Multiple forms of phosphofructokinase in developing rabbit and guinea pig tissues

Novel testis- and embryo-specific isoforms of the phosphofructokinase-1 muscle type gene

We have identified novel transcriptional isoforms of the human and mouse genes encoding muscle type phosphofructokinase-1 (PFK-M). These isoforms are expressed specifically in the testis and in the mid-gestation embryo, and have been termed TE-PFK-M (testis- and embryo-specific PFK-M). The 5'UTR of TE-PFK-M is composed of three newly identified exons that lie much farther upstream of the PFK-M coding region than the previously characterized 5'UTR. In addition, this upstream region encodes a series of small polyadenylated transcripts, some of which share the same exons found in the 5'UTR of TE-PFK-M, and which may play some role in regulating TE-PFK-M expression. These findings indicate an even more complex level of control of PFK-M expression than previously thought.

Substrate metabolism in the developing heart

The developing heart undergoes a remarkable metabolic transformation as it adjusts to the higher-oxygen, extrauterine environment. During gestation, glycolysis and lactate oxidation constitute the major sources of adenosine triphosphate (ATP) for the fetal heart. After birth, however, there is a rapid shift from carbohydrate to fatty acid utilization. Despite the transition to primarily aerobic metabolism, the neonatal heart retains an enhanced capacity for anaerobic energy production. This unique metabolic adaptation is important when assessing the immature heart's responses to states of oxygen insufficiency, such as ischemia, hypoxia, and tachycardia. This article reviews the dramatic changes in enzyme activities, mitochondrial morphology and function, and substrate availability that underlie this change in metabolism in the maturing heart.

Changes in phosphofructokinase isozymes during development of myoblasts to myotubes

The regulation of phosphofructokinase during development of C2C12 myoblasts to myotubes was investigated. Enzyme activity was markedly increased during myogenic development. The increase was observed when enzyme activity was measured under optimal conditions and was not due to changes in the allosteric kinetic properties of the enzyme. Immunoprecipitation of phosphofructokinase from [35S]methionine-labeled myogenic cells revealed that equal amounts of liver and muscle isozymes are present in myoblasts, while in myotubes there was a much higher level of the muscle isozyme. These results were confirmed using an immunoblotting technique. The increase in the level of muscle isozyme in myotubes is due to an increase in the rate of synthesis of the muscle isozyme and occurs in spite of a measurably small increase in its degradation rate. Northern blot analysis using a synthetic oligonucleotide probe showed a 25-fold increase in the level of muscle phosphofructokinase mRNA in myotubes. The conclusion is drawn that the increase in muscle isozyme in myotubes during myogenesis is due to an increase in its mRNA level.

Changes of glycogen metabolism in phosphorylcreatine-depleted muscles taken from rats fed with beta-guanidine propionate

Changes in glycogen metabolism were explored in fast and slow muscles taken from rats fed with a diet containing 1% beta-guanidine propionate (GPA), a synthetic analog that inhibits the entry of creatine into muscle cells competitively and causes phosphorylcreatine depletion. Feeding with the GPA-containing diet increased glycogen levels in the two types of muscles to a different extent and with different temporal patterns; it did not change significantly the rate of glycogen turnover both at rest and during exercise; it did not affect the net degradation of glycogen during exercise. Diet could affect the activity of several enzymes of sugar metabolism. These latter changes too were different in fast-twitch and in slow-twitch muscles.

On the ontogeny and interactions of phosphofructokinase in mouse tissues

The distribution and interactions of phosphofructokinase isozymes with cellular structure have been studied in the major tissues of the mouse during development. The ontogenic patterns of isozymes which were obtained were consistent with those observed for other species and are interpreted in terms of the presence of three genes and three homotetrameric forms of the enzyme (A4, B4 and C4) in the tissues of the mouse. In addition, the data provides a clear indication that interactions between the enzyme and cellular structure are appreciable in all major tissues and at all stages of development, with all isozyme types exhibiting such interactions. The significance of the study of subcellular interactions of these isozymes in contributing to a comprehensive physiological rationale for this mammalian enzyme and its multiple forms is discussed.

Alterations in phosphofructokinase isoenzymes during early human development. Establishment of adult organ-specific patterns

Human 6-phosphofructokinase (EC 2.7.1.11) exists in tetrameric isoenzymic forms composed of muscle (M), liver (L) and platelet (P) subunits, which are under separate genetic control. In the adult, the proportion of these subunits in different organs reflects the relative activity of glycolysis versus gluconeogenesis. To elucidate the developmental basis for the observed distribution, we investigated the isoenzymic transitions of phosphofructokinase in human foetuses (12-40 weeks' gestation) by using high-resolution chromatography and monoclonal antibodies. We studied skeletal muscle, heart, liver and brain because these organs show very different glycolytic fluxes and isoenzymic patterns in adult individuals. Our results demonstrate that there is no unique 'foetal' form of phosphofructokinase in humans, but all three loci are variably expressed in all foetal organs during early gestation. As development proceeds, muscle and liver isoenzyme patterns show dramatic changes, with disappearance of P and L subunits in muscle and transient reappearance of M and P subunits in liver; in contrast, phosphofructokinase isoenzymes change little in brain and heart. Most changes occur at mid-gestation and near term, and adult isoenzyme patterns are expressed at birth, indicating that organ differentiation is complete. These studies show that phosphofructokinase undergoes changes of isoenzyme patterns similar to, but not identical with, those of other multilocus isoenzyme systems of glycolysis. The observed changes probably reflect changing patterns of gene expression, with repression of some loci and activation of others.

Purification of F4 phosphofructokinase from human platelets and comparison with the other phosphofructokinase forms

The phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) tetramers F4, F3L and F2L2 have been separated from human platelets, and purified to homogeneity by affinity chromatography on Dextran Blue-Sepharose 4B. The F subunits have a molecular weight of 85 000, identical to that of the M subunits. By contrast with L-type phosphofructokinase, the F-type enzyme seems to exist predominantly in a tetrameric form and not to aggregate to high molecular weight polymers. Specific activity of pure F4 phosphofructokinase is about 140 IU/mg of protein. Immunologically, it is easy to distinguish all the basic phosphofructokinase forms (i.e. M, L and F types); nevertheless a slight immunological cross-reactivity seems to exist between all these forms.