%, whereas for 2,3DPG they were 84, 105, and 179%; at 1 mM ATP, the F2,6DP effect was 88, 99, and 121%, and for 2,3DPG it was 52, 89, and 96%). Stimulation by these compounds was less obvious at higher glycolytic flux rates (saturating amounts of G6P). Amongst this group, and only at 1.0 mM ATP, the sole other positive effector was 25 microM R1,5DP. The measured fat cell content of G1,6DP was 24 +/- 4 microM (n = 3); at this concentration no significant effect on glycolysis was observed. Examining the effects of 2,3DPG (25 microM) on proximal glycolysis (to triose phosphates) revealed there was a modest, but significant, 41% increase over basal; in contrast, under the exact same conditions, F2,6DP caused a 123% increase. Separate experiments also examined the effect of F2,6DP, 2,3DPG, and G1,6DP on glycolysis at 5 and 25 microM in the presence of a physiologic cytosolic ATP/ADP ratio and free cation concentrations. Under these conditions, F2,6DP and 2,3DPG remained pre-eminent in their stimulatory prowess, inducing 27-71% increases over control, while G1,6DP remained ineffectual. These studies support a locus of action of 2,3DPG on overall glycolysis which is distal to the triose phosphates. M6P was ineffective at all concentrations. In conclusion, F2,6DP is the pre-eminent in situ regulator of in situ adipocyte glycolysis, especially at higher ATP levels, although other sugars containing two phosphoryl groups may under certain conditions cause activation.