Biochemical properties of duplicated isozymes of phosphoglucose isomerase in the plant Clarkia xantiana

The structural gene locus specifying subunits of the cytoplasmic isozymes of phosphoglucose isomerase (PGI) is present in duplicate in many diploid species of Clarkia (Onagraceae), a genus of annual plants native to California. We studied the kinetic properties and molecular weights of a large number of genetically defined and highly purified PGIs in C. xantiana, a species with the duplication, as a means of examining the biochemical consequences of the evolution of a new gene locus. This species is primarily outcrossing, but also includes several previously described predominantly self-pollinating populations. Both cytoplasmic PGI loci in the outcrossing populations are polymorphic and their enzyme products are readily separated by electrophoresis. The PGIs from the outcrossing populations were generally closely similar in molecular weight, pH optimum, heat sensitivity, energy of activation, and apparent K _m (fructose-6-phosphate). The PGI loci in the selfing populations are monomorphic and specify enzymes having identical electrophoretic mobilities to those coded by the most frequent alleles of the outcrosser. The PGI isozymes in the selfers differed fivefold in K _m , suggesting that they have a very different catalytic effectiveness. The high K _m of the PGI-3A isozyme (1.1mm) was anomalous among the examined and would likely be disadvantageous in a species which lacked other more normally functioning PGIs. But in the cytoplasm of the selfing plants, it is present with other PGIs that have low K _m values. The PGI-3A enzyme is a good candidate for a gene product coded by a forbidden mutation that could not have been established in the absence of the duplication. The rationale for this suggestion is described and it is also pointed out that the divergence of duplicated genes is influenced by many factors such as the breeding system and other population factors as well as the effect of particular mutations.This research was supported by NSF Grant DEB77-08448.     

Evidence from subunit molecular weight suggests hybridization was the source of the phosphoglucose isomerase gene duplication in Clarkia

Summary The apparent molecular weight of cytosolic phosphoglucose isomerase (PGI) subunits was evaluated in 18 species of Clarkia which have or do not have duplicated genes specifying this glycolytic enzyme. Species that lack the duplication had subunits of 59,000 or 60,400 whereas species with the duplication generally possessed two types of PGI subunits with these or closely similar molecular weights. The additive pattern in the species with the duplication suggests that the molecular weight divergence preceded the origin of the duplication, and that the duplication arose following hybridization between taxa that represented different lineages within Clarkia.     

Escherichia coli phosphoglucose isomerase can be substituted by members of the PGI family, the PGI/PMI family, and the cPGI family

The Escherichia coli strain Pgi-UdhA, a mutant of the strain MG1655, is deficient in both the pgi gene and the udhA gene and cannot grow on glucose as carbon and energy source. This strain was transformed with different pET-plasmids containing archaeal or bacterial pgi, cpgi or pgi/pmi genes from the three known PGI families (PGI, PGI/PMI, cPGI). Growth could be restored upon plasmid-based expression of pgi, pgi/pmi or cpgi genes indicating that these heterologous proteins can substitute for E. coli PGI. However, complete restoration of the growth rate could not be obtained by any of the PGIs, PGI/PMIs, or cPGIs used. The data indicate that the PGI function of the three PGI families is functionally exchangeable in glycolysis.     

Biochemical characterization of recombinant phosphoglucose isomerase of Mycobacterium tuberculosis

Phosphoglucose isomerase (PGI) is a well-characterized ubiquitous enzyme involved in the glycolytic pathway. It catalyzes the reversible isomerization of D-glucopyranose-6-phosphate and D-fructofuranose-6-phosphate and is present in all living cells. However, there is interspecies variation at the level of the primary structure which sometimes produces heterogeneity at the structural and functional levels. In order to evaluate and characterize the mycobacterial PGI, the gene encoding the PGI from Mycobacterium tuberculosis H37Rv was cloned in pET-22b(+) vector and expressed in Escherichia coli. The target DNA was PCR amplified from the bacterial artificial chromosome using specific primers and cloned under the control of T7 promoter. Upon induction with IPTG, the recombinant PGI (rPGI) expressed partly as soluble protein and partly as inclusion bodies. The rPGI from the soluble fraction was purified to near homogeneity by ion-exchange chromatography. Mass spectrum analysis of the purified rPGI revealed its mass to be 61.45 kDa. The purified rPGI was enzymatically active and the specific activity was 600 U/mg protein. The K(m) of rPGI was determined to be 0.318 mM for fructose-6-phosphate and the K(i) was 0.8 mM for 6-phosphogluconate. The rPGI exhibited optimal activity at 37 degrees C and pH 9.0, and did not require mono- or divalent cations for its activity.     

Duplicated phosphoglucose isomerase genes in avocado

Summary Avocado (Persea americana) cultivars were assayed for phosphoglucose isomerase (PGI) isozymes using starch gel electrophoresis. Three PGI genes were identified: one monomorphic locus, Pgi-I, coding for the plastid isozyme and two independently assorting loci, Pgi-2 and Pgi-3, coding for the cytosolic isozymes. The genetic analysis was based on comparisons of PGI zymograms from somatic and pollen tissue and on Mendelian analysis of progeny from selfed trees. The isozymic variability for PGI can be used for cultivar identification and for differentiating between hybrid and selfed progeny in avocado breeding.     

Glucose-6-phosphate isomerase from the hyperthermophilic archaeon<Emphasis Type="Italic"> Methanococcus jannaschii:</Emphasis> characterization of the first archaeal member of the phosphoglucose isomerase superfamily

ORF MJ1605, previously annotated as pgi and coding for the putative glucose-6-phosphate isomerase (phosphoglucose isomerase, PGI) of the hyperthermophilic archaeon Methanococcus jannaschii, was cloned and functionally expressed in Escherichia coli. The purified 80-kDa protein consisted of a single subunit of 45 kDa, indicating a homodimeric (₂) structure. The K _m values for fructose 6-phosphate and glucose 6-phosphate were 0.04 mM and 1 mM, the corresponding V _max values were 20 U/mg and 9 U/mg, respectively (at 50 °C). The enzyme had a temperature optimum at 89 °C and showed significant thermostability up to 95 °C. The enzyme was inhibited by 6-phosphogluconate and erythrose-4-phosphate. RT-PCR experiments demonstrated in vivo expression of ORF MJ1618 during lithoautotrophic growth of M. jannaschii on H₂/CO₂. Phylogenetic analyses indicated that M. jannaschii PGI was obtained from bacteria, presumably from the hyperthermophile Thermotoga maritima.     

Control of photosynthate partitioning in spinach leaves

Experiments were carried out to estimate the elasticity coefficients and thence the distribution of control of sucrose synthesis and photosynthate partitioning between cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase (SPS), by applying the dualmodulation method of Kacser and Burns (1979, Biochem. Soc. Trans. 7, 1149–1161). Leaf discs of spinach (Spinacia oleracea L.) were harvested at the beginning and end of the photoperiod and illuminated at five different irradiances to alter (i) the extent of feedback inhibition and (ii) the rate of photosynthesis. The rate of CO₂ fixation, sucrose synthesis and starch synthesis were measured and compared with the activation of SPS, and the levels of fructose-2,6-bisphosphate (Fru2,6bisP) and metabolites. Sucrose synthesis increased progressively with increasing irradiance, accompanied by relatively large changes of SPS activity and Fru2,6bisP, and relatively small changes of metabolites. At each irradiance, leaf discs harvested at the end of the photoperiod had (compared with leaf discs harvested at the beginning of the photoperiod) a decreased rate of sucrose synthesis, increased starch synthesis, decreased SPS activity, increased Fru2,6bisP, a relatively small (20%) increase of most metabolites, no change of the glycerate-3-phosphate: triose-phosphate ratio, a small increase of NADPmalate dehydrogenase activation, but no inhibition of photosynthesis. The changes of sucrose and starch synthesis were largest in low light, while the changes of SPS and Fru2,6bisP were as large, or even larger, in high light. It is discussed how these results provide evidence that the control of sucrose synthesis is shared between SPS and fructose-1,6-bisphosphatase, and provide information about the in-vivo response of these enzymes to changes in the levels of their substrates and effectors. At low fluxes, feedback regulation is very effective at altering partitioning. In high light, changes of SPS activation and Fru2,6bisP can be readily overriden by increasing levels of metabolites.     

Molecular analysis of the plant gene encoding cytosolic phosphoglucose isomerase

The gene encoding a cytosolic isozyme of phosphoglucose isomerase (PGI, EC 5.3.1.9) was isolated from Clarkia lewisii, a wild flower native to California, and the structure and sequence of the entire coding region determined. PGI catalyzes an essential step in glycolysis and carbohydrate biosynthesis in plants. Spanning about 6 kb, the gene has 23 exons and 22 introns, the highest number yet reported in plants. The exons range in size from 43 to 156 nt and encode a protein of 569 amino acids. The protein is about 44–46% identical to the inferred protein sequences of pig, Escherichia coli and Saccharomyces cerevisiae. All of the introns are bordered with the consensus 5-GT...AG-3 dinucleotides. The longest intron includes a large stem-loop structure bounded by a perfect 9 nt direct repeat. We cloned the PGI gene from a genomic library prepared from a single plant of known PGI genotype. The locus and allele of the clone were identified by matching restriction fragments to fragments from genetically defined genomic DNAs by Southern hybridization.     

Evolutionary genetics of Metridium senile. I. Kinetic differences in phosphoglucose isomerase allozymes

Populations of the sea anemone Metridium senile from the northeast coast of the United States exhibit a one-locus, two-allele polymorphism for phosphoglucose isomerase. No additional hidden variation is exposed by changes in pH, gel pore size, or heat denaturation. The allozymes are similar in pH optimum, sensitivity of K _m to pH, and sensitivity of K _m and V _max to temperature. In other respects they are functionally different, with the fast allozyme having a 3.5-fold higher specific activity and a slightly higher K _m of fructose-6-phosphate than the slow form. In these respects, heterozygotes produce a mixture of enzymes that appears to function roughly as the sum of its component parts. Comparisons of V _max/K _m ratios reveal significant differences among genotypes, with the fast form having higher values at all temperatures than the slow form and heterozygotes falling intermediate. In addition, there is a significant difference among genotypes in sensitivity of this parameter to temperature, with the fast homozygote and heterozygote displaying greater sensitivity than the slow homozygote. Temperature is probably an important selective agent in maintaining this polymorphism.Supported by Grant T-4 from the Health Research and Services Foundation, NSF DEB77-14442, NIH GM25809, and NIH GM28024.     

Isolation and molecular analysis of the phosphoglucose isomerase structural gene of Saccharomyces cerevisiae

Summary The PGI1 gene of Saccharomyces cerevisiae coding for the glycolytic enzyme phosphoglucose isomerase has been cloned by complementation of a mutant strain (pgi1) with a strongly reduced phosphoglucose isomerase activity. A genomic library constructed in the yeast multicopy vector YEp13 (Nasmyth and Tatchell 1980) was used. Four plasmids containing an overlapping region of 4.1 kb were isolated and characterized by restriction endonuclease mapping. Southern analysis of genomic digests prepared with different restriction enzymes confirmed the same pattern for the chromosomal sequences. Transformants with the isolated plasmids had a phosphoglucose isomerase activity increased by a factor of 7. The cloned sequence hybridized to a constitutively synthesized 2.2 kb RNA in Northern analysis. The coding region includes a 2.05 kb EcoRI fragment common to all four inserts. A fragment including part of the PGI1 region was subcloned into vector YRp7 and used to induce integration at the PGI1 locus. Genetical and Southern analysis of stable transformants showed that single as well as tandem integration took place at this locus. This showed that the PGI1 gene had been isolated. Finally, and in contrast to the results of Kempe et al. (1974a, b) who reported three isoenzymes in yeasts, only one copy of the PGI1 gene per genome was found in several laboratory strains tested by Southern analysis.     

A phosphoglucose isomerase gene is involved in the Rag phenotype of the yeast Kluyveromyces lactis

Summary The rag2 mutant of Kluyveromyces lactis cannot grow on glucose when mitochondrial functions are blocked by various mitochondrial inhibitors, suggesting the presence of a defect in the fermentation pathway. The RAG2 gene has been cloned from a K. lactis genomic library by complementation of the rag2 mutation. The amino acid sequence of the RAG2 protein deduced from the nucleotide sequence of the cloned RAG2 gene shows homology to the sequences of known phosphoglucose isomerases (PGI and PHI). In vivo complementation of the pgi1 mutation in Saccharomyces cerevisiae by the cloned RAG2 gene, together with measurements of specific PGI activities and the detection of PGI proteins, confirm that the RAG2 gene of K. lactis codes for the phosphoglucose isomerase enzyme. Complete loss of PGI activity observed when the coding sequence of RAG2 was disrupted leads us to conclude that RAG2 is the only gene that codes for phosphoglucose isomerase in K. lactis. The RAG2 gene of K. lactis is expressed constitutively, independently of the growth substrates (glycolytic or gluconeogenic). Unlike the pgi1 mutants of S. cerevisiae, the K. lactis rag2 mutants can still grow on glucose, however they do not produce ethanol.     

Functional phosphoglucose isomerase from Mycobacterium tuberculosis H37Rv: rapid purification with high yield and purity

Phosphoglucose isomerase (PGI) EC 5.3.1.9, is a housekeeping enzyme that catalyzes the reversible isomerization of d-glucopyranose-6-phosphate and d-fructofuranose-6-phosphate. We have previously reported expression and multistep purification of recombinant PGI from Mycobacterium tuberculosis using conventional methods. We now describe an improved and simplified single step approach for purification of functionally active mycobacterial rPGI. The gene encoding PGI from M. tuberculosis H37Rv was cloned in bacterial expression vector pET22b(+). Expression of recombinant PGI with six-histidine-tag protein was observed both in the soluble fraction and inclusion bodies. Approximately 116mg of recombinant enzyme was purified to near homogeneity with approximately 80% yield from the soluble fraction of 1L culture at shake flask level using one step Ni-NTA affinity chromatography. The specific activity of the purified six-histidine-tagged recombinant PGI (rPGI-His(6)) was approximately 800U/mg of protein. The apparent K(m) value of the active recombinant protein followed Michaelis-Menten kinetics and was 0.27+/-0.03mM. K(i) for the competitive inhibitor 6-phosphogluconate was 0.75mM. The enzyme had pH optima in the range of pH 7.6-9.0 and was stable up to 55 degrees C. rPGI-His(6) exhibited enzyme activity almost equal to that of enzyme without histidine tag.     

Inhibition of phosphoglucose isomerase allozymes from the wing polymorphic waterstrider,Limnoporus canaliculatus,by pentose shunt metabolites

Inhibition of phosphoglucose isomerase (PGI) allozymes from the wing-polymorphic waterstrider, Limnoporus canaliculatus, by three pentose-shunt metabolites was studied at several different temperatures. This was done to determine if the allozymes exhibited a differential ability to participate in lipid biosynthesis via differential partitioning of carbon flux through the pentose shunt versus glycolysis. 6-Phosphogluconate and erythrose-4-phosphate proved to be strong competitive inhibitors of PGI, while sedoheptulose-7-phosphate was a very weak inhibitor. The PGI allozymes from L. canalicualtus were differentially inhibited by 6-phosphogluconate at two of the three temperatures studied. However, this property does not appear to be an adaptive difference between the allozymes but, rather, a correlated effect resulting from variation in substrate binding. Estimates of reaction rates for the allozymes indicate that the differences in inhibition result in no detectable differences in reaction velocities. Thus, no evidence in support of the hypothesis that PGI allozymes from Limnoporus canaliculatus were adapted to function in different metabolic capacities via differential inhibition was obtained in this study. However, the importance of this characteristic in allozymic adaptation in natural populations remains an open question.Supported by NSF Grant DEB 7908802 and UPHS Grant GM 21133 to R. K. Koehn and an NSF dissertation improvement grant to A. J. Zera.     

Bifunctional phosphoglucose/phosphomannose isomerase from the hyperthermophilic archaeon <Emphasis Type="Italic">Pyrobaculum aerophilum</Emphasis>

ORF PAE1610 from the hyperthermophilic crenarchaeon Pyrobaculum aerophilum was first annotated as the conjectural pgi gene coding for hypothetical phosphoglucose isomerase (PGI). However, we have recently identified this ORF as the putative pgi/pmi gene coding for hypothetical bifunctional phosphoglucose/phosphomannose isomerase (PGI/PMI). To prove its coding function, ORF PAE1610 was overexpressed in Escherichia coli, and the recombinant enzyme was characterized. The 65-kDa homodimeric protein catalyzed the isomerization of both glucose-6-phosphate and mannose-6-phosphate to fructose-6-phosphate at similar catalytic rates, thus characterizing the enzyme as bifunctional PGI/PMI. The enzyme was extremely thermoactive; it had a temperature optimum for catalytic activity of about 100°C and a melting temperature for thermal unfolding above 100°C.     

Malic enzyme,phosphoglucomutase and phosphoglucose isomerase isozymes inTrichogramma [Hym.: Trichogrammatidae]

ME, PGM and PGI electrophoretic banding patterns in 20 laboratory cultures representing 14 species ofTrichogramma were studied. Variations in PGM were found inT. exiguum, T. marylandense, andT. pretiosum. PGI also showed variation inT. exiguum, T. marylandense, T. minutum, andT. parkeri. However, ME variations were found only inT. pretiosum. Based on progeny analyses, we concluded that ME is a tetramer inTrichogramma with fast and slow alleles at a single locus, and that both PGM and PGI have a single locus and each has four alleles. PGM is a monomer, but PGI is a dimer.

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Résumé Les bandes électrophorétiques de l'enzyme malique, de la P.G.M. et de la P.G.I. ont été étudiées chez 20 souches de laboratoire représentant 14 espèces deTrichogramma. Des variations de la P.G.M. ont été trouvées chezT. exiguum, T. marylandense etT. pretiosum. La. P.G.I. montre aussi des variations chezT. exiguum, T. marylandense, T. minutum etT. parkeri. Par contre, des variations de l'enzyme malique ne sont trouvées que chezT. pretiosum. En nous basant, sur l'analyse de progénitures, nous avons conclu que l'enzyme malique est un tétramère chezTrichogramma comprenant un allèle “lent” et un alléle “rapide”, à un seul locus, et que la P.G.M. et la P.G.I. ont chacune un seul locus à quatre allèles. La P.G.M. est un monomère mais la P.G.I. est un dimère.

     

Coordinate control of sucrose formation in soybean leaves by sucrose-phosphate synthase and fructose-2,6-bisphosphate

Net photosynthesis (CER), assimilate-export rate, sucrose-phosphate-synthase (EC 2.4.1.14) activity, fructose-2,6-bisphosphate content, and 6-phosphofructo-2-kinase (EC 2.7.1.105) activity were monitored in leaves of soybean (Glycine max (L.) Merr.) plants during a 12:12 h day-night cycle, and in plants transferred, at regular intervals throughout the diurnal cycle, to an illuminated chamber for 3 h. In the control plants, assimilate-export rate decreased progressively during the day whereas in transferred plants, a strongly rhythmic fluctuation in both CER and export rate was observed over the 24-h test period. Two maxima during the 24-h period for both processes were observed: one when plants were transferred during the middle of the normal light period, and a second when plants were transferred during the middle of the normal dark period. Overall, the results indicated that export rate was correlated positively with photosynthetic rate and sucrose-phosphate-synthase activity, and correlated negatively with fructose-2,6-bisphosphate levels, and that coarse control and fine control of the sucrose-formation pathway are coordinated during the diurnal cycle. Diurnal changes in sucrose-phosphate-synthase activity were not associated with changes in regulatory properties (phosphate inhibition) or substrate affinities. The biochemical basis for the diurnal rhythm in sucrose-phosphate-synthase activity in the soybean leaf thus appears to involve changes in the amount of the enzyme or a post-translational modification that affects only the maximum velocity.Abbreviations FBPase fructose-1,6-bisphosphatase - SPS sucrose-phosphate synthase - F26BPase fructose-2,6-bisphosphatase - PGI glucose-6-phosphate isomerase - F6P fructose-6-phosphate - F26BP fructose-2,6-bisphosphate - G6P glucose-6-phosphate - CER net carbon exchange rate - Pi inorganic phosphate - DHAP dihydroxyacetone phosphate - PGA glycerate 3-phosphate - F6P,2-kinase 6-phosphofructo-2-kinase Cooperative investigations of the U.S. Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh. Paper No. 10503 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601     

Decreased-activity mutants of phosphoglucose isomerase in the cytosol and chloroplast of Clarkia xantiana. Impact on mass-action ratios and fluxes to sucrose and starch, and estimation of Flux Control Coefficients and Elasticity Coefficients.

1. Subcellular-compartment-specific decreased-activity mutants of phosphoglucose isomerase in Clarkia xantiana were used to analyse the control of sucrose and starch synthesis during photosynthesis. Mutants were available in which the plastid phosphoglucose isomerase complement is decreased to 75% or 50% of the wild-type level, and the cytosol complement to 64%, 36% or 18% of the wild-type level. 2. The effects on the [product]/[substrate] ratio and on fluxes to sucrose or starch and the rate of photosynthesis were studied with the use of saturating or limiting light intensity to impose a high or low flux through these pathways. 3. Removal of a small fraction of either phosphoglucose isomerase leads to a significant shift of the [product]/[substrate] ratio away, from equilibrium. We conclude that there is no 'excess' of enzyme over that needed to maintain its reactants reasonably close to equilibrium. 4. Decreased phosphoglucose isomerase activity can also alter the fluxes to starch or sucrose. However, the effect on flux does not correlate with the extent of disequilibrium, and also varies depending on the subcellular compartment and on the conditions. 5. The results were used to estimate Flux Control Coefficients for the chloroplast and cytosolic phosphoglucose isomerases. The chloroplast isoenzyme exerts control on the rate of starch synthesis and on photosynthesis in saturating light intensity and CO2, but not at low light intensity. The cytosolic enzyme only exerts significant control when its complement is decreased 3-5-fold, and differs from the plastid isoenzyme in exerting more control in low light intensity. It has a positive Control Coefficient for sucrose synthesis, and a negative Control Coefficient for starch synthesis. 6. The Elasticity Coefficients in vivo of the cytosolic phosphoglucose isomerase were estimated to lie between 5 and 8 in the wild-type. They decrease in mutants with a lowered complement of cytosolic phosphoglucose isomerase. 7. The implications of these results for regulation and for evolution are discussed.     

Control of photosynthetic sucrose synthesis by fructose-2,6-bisphosphate

The metabolite levels in the mesophyll of leaves of Zea mays L. have been compared with the regulatory properties of the cytosolic fructose-1,6-bisphosphatase from the mesophyll to show how withdrawal of triose phosphate for sucrose synthesis is reconciled with generation of the high concentrations of triose phosphate which are needed to allow intercellular diffusion of carbon during photosynthesis. i) A new technique is presented for measuring the intercellular distribution of metabolites in maize. The bundle-sheath and mesophyll tissues are partially separated by differential homogenization and filtration through nylon nets under liquid nitrogen. ii) considerable gradients of 3-phosphoglycerate, triose phosphate, malate and phosphoenolpyruvate exist between the mesophyll and bundle sheath which would allow intercellular shuttles to be driven by diffusion. These gradients could result from the distribution of electron transport and the Calvin cycle in maize leaves. iii) consequently, the mesophyll contains high concentrations of triose phosphate and fructose-1,6-bisphosphate. iv) Most of the regulator metabolite fructose-2,6-bisphosphate, is present in the mesophyll. v) The cytosolic fructose-1,6-bisphosphatase has a lower substrate affinity than that found for the enzyme from C₃ species, especially in the presence of inhibitors like fructose-2,6-bisphosphate. vi) This lowered affinity for substrate makes it possible to reconcile use of triose phosphate for sucrose synthesis with the maintenance of the high concentration of triose phosphate in the mesophyll needed for operation of photosynthesis in this species.Abbreviations DHAP Dihydroxyacetonephosphate - Fru1,6-bisP fructose-1,6-bisphosphate - Fru2,6bisP fructose-2,6-bisphosphate - PEP(Case) phosphoenolpyruvate (carboxylase) - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase     

Phenotypes of Phosphoglucose isomerase,phosphoglucose mutase and general protein in some freshwater molluscs from Basrah,Iraq

Electrophoretic variants of phosphoglucose isomerase (EC.5.3.1.9) and phosphoglucose mutase (EC.2.7.5.1) have been studied in eight species of freshwater molluscs. Two phenotypes of phosphoglucose isomerase were observed in Melanopsis nodosa and one phenotype was observed in the rest of the species. One phenotype of phosphoglucose mutase was observed in all the species of molluscs studied. Phosphoglucose isomerase is inferred to be a dimer encoded at a single polymorphic locus in Melanoides nodosa. There are two alleles at this locus. Phosphoglucose mutase is inferred to be a monomer encoded at a single monomorphic locus in all species. The electrophoretic analysis revealed that phosphoglucose isomerase enzyme cannot be considered a good taxonomic criterion to differentiate the different members of the six families studied but, on the other hand, it is considered a good taxonomic criterion to differentiate Melanopsis nodosa and Theodoxus jordani. Phosphoglucose mutase is considered a good taxonomic criterion to differentiate the family Melanidae from the remaining five families studied. General protein can be considered a good taxonomic criterion to differentiate the family Corbicullidae from Melanidae, Viviparidae and Neritidae but, on the other hand, it seems to be a less useful taxonomic criterion to differentiate between the Viviparidae and Neritidae.