Different Functional and Structural Properties of Lactate Dehydrogenase Isozymes at Different Stages of Danio rerioOntogenesis

We studied properties of lactate dehydrogenase isozymes expressed at different stages of Danio rerioontogenesis. H₄-LDH and a minor fraction H₃M₁are expressed during embryonic development. The muscle isozyme (₄) appears after the beginning of muscle contractions in the embryo. ₄and ₄isozymes isolated from the heart and skeletal muscle of the adult fish, respectively, show significant differences in terms of Michaelis constant (K _m) activation energy (AE), and inactivation temperature. H₄-LDH isozymes isolated from unfertilized eggs, the skeletal muscle of larvae, and the heart of the adult fish differ inK _mand activation energy, as well as in inactivation temperature. We propose that these differences may be associated with a ligand interacting with the H₄isozyme at different steps of ontogenesis.     

The isozymes of glucose-phosphate isomerase (GPI-A2 and GPI-B2) from the teleost fish Fundulus heteroclitus (L.)

The fish, Fundulus heteroclitus (L.), like most advanced teleosts, possesses duplicate loci for the glycolytic enzyme, glucose-phosphate isomerase (D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9). The locus for the GPI-A2 (where GPI represents glucose-phosphate isomerase) isozyme is preferentially expressed in anaerobic tissues such as white skeletal muscle, while GPI-B2 predominates in aerobic tissues like liver and red muscle. We questioned whether this tissue specificity would be reflected in unique structural and functional characteristics of the respective isozymes. Consequently, an analysis of the two isozymes was undertaken. The enzymes were purified by a combination of ion-exchange chromatography and isoelectric focusing. Each isozyme was characterized as to native and subunit molecular weight, isoelectric pH, and susceptibility to thermal denaturation. Both were dimeric enzymes, with native molecular masses of 110 kDa. The isoelectric pH values for GPI-A2 and GPI-B2 were 7.9 and 6.4, respectively. Differences were apparent in thermal stability, i.e. GPI-A2 was more stable than GPI-B2. Kinetic properties were investigated as a function of both pH and temperature. The Km values for fructose 6-phosphate (Fru-6-P) differed between the isozymes at low pH, but no significant differences were observed at higher pH. The inhibition constant (Ki) for 6-phosphogluconate (6-P-gluconate) was pH dependent. GPI-A2 was slightly more sensitive to 6-P-gluconate inhibition than GPI-B2 between pH 7.0 and 8.5. The Km for Fru-6-P was temperature dependent for the GPI-B2 isozyme, but relatively temperature independent for GPI-A2 between 10 and 35 degrees C. The Ki for 6-P-gluconate was temperature dependent for both isozymes. The Ki values for GPI-A2 were consistently lower than those for GPI-B2. Energies of activation differed between the two isozymes by 4.4 kcal with GPI-A2 having the lower value. While delta G values were identical for the isozymes, their delta H and delta S values differed significantly. The structural and kinetic differences that exist between the glucose-phosphate isomerase isozymes appear to be tailored to the unique metabolic demands of the tissues in which these Gpi loci are expressed.     

A multilocus system for studying tissue and subcellular specialization. The pH and temperature dependence of the two major NADP-dependent isocitrate dehydrogenase isozymes of the fish Fundulus heteroclitus

In the teleost fish Fundulus heteroclitus, there are three NADP-dependent isocitrate dehydrogenase isozymes. IDH-B2 is the only cytoplasmic isozyme, and IDH-C2 dominates the mitochondria of all tissues other than liver, where IDH-A2 is expressed. Since fish are ectotherms, their intracellular temperature and pH change directly with environmental temperature. In order to evaluate the influence of these environmental parameters on a model fish NADP-isocitrate dehydrogenase system, the major cytoplasmic (IDH-B2) and mitochondrial (IDH-C2) isozymes were kinetically evaluated as a function of pH and temperature. Whereas Vfmax and KmISOCm (where ISOC is isocitrate) were pH-independent, the Km for NADP was pH-dependent for both isozymes. The cytoplasmic isozyme (IDH-B2) had smaller KmNADP values between pH 7.0 and pH 8.0 than the mitochondrial form (IDH-C2). Vfmax and Km for substrate and coenzyme were temperature-dependent. Energy of activation for IDH-B2 and IDH-C2 was 10.6 and 12.8 kcal/mol, respectively. Both proteins had delta G not equal to values of about 15.8 kcal/mol, with significantly different distributions between delta H not equal to and delta S not equal to. The cytoplasmic isozyme (IDH-B2) appears to have a greater rate of catalysis than the mitochondrial enzyme (IDH-C2) at temperatures less than 30 degrees C. Moreover, the IDH-B2 isozyme had lower KmNADP values than the IDH-C2 isozyme at all temperatures, whereas the KmISOC values for the two isozymes were indistinguishable. Our data suggest that the two major NADP-dependent isocitrate dehydrogenase isozymes have unique physiological and metabolic functions that are adapted to the tissues and cellular compartments in which they are expressed.     

Developmental changes in heart and muscle phosphofructokinase isozymes

Phosphofructokinase isozymes of fetal, neonatal, and adult rat heart and skeletal muscle were characterized by DEAE-cellulose chromatography, agarose gel electrophoresis, and immunodiffusion with specific antisera. The results of these studies indicate that in skeletal muscle and heart the levels of the major liver phosphofructokinase isozyme (PFK-L2) and the muscle phosphofructokinase isozyme (PFK-M) are dependent on the developmental status of the rat. For example, PFK-L2 and PFK-M are present in fetal and early neonatal skeletal muscle; whereas in adult skeletal muscle, only PFK-M is detectable. By DEAE- cellulose chromatography, PFK-L2 activity was estimated to be 2.4 units/g (41% of total phosphofructokinase activity) in fetal muscle, very low and not resolved from PFK-M in 7-day neonatal muscle, and not detectable in adult muscle. Further, PFK-M activity was found to be 3.4 units/g (59% of total phosphofructokinase activity), 10 units/g, and 31.6 units/g in fetal, 7-day neonatal, and adult skeletal muscle, respectively. The developmental changes of heart phosphofructokinase isozymes differ considerably from that of the skeletal muscle phosphofructokinase isozymes. In fetal heart, PFK-L2 is the major phosphofructokinase isozyme (5.6 units/g), constituting 67% of total phosphofructokinase activity. Further, in fetal heart another phosphofructokinase isozyme (33% of total phosphofructokinase activity) was found by DEAE-cellulose chromatography which is different from PFK-M and PFK-L2. In 7-day neonatal and adult heart, PFK-M and PFK-L2 are the only detectable phosphofructokinase isozymes. Varying from 5.6 units/g (44% of total) in 7-day neonatal to 5.9 units/g (40% of total) in adult heart, PFK-L2 activity remains fairly constant. Also, PFK-M is very low in fetal heart but increases within 1 week postpartum to 5.5 units/g (50% of total activity) and to 8.9 units/g (60% of total activity) in adult heart.     

Physiological relevance of the changing subunit composition and regulatory properties of the 6-phosphofructo-1-kinase isozyme pools during heart and muscle development

During postnatal development, the subunit compositions of the 6-phosphofructo-l-kinase isozyme pools of heart and skeletal muscle are known to change. The isozyme pools from fetal muscle were composed of the L-type (60%), and M-type (36%) and C-type (4%) subunits and the isozymes from fetal and early neonatal heart contain nearly equal amounts of all three subunits. During postnatal development of both tissues, the proportion of the M-type subunit increases until it is the only type present in adult muscle and the major subunit in adult heart (7507o). The isozyme pool from fetal muscle exhibit a decreased affinity for fructose-6-P and a greater susceptibility to ATP inhibition compared to the M-rich isozymes which are subsequently present. The isozyme pools from fetal and early neonatal heart, if compared to the M-rich isozymes which are present later during heart development and to the fetal muscle isozymes, exhibited the least affinity for fructose-6-P and the greatest susceptibility to ATP inhibition. Comparison of the isozyme pools containing little or no C-type subunit with those from fetal and early neonatal heart clearly indicates that the presence of substantial levels of the C-type subunit imposed a decreased ability for fructose-2,6-P₂ to both lower affinity for fructose-6-P and antagonize sensitivity to ATP inhibition. Although still not thoroughly appreciated, it appears that the changing nature of the isozyme pools in these tissues permits regulation of glucose metabolism in a manner which allows efficient utilization of nutritional opportunities and which adequately meets the energy requirements of each tissue at different stages of development.Abbreviations PFK 6-phosphofructo-l-kinase - fructose-6-P D-fructose-6-phosphate - fr-t_ose-2,6-P₂ D-fructose-2,6-bisphosphate     

Differential expression of creatine kinase isozymes during development of Xenopus laevis: An unusual heterodimeric isozyme appears at metamorphosis

The creatine kinase (CK) repertoire of Xenopus laevis, which is more complex than that of most other vertebrates, involves at least four genomic loci, all showing developmental and tissue-specific expression. The differential expression of this multilocus CK isozyme system was investigated by immunohistology. Specific monoclonal antibodies (mAb) against the three cytoplasmic CK isozymes of Xenopus laevis were isolated and characterized. Two of these mAbs, anti-CK-IV (DM16) and anti-CK-III (JRM4), were specific for CK-IV and CK-III subunits respectively, as well as for the corresponding homodimeric isozymes, CK-IV/IV and CK-III/III. Anti-CK-II (MRX7) mAb recognizes CK-II subunits and CK-II/III heterodimers; the homodimeric CK-II/III does not occur. Immunohistological localization on larval and adult tissue sections reveals that CK-IV epitopes, beside a generalized tissue distribution, are especially concentrated in the cytoplasm of some particular cells such as the photoreceptors in the outer segment of the retina, certain nerve cells of the spinal cord and spinal ganglia, and in larval hepatocytes. The CK-III III isozyme is specifically expressed in skeletal muscle, its appearance and accumulation occurring in parallel with myoblast differentiation. The CK-II antigen is detected first at the time of metamorphosis is skeletal muscles, as well as in the heart, eyes and brain. In striated musculature the expression of CK-II subunits during metamorphosis results in almost complete replacement of CK-III/III homodimers by CK-II/III heterodimers, as indicated by the progressive masking of CK-III epitope and the corresponding appearance of CK-II antigen. In the adult eyes, CK-II antigens localize at the same particular site of photoreceptors as do CK-IV antigens. Since that antigen represents a heterodimeric CK-II/III isozyme, this implies the activation of both CK-II and CK-III genes, none of which is expressed in larval retina.     

Purification and characterization of adenylate kinase isozymes from rat muscle and liver

Isozymes of adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) were purified from skeletal muscle and liver of rats to essentially homogeneous states by acrylamide gel electrophoresis and sodium dodecyl sulfate gel electrophoresis. The isozyme from muscle was purified by acidification to pH 5.0, and column chromatography on phosphocellulose, Sephadex G-75 and Blue Sepharose CL-6B, while that from liver was purified by column chromatography on Blue Sepharose CL-6B, Sephadex G-75 and carboxymethyl cellulose. By these procedures the muscle isozyme was purified about 530-fold in 29% yield, and the liver isozyme about 3600-fold in 27% yield from the respective tissue extracts. The molecular weights of the muscle and liver isozymes were estimated as about 23 500 and 30 500, respectively, by both sodium dodecyl sulfate gel electrophoresis and molecular sieve chromatography, and no subunit of either isozyme was detected. The isoelectric points of the muscle and liver isozymes were 7.0 and 8.1, respectively. The Km values of the respective enzymes for ATP and ADP were similar, but the Km(AMP) of the liver isozyme was about one-fifth of that of the muscle isozyme. Immunological studies with rabbit antiserum against the rat muscle isozyme showed that the muscle isozyme was abundant in muscle, heart and brain, while the liver isozyme was abundant in liver and kidney.     

Decreases in activation energy and substrate affinity in cold-adapted A4-lactate dehydrogenase: evidence from the Antarctic notothenioid fish Chaenocephalus aceratus

Enzyme function is strongly affected by temperature, and orthologs from species adapted to different thermal environments often show temperature compensation in kinetic properties. Antarctic notothenioid fishes live in a habitat of constant, extreme cold (-1.86 +/- 2 degrees C), and orthologs of the enzyme A4-lactate dehydrogenase (A4-LDH) in these species have adapted to this environment through higher catalytic rates, lower Arrhenius activation energies (Ea), and increases in the apparent Michaelis constant for the substrate pyruvate (Km(PYR)). Here, site-directed mutagenesis was used to determine which amino acid substitutions found in A4-LDH of the notothenioid Chaenocephalus aceratus, with respect to orthologs from warm-adapted teleosts, are responsible for these adaptive changes in enzyme function. Km(PYR) was measured in eight single and two double mutants, and Ea was tested in five single and two double mutants in the temperature range 0 degrees C-20 degrees C. Of the four mutants that had an effect on these parameters, two increased Ea but did not affect Km(PYR) (Gly224Ser, Ala310Pro), and two increased both Ea and Km(PYR) (Glu233Met, Gln317Val). The double mutants Glu233Met/Ala310Pro and Glu233Met/Gln317Val increased Km(PYR) and Ea to levels not significantly different from the A4-LDH of a warm temperate fish (Gillichthys mirabilis, habitat temperature 10 degrees C-35 degrees C). The four single mutants are associated with two alpha-helices that move during the catalytic cycle; those that affect Ea but not Km(PYR) are further from the active site than those that affect both parameters. These results provide evidence that (1) cold adaptation in A4-LDH involves changes in mobility of catalytically important molecular structures; (2) these changes may alter activation energy alone or activation energy and substrate affinity together; and (3) the extent to which these parameters are affected may depend on the location of the substitutions within the mobile alpha-helices, perhaps due to differences in proximity to the active site.     

Kinetic and immunological differences between the retinal specific C4- and B4-lactate dehydrogenase of the cichlid fish Oreochromis mossambicus

The eye-specific C₄-lactate dehydrogenase (LDH) and the widely distributed B₄-LDH isozymes from the fish Oreochromis mossambicus were purified to homogeneity using DEAE Sepharose ion-exchange chromatography and oxamate-linked sepharose affinity-chromatography. Kinetic analysis was performed on pure B₄- and C₄-LDH. The Michaelis-Menten constant (K_m) for B₄-LDH and pyruvate was 32.3μM, for B₄-LDH and lactate 717 μM for C₄-LDH and pyruvate 14.1 μM and for C₄-LDH and lactate 1898 μM. The pure C₄-isozyme was subjected to SDS-polyacrylamide gel electrophoresis and the Coomassie Brilliant Blue-stained band injected into a rat to produce antiserum. The antiserum proved to be C₄-LDH monospecific, which will allow using it to localize the isozyme in the retina at a light and electron microscopical level.     

The allelic isozymes of hexose-6-phosphate dehydrogenase isolated from Fundulus heteroclitus: physical characteristics and kinetic properties

Hexose-6-phosphate dehydrogenase (H6PDH-A2; beta-D-glucose:NAD(P)+ oxido-reductase; E.C. 1.1.1.47) of the teleost Fundulus heteroclitus (L.) shows clinal allelic variation along the east coast of North America. Three of the major allelic isozymes have been purified and compared for native molecular weight, subunit molecular weight, isoelectric point, thermal stability, and steady-state kinetic properties (pH 8.0 and 25 degrees C). Significant differences were found among the allelic isozymes for isoelectric point, thermal stability, and some kinetic parameters. The predominant allelic isozyme in northern populations (H6PDH-AcAc) was found to be more sensitive to heat denaturation than were the predominant homozygous allelic isozymes isolated from southern populations (H6PDH-AaAa and H6PDH-AbAb). The H6PDH-AcAc allelic isozyme had both a significantly greater Km for glucose-6-phosphate than did either of the southern phenotypes and a significantly greater Km for NADP+ and Ki of NAD+ than did one of the southern phenotypes (H6PDH-AaAa). While the allelic isozymes are functionally nonequivalent, it is not yet known whether these differences are reflected at higher levels of biological organization.      

Quantitation of muscle glycogen phosphorylase mRNA and enzyme amounts in adult rat tissues

Mammalian glycogen phosphorylases comprise a family of isozymes that are expressed selectively in a variety of cell types. As an initial step towards understanding the molecular processes that regulate the differential expression of the phosphorylase family, we have begun a quantitative examination of isozyme expression in vivo. In this paper, we report quantitative estimates of the amounts of the muscle (M) isozyme and its mRNA in adult rat tissues. Quantitative estimates of the amount of M-phosphorylase were obtained by an analysis involving electrophoretic transfer of proteins from polyacrylamide gels to nitrocellulose filters and sequential treatment with M-isozyme specific antibody and radioactively- labeled protein A. M-phosphorylase mRNA amounts were determined by an analysis involving transfer of RNA from agarose gels to nitrocellulose filters and subsequent hybridization with radioactively labelled rat M-phosphorylase cDNA. These studies indicate that M-phosphorylase is present in all tissues tested with the possible exception of liver. These are skeletal muscle, heart, brain, stomach, lung, kidney, spleen and testis. Quantitation of M-phosphorylase amounts indicate that there is a wide spectrum of variation (over 1000-fold range) in the relative amounts of the M-isozymes in these tissues. Relative mRNA levels parallel isozyme levels indicating that the major control of expression of this isozyme is governed by mRNA accumulation.     

Phylogeny and ontogeny of the phosphoglycerate mutases - III. Inactivation of rabbit muscle phosphoglycerate mutase (type M isozyme) by the sulfhydryl group reagents

1. The three phosphoglycerate mutase isozymes from mammals (types M, B and MB isozymes) differ in their sensitivity to the - SH group reagents. 2. Rabbit muscle phosphoglycerate mutase (type M isozyme) is reversibly inactivated by tetrathionate, rho-chloromercuribenzoate and Hg2+. 3. Titration with rho-chloromercuribenzoate shows the existence of two sulfhydryl groups per enzyme subunit, the modification of which produces a progressive decline in enzyme activity. 4. The apparent Km values for substrate and cofactor are not affected by tetrathionate treatment. 5. Phosphoglycerate mutase inactivated by tetrathionate and by rho-chloromercuribenzoate is unable to form the functionally active phosphorylenzyme when mixed with glycerate-2,3-P2, and is not protected by the cofactor against heating. 6. Glycerate-2,3-P2 protects against tetrathionate treatment, but fails to protect against Hg2+ and rho-chloromercuribenzoate inactivation.     

AMP deaminase isozymes in human tissues

In human, there are four AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) isozymes: E1, E2, M and L. Chromatographic, electrophoretic and immunological studies showed the existence of isozymes E1 and E2 in erythrocytes, isozyme M in muscle and isozyme L in liver and brain. The tissues such as heart, kidney and spleen contained isozymes E1, E2 and L. Isozymes E1, M and L were isolated as apparently homogeneous preparations. The three isozymes were all tetramers composed of identical subunits, but differing slightly in molecular weight; isozyme E1 showed a subunit molecular weight of 80 000, isozyme M 72 000 and isozyme L 68 000. They were immunologically different from one another. The antisera precipitated only the corresponding enzyme and did not precipitate any other isozyme. The three isozymes were also different in kinetic and regulatory properties. Isozyme E2 was very similar to isozyme E1 in immunological and kinetic properties, although isozyme E2 could be separated from isozyme E1 by phosphocellulose chromatography, and zonal electrophoresis.     

Patterns of gene expression during teleost embryogenesis: Lactate dehydrogenase isozyme ontogeny in the medaka (Oryzias iatipes)

The ontogeny of the lactate dehydrogenase (LDH; EC 1.1.1.27) isozymes during medaka (Oryzias latipes) embryogenesis was determined after the genetic and molecular bases of this multilocus isozyme system were established. Three LDH loci are differentially expressed among the tissues of the adult medaka. The LDH-A locus was expressed almost exclusively in the white skeletal muscle, the LDH-B locus in all tissues examined, and the LDH-C locus in the eye and brain. The contribution of each of these LDH loci was quantitatively determined throughout early medaka embryogenesis by using a combination of electrophoretic, immunochemical, and spectrophotometric procedures. LDH-B₄ is present throughout embryogenesis and is the predominant LDH isozyme during this period. LDH-C subunit activity was first detected 146 hr after fertilization (26°C), 142 hr prior to hatching. LDH-A subunit activity, however, was not detected until after hatching and, then, only as heterotetramers containing LDH-B subunits. The pattern of LDH gene expression during medaka embryogenesis was compared with the patterns of LDH gene expression during early development in five other teleost species. Some common patterns of differential LDH gene expression appear to exist among the teleosts. In all species examined, isozymes encoded in at least one LDH locus, A and/or B, were present throughout development. Those isozymes present continually during embryogenesis also tend to be active in a wide variety of differentiated tissues in the adult fish. Conversely, LDH isozymes which are active in a restricted number of adult tissues are detected only later in embryogenesis. The initiation of LDH-C gene expression, however, is closely coupled with morphological and functional differentiation of those cells in which this locus is predominantly expressed in the adult.     

Chinook salmon NADP+-dependent cytosolic isocitrate dehydrogenase: Electrophoretic and genetic dissection of a complex isozyme system and geographic patterns of variation

Species in the genus Oncorhynchus express complicated isocitrate dehydrogenase (IDHP) isozyme patterns in many tissues. Subcellular localization experiments show that the electrophoretically distinct isozymes of low anodal mobility expressed predominantly in skeletal and heart muscle are mitochondrial forms (mIDHP), while the more anodal, complex isolocus isozyme system predominant in liver and eye is cytosolic (sIDHP). The two loci encoding sIDHP isozymes are considered isoloci because the most common allele at one of these loci cannot be separated electrophoretically from the most common allele of the other. Over 12 electrophoretically detectable alleles are segregating at the two sIDHP* loci in chinook salmon. Careful electrophoretic comparisons of the sIDHP isozyme patterns of muscle, eye, and liver extracts of heterozygotes reveal marked differences between the tissues with regard to both relative isozyme staining and the expression of several common alleles. Presumed single-dose heterozygotes at the sIDHP isolocus isozyme system exhibit approximate 9:6:1 ratios of staining intensity in liver and eye, while they exhibit approximate 1:2:1 ratios in skeletal muscle. The former proportions are consistent with the equal expression of two loci (isolocus expression), while the latter are consistent with the expression of a single locus. Screening of over 10,000 fish from spawning populations and mixed-stock fishery samples revealed that certain variant alleles (*127, *50) are detectable only in liver and eye, while other alleles (*129, *94, and *74) are strongly expressed in muscle, eye, and liver. The simplest explanation for these observations is that the "isolocus" sIDHP system of chinook salmon (and that of steelhead and rainbow trout) results from the expression of two distinct loci (sIDHP-1* and sIDHP-2*) that have the same common allele (as defined by electrophoretic mobility). IDHP expression in skeletal muscle is due to the nearly exclusive expression of the sIDHP-1* locus, while IDHP expression in eye and liver tissues is due to high levels of expression of both sIDHP-1* and sIDHP-2*--giving rise to the isolocus situation in these latter tissues. Direct inheritance studies confirm this model of two genetically independent (disomic) loci encoding sIDHP in chinook salmon. Extensive geographic surveys of chinook salmon populations from California to British Columbia reveal marked differences in allele frequencies at both sIDHP-1* and sIDHP-2* and considerably more interpopulation differentiation than was recognized previously when sIDHP was treated as an isolocus system with only five recognized alleles.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cold-hardiness-specific glutathione reductase isozymes in red spruce. Thermal dependence of kinetic parameters and possible regulatory mechanisms.

The thermal dependence of kinetic parameters has been determined in purified or partially purified preparations of cold-hardiness-specific glutathione reductase isozymes from red spruce (Picea rubens Sarg.) needles to investigate a possible functional adaptation of these isozymes to environmental temperature. We have previously purified glutathione reductase isozymes specific for nonhardened (GR-1NH) or hardened (GR-1H) needles. Isozymes that were distinct from GR-1NH and GR-1H, but appeared to be very similar to each other, were also purified from nonhardened (GR-2NH) or hardened (GR-2H) needles (A. Hausladen, R.G. Alscher [1994] Plant Physiol 105: 205-213). GR-1NH had 2-fold higher Km values for NADPH and 2- to 4-fold lower Km values for oxidized glutathione (GSSG) than GR-2NH, and a similar difference was found between GR-1H and GR-2H. However, no differences in Km values were found between the hardiness-specific isozymes GR-1NH and GR-1H. There was only a small effect of temperature on the Km(GSSG) of GR-1H and GR-2H, and no significant temperature effect on Km(NADPH) or Km(GSSG) could be found for the other isozymes. These results are discussed with respect to "thermal kinetic windows," and it is proposed that the relative independence of Km values to temperature ensures adequate enzyme function in a species that is exposed to extreme temperature differences in its natural habitat. A variety of substrates has been tested to characterize any further differences among the isozymes, but all isozymes are highly specific for their substrates, NADPH and GSSG. The reversible reductive inactivation by NADPH (redox interconversion) is more pronounced in GR-1H than in GR-2H. Reduced, partially inactive GR-1H is further deactivated by H2O2, whereas GR-2H is fully reactivated by the same treatment. Both isozymes are reactivated by GSSG or reduced glutathione. It is proposed that this property of GR-2H ensures enzyme function under oxidative conditions, and that in vivo the enzyme may exist in its partially inactive form and be activated in the presence of increased levels of GSSG or oxidants.     

Molecular forms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase expressed in rat skeletal muscle.

The rat cDNA for the muscle-type (M) isozyme of 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (FBPase-2) contains two putative translation initiation sites. To determine whether the M isozyme expressed in rat skeletal muscle corresponds to the short (PFK2M-sf) or the long (PFK2M-lf) isoform, we have expressed them in Escherichia coli. A third construction was also expressed in which the second ATG codon was deleted (PFK2M-lf delta ATG) to ensure that initiation started at the first ATG. The properties of these recombinant proteins were compared with those of the PFK-2/FBPase-2 present in rat skeletal muscle and liver. The recombinant proteins displayed PFK-2 and FBPase-2 activities and the M(r) values of the subunits measured by SDS-polyacrylamide gel electrophoresis were compatible with the calculated ones. The purified recombinant lf form contained not only the expected lf band (54,500 M(r)) but also the sf band (52,000 M(r)), indicating that the expression system could synthesize the long and the short isoforms from the same mRNA. The kinetic properties of the recombinant sf form were not different from those of the rat muscle enzyme. By contrast, lf delta ATG PFK-2 displayed a higher Km for its substrates and a lower Vmax. Immunoblotting with an antibody directed against the long isoform revealed a 54,500 M(r) band both in the lf and the lf delta ATG recombinant, but no band in rat skeletal muscle extracts. In these extracts, one band of 52,000 and a minor one of 54,500 M(r) were detected by an anti PFK-2/FBPase-2 antibody. The 54,500 M(r) band was recognized by an antibody directed against the L isozyme, suggesting that a small amount of the latter is expressed in skeletal muscle. Thus, the M isozyme differs from the L isozyme by replacement of the first 32 amino acids of the L isozyme by an unrelated nonapeptide.