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., M4

A B S T R A C T Human phosphofructokinase (PFK; EC 2.7.1.11) exists in tetrameric isozymic forms. Muscle and liver contain the homotetramers M4 and L4, whereas erythrocytes contain five isozymes composed of M (muscle) and L (liver) subunits, i.e., M4, M3L, M2L2, ML3, and L4. Inherited defects of erythrocyte PFK are usually partial and are described in association with heterogeneous clinical syndromes. To define the molecular basis and pathogenesis of this enzymopathy, we investigated four unrelated individuals manifesting myopathy and hemolysis (glycogenosis type VII), isolated hemolysis, or no symptoms at all.The three symptomatic patients showed high-normal hemoglobin levels, despite hemolysis and early-onset hyperuricemia. They showed total lack of muscle-type PFK and suffered from exertional myopathy of varying severity. In the erythrocytes, a metabolic crossover was evident at the PFK step: the levels of hexose monophosphates were elevated and those of 2,3-diphosphoglycerate (2,3-DPG) were depressed, causing strikingly increased hemoglobin-oxygen affinity. In all cases,

In man and the rabbit, 6-phosphofructokinase (PFK, EC 2.7.1.11) exists in tetrameric isoenzymic forms composed of muscle (M or A), liver (L or B) and platelet or brain (P or C) subunits, which are under separate genetic control. In contrast, the genetic control of the rat PFK has not yet been conclusively established; it is unclear whether the P-type or C-type subunit exists in this species. To resolve this question, we investigated the enzyme from the skeletal muscle, liver and brain of rats of Wag/Rij strain. Our studies demonstrate that the rat PFK is also under the control of three structural loci and that the homotetramers M4, P4 and L4 exhibit unique chromatographic, immunological and kinetic-regulatory properties. Skeletal-muscle and brain PFKs consist of isolated M4 and P4 homotetramers respectively. Although liver PFK consists predominantly of L4 homotetramer, it also contains small amounts of PL3 and P2L2 species. All three PFKs exhibit allosteric properties: co-operativity with fructose 6-phosphate and inhibition by ATP decrease in the order P4 greater than L4 greater than M4. P4 and M4 tetramers are the most sensitive to citrate inhibition, whereas L4 tetramer is the least sensitive. More importantly, P4 and L4 isoenzymes are the most sensitive to activation by fructose 2,6-bisphosphate, whereas M4 isoenzyme is the least sensitive. These results indicate that the brain PFK in this strain of rat is a unique tetramer, P4, which also exhibits allosteric kinetics, as do the well-studied M4 and L4 isoenzymes. The reported differences in the number and nature of isoenzymes present in the rat brain and liver most probably reflect the differences in the strains studied by previous investigators. Since the nature of the rat PFK isoenzymes and nomenclatures reported by previous investigators have been now reconciled, it is proposed that, for the sake of uniformity, only well-established nomenclatures used for the rabbit or human PFK isoenzymes be used for the rat isoenzymes.

Mammalian phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) exists in multimolecular forms, which result from random tetramerization of three distinct subunits, M (muscletype), L (liver-type), and P (platelet-type), each under a separate genetic control. Human muscle and liver contain homotetramers M4 and L4, respectively, whereas erythrocytes contain a mixture of M4, M3L, M2L2, ML3, and L4 isozymes.Homozygous deficiency of the M subunit in man results in glycogen storage disease (GSD) type VII, which is characterized by exertional muscle weakness and compensated hemolysis; the residual erythrocyte PFK consists of isolated L4 isozyme. Recently, PFK deficiency associated with isolated hemolytic anemia has been identified among English springer spaniel dogs. We investigated the genetic control of the dog PFK system and the nature of the enzymatic defect in two PFK-deficient animals, using chromatographic and immunological techniques. Our studies indicate the existence of a trilocus isozyme system for the dog, as is the case with other mammals. Muscle PFK consists of M4 isozyme, whereas the predominant species of liver and platelet consists, respectively, of the L4 and P4 isozyme; erythrocyte PFK consists of a threeor four-membered set composed of M and P subunits. PFK deficiency in the dogs was found to result from a total and universal lack of the M subunit, as is the case in man. However, the probands consistently exhibited L4 isozyme in their muscle; P4, L4, and hybrids thereof in their erythrocytes; and an increase in the L-containing isozymes in their platelets, indicating a generalized anomalous presence of the L subunit. The apparent absence of muscle disease in these animals is most likely accounted for by both the well-known high oxidative potential of the canine muscle in general and the presence of liver PFK in the M-deficient muscle in particular. In contrast, presence of hemolysis despite residual P4 and hybrids of P and L in the erythrocytes may be inferred to result in severe glycolytic handicap under existing intraerythrocytic conditions.

No abstract