Purification of human erythrocyte phosphofructokinase

Stimulatory effect of fatty acid treatment on glucose utilization in human erythrocytes

We previously reported that treatment of human erythrocytes with bee venom phospholipase A2 increased the rate of lactate production from glucose. This increase was suggested to be mediated through liberation of free fatty acids from membrane phospholipids. So, in the present study we examined the mechanism of stimulation of glycolysis by fatty acids. Treatment of intact erythrocytes with most of the 15 fatty acids tested resulted in stimulation of lactate production from glucose. Among the fatty acids tested, myristoleic acid showed the highest stimulatory activity. The ratio of moles of lactate produced to those of glucose utilized was about 1.9 in both myristoleic acid-treated and untreated cells. Treatment of erythrocytes with myristoleic acid did not affect the amount of 2,3-bisphosphoglycerate. Lactate production from D-glyceraldehyde, which is thought to be phosphorylated to D-glyceraldehyde 3-phosphate and then metabolized in the glycolytic pathway, was not at all affected by treatment of cells with myristoleic acid. The cross-over plot of glycolytic intermediates suggested that the enhancement of glycolysis was induced by activation of the 6-phosphofructokinase (PFK) step. Fatty acids incorporated into erythrocytes were found to be present predominantly in the cytoplasm rather than in the plasma membrane. The PFK activity, but not the hexokinase activity, in hemolysates was clearly increased by a set of fatty acids, and myristoleic acid was again the most potent. However, partially purified human erythrocyte PFK was not activated by the acid. We conclude that fatty acids stimulate glycolysis through activation of PFK in cooperation with some other component(s) in erythrocytes.

4-Hydroxy-2-nonenal hardly affects glycolysis

4-Hydroxy-2-nonenal (HNE), one of the major products of lipid peroxidation, inactivated the rate-limiting enzymes (from animal sources) of the glycolytic pathway and the pentose phosphate pathway when incubated at 37 degrees C for 1 h in the absence of glutathione (GSH). The HNE concentration for half-maximal inactivation of 6-phosphofructokinase (PFK) and glyceraldehyde-3-phosphate dehydrogenase was 3-10 microM; and that value for pyruvate kinase, glucose-6-phosphate dehydrogenase, and hexokinases I and II was 0.15-0.6 mM. In the presence of 5 mM GSH, however, only PFK, irrespective of the source (muscle, liver, or erythrocyte), was inactivated by 40-50% when incubated with 0.1 mM HNE for 1 h. Even PFK was not inactivated in the presence of both GSH and its substrate, ATP (2 mM). Glycolysis in human erythrocytes was not affected by treatment of cells with 0.1 mM HNE at 37 degrees C for 30 min. The results suggest that HNE, at concentrations observable under physiological and pathological conditions, hardly affects glycolysis in cells.

Cellular concentrations of enzymes and their substrates

The activity of crude and pure enzyme preparations as well as the molecular weight of these enzymes were obtained from the literature for several organisms. From these data enzyme concentrations were calculated and compared to the concentration(s) of their substrates in the same organism. The data are expressed as molar ratios of metabolite concentration to enzyme site concentration. Of the 140 ratios calculated, 88% were one or greater, indicating that in general substrates exceed their cognate enzyme concentrations. Of the 17 cases where enzyme exceeds metabolite concentration, 16 were in glycolysis. The data in general justify the use of enzyme kinetic mechanisms determined in vitro in the construction of dynamic models which simulate in vivo metabolism.

Isozymes of human phosphofructokinase: identification and subunit structural characterization of a new system

The existence of a five-membered isozyme system for human phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) has been demonstrated. These multimolecular forms result from the random polymerization of two distinct subunits, M (muscle type) and L (liver type), to form all possible tetrameters-i.e., M(4), M(3)L, M(2)L(2), ML(3), and L(4). Partially purified muscle and liver PFKs were hybridized by dissociation at low pH and then recombination at neutrality. Three hybrid species were generated in addition to the two parental isozymes, to yield an entire five-membered set. The various species could be consistently and reproducibly separated from one another by DEAE-Sephadex chromatography at pH 8.0 with a concave elution gradient of salt. Under similar experimental conditions, erythrocyte PFK from hemolysates was also resolved into five species chromatographically indistinguishable from those produced in the above experiment. Immunological and kinetic studies of the isozymes provided corroborative evidence to support the proposed subunit structures. Erythrocyte PFK was found to have kinetic properties intermediate between those of muscle and liver PFK and was neutralized only 50% by an antiserum against muscle PFK that completely neutralized muscle PFK. These data demonstrate that muscle and liver PFKs are distinct homotetramers-i.e., M(4) and L(4), respectively-whereas erythrocyte PFK is a heterogeneous mixture of all five isozymes. The structural heterogeneity of erythrocyte PFK provides a molecular genetic basis for the differential organ involvement observed in some inherited PFK deficiency states in which myopathy or hemolysis or both can occur.

Self-association of human erythrocyte phosphofructokinase. Kinetic behaviour in dependence on enzyme concentration and mode of association

The kinetic behaviour of human erythrocyte phosphofructokinase has been analyzed over a relative wide range of enzyme concentration (0.01 -- 1.7 mug/ml). The kinetic cooperativity which becomes apparent when the enzymic reaction rate is plotted versus the fructose 6-phosphate concentration decreases with increasing enzyme concentration. Simultaneously, a decrease of the half-saturation concentration for fructose 6-phosphate [S]0.5 is observed. Maximum velocity passes through a maximum at increasing enzyme concentrations. Sets of curves representing specific enzymic activity of phosphofructokinase versus enzyme concentration obtained at various fixed concentrations of fructose 6-phosphate and ATP are analyzed. The shapes of these curves are interpreted in terms of an association model of human erythrocyte phosphofructokinase, in which an inactive dimer (Mr 190000) and active multimers of the dimeric form are involved. The conclusion is drawn that the sigmoidal shape of the plots of the enzymic reaction rate versus fructose 6-phosphate concentration is partially caused by a displacement of the equilibrium between different states of association of phosphofructokinase to multimers by this substrate. On the other hand, the inhibition of the enzyme by high concentrations of ATP may be partially caused by a shift of this equilibrium to the state of the inactive dimer.