Kinetic analysis of duck epsilon-crystallin, a lens structural protein with lactate dehydrogenase activity

Simultaneous separation of taxon-specific crystallins from Mule duck and characterization of their enzymatic activities and structures

Methods to obtain pure proteins in large amounts are indispensible in protein research. We report here a large-scale/simultaneous isolation of taxon-specific crystallins (ɛ- and δ-crystallin) from the eye lenses of Mule duck. We also investigate the compositions, enzymatic activities, and structures of these purified taxon-specific proteins. A relatively mild method of ion-exchange chromatography was developed to fractionate ɛ-crystallin and δ-crystallin in large amount, ca. ∼6.60mg/g-lens and ∼41.0mg/g-lens, respectively. Both crystallins were identified by electrophoresis, HPLC, and MALDI-TOF-MS. ɛ-Crystallin, with native composition of M_r 142kDa, consisted of two subunits of 35kDa and 36kDa, while δ-Crystallin, with native molecular mass of 200kDa, comprised single subunit of M_r ∼50kDa. Both ɛ- and δ-crystallin were tetramers. The former showed lactate dehydrogenase (LDH) activity, while the latter appeared slightly active in an argininosuccinate lyase (ASL) assay. Raman spectroscopic results indicated that the secondary structures of ɛ- and δ-crystallin were predominantly α-helix as evidenced by the vibrational stretching of amide III over 1260cm^-1 and amide I at 1255cm^-1, in greatly contrast to the anti-parallel β-sheet of α- and β-crystallin as demonstrated by amide III at 1238cm^-1 and amide I at 1672cm^-1. The microenvironments of aromatic amino acids and the status of thiol groups also vary in different crystallins. The compositions, enzyme activities, and structures of the ɛ- and δ-crystalline of Mule duck are different from those of Muscovy duck (Cairina moschata) or Kaiya duck (Anas Platyrhynchos var. domestica), which reflect faithfully species specificity.

Characterization of gamma-crystallin from the eye lens of bullfrog: complexity of gamma-crystallin multigene family as revealed by sequence comparison among different amphibian species

gamma-Crystallin is the major and most abundant lens protein present in the eye lens of lower vertebrates such as amphibian and piscine species. To facilitate structural characterization of gamma-crystallins isolated from the lens of the bullfrog (Rana catesbeiana), a cDNA mixture was synthesized from the poly(A)+mRNA isolated from fresh eye lenses. cDNA encoding gamma-crystallin was then amplified using polymerase chain reaction (PCR) based on two primers designed according to the relatively conserved N- and C-terminal sequences of known gamma-crystallins from teleostean fishes. PCR-amplified product corresponding to gamma-crystallin isoforms was obtained, which was then subcloned in pUC18 vector and transformed into Escherichia coli strain JM109. Plasmids containing amplified gamma-crystallin cDNAs were purified and prepared for nucleotide sequencing by the dideoxynucleotide chain-termination method. Sequencing several clones containing DNA inserts of about 0.54 kb revealed the presence of two isoforms with an open reading frame of 534 base pairs, covering two gamma-crystallins each with a deduced protein sequence of 177 amino acids including the translation-initiating methionine. These gamma-crystallins of pI 6.364 and 6.366 contain a low-methionine content of 2.81%, in contrast to 11-16% obtained for those gamma-crystallins with high-methionine content from most teleostean lenses. Pairwise sequence comparison of bullfrog gamma-crystallins with those published sequences of gamma-crystallins from carp, shark, Xenopus and another Rana frog, bovine, and human lenses indicates that there is only 46-63% sequence similarity among these species, revealing that amphibians possess a very complex and heterogeneous group of gamma-crystallins even from closely related species of Rana frogs. The sequence analysis and comparison of various isoforms of the frog gamma-crystallin family provide a firm basis for identifying these lens proteins as members of a multigene family more complex than that reported for mammalian gamma-crystallins.

Octopus S-crystallins with endogenous glutathione S-transferase (GST) activity: sequence comparison and evolutionary relationships with authentic GST enzymes

S-Crystallin is a major protein present in the lenses of cephalopods (octopus and squid). To facilitate the cloning of this crystallin gene, cDNA was constructed from the poly(A)+ mRNA of octopus lenses, and amplified by PCR for nucleotide sequencing. Sequencing of 10 of 15 positive clones coding for this crystallin revealed three distinct S-crystallin isoforms with 61-64% identity in nucleotide sequences and 42-58% similarity in amino acid sequences when compared with homologous crystallins in squid lenses. These charge-isomeric crystallins also show between 26 and 33% amino acid sequence identity to four major classes of glutathione S-transferase (GST), a major detoxification enzyme present in most mammalian tissues. For further analysis, expression of one of the S-crystallin cDNAs was carried out in the bacterial expression system pQE-30, and the S-crystallin protein produced in Escherichia coli was purified to homogeneity to determine the enzymic properties. We found that the expressed octopus S-crystallin possessed much lower GST activity than the authentic GSTs from other tissues. Sequence comparison and construction of phylogenetic trees for S-crystallins from squid and octopus lenses and various classes of GSTs revealed that S-crystallins represent a multigene family which is structurally related to Alpha-class GSTs and probably derived from the ancestral GST by gene duplication and subsequent multiple mutational substitutions.

Effect of catalytically active recruited crystallins on lens metabolism

The impact on duck lens metabolism of recruited epsilon-crystallin/LDH-B4 and tau-crystallin/enolase was investigated by NMR spectroscopy. A comparison of the duck lens metabolite profile with that of the calf in which recruited crystallins are absent revealed significant increases in ATP, alpha-glycerophosphate (alpha-GP) and pyridine dinucleotide concentrations. The alterations in the concentrations of ATP and alpha-GP appear to be related to both the high concentration of NAD and the elevated reduced to oxidized NADH/NAD+ ratio.

Comparison of stability properties of lactate dehydrogenase B4/epsilon-crystallin from different species

epsilon-Crystallin occurs as an abundant lens protein in many birds and in crocodiles and has been identified as heart-type lactate dehydrogenase (LDH-B4). Lens proteins have, due to their longevity and environmental conditions, extraordinary requirements for structural stability. To study lens-protein stability, we compared various parameters of LDH-B4/epsilon-crystallin from lens and/or heart of duck, which has abundant amounts of this enzyme in its lenses, and of chicken and pig, which have no epsilon-crystallin. Measuring the thermostability of LDH-B4 from the different sources, the t50 values (temperature at which 50% of the enzyme activity remains after a 20-min period) for LDH-B4 from duck heart, duck lens and chicken heart were all found to be around 76 degrees C, whereas pig heart LDH-B4 was less thermostable, having a t50 value of 62.5 degrees C. A similar tendency was found with urea inactivation studies. Plotting the first-order rate constants obtained from inactivation kinetic plots against urea concentration, it was clear that LDH-B4 from pig heart was less stable in urea than the homologous enzymes from duck heart, chicken heart and duck lens. The duck and chicken enzymes were also much more resistant against proteolysis than the porcine enzyme. Therefore, it is concluded that avian LDH-B4 is structurally more stable than the homologous enzyme in mammals. This greater stability might make it suitable to function as a crystallin, as in duck, but is not necessarily associated with high lens expression, as in chicken.

Biochemical characterization of crystallins from pigeon lenses: structural and sequence analysis of pigeon delta-crystallin

Crystallins from pigeon eye lenses were isolated and purified by gel-permeation chromatography and characterized by gel electrophoresis, amino-acid composition and sequence analysis. Alpha- and beta-crystallins could be obtained in relatively pure forms by single-step size-exclusion chromatography whereas an extra step of ion-exchange chromatography was needed for the separation of delta-crystallin from the beta-crystallin fraction. In contrast to most characterized vertebrate species, a large amount of glycogen is eluted as a high molecular form in the first peak of the gel filtration column. Pigeon delta-crystallin, similar to duck and reptilian delta-crystallins, exists as a tetrameric structure of about 200 kDa in the native form and is composed of one major subunit of 50 kDa with heterogeneous isoelectric points spreading in a range of 4.7 to 6.8. In contrast to those obtained from duck, goose and caiman, delta-crystallin isolated from the pigeon lens possessed very little argininosuccinate lyase activity. However, pigeon delta-crystallin can still cross-react with the antibody against enzymically active duck delta-crystallin as revealed by the sensitive immunoblotting technique. It was also shown that the delta-crystallin content of the total pigeon soluble proteins decreased with the age of the animal. Structural analysis of purified delta-crystallin fraction was made with respect to its amino-acid composition and protein primary sequence. N-terminal sequence analysis indicated the presence of blocked amino-termini in all crystallin fractions of pigeon lenses. Therefore, a sequence analysis of PCR (polymerase chain reaction) amplified delta-crystallin cDNA was employed to deduce the protein sequence of this crystallin. Structural comparison of delta-crystallin sequences from pigeon, chicken and duck lenses casts some doubts on the recent claim that His-89-->Gln mutation in the chicken delta-crystallin may account for the loss of argininosuccinate lyase activity in this avian species, as compared to high enzymic activity in the duck crystallin (Barbosa et al. (1991) J. Biol. Chem. 266, 5286-5290).

Sequence analysis of pigeon delta-crystallin gene and its deduced primary structure. Comparison of avian delta-crystallins with and without endogenous argininosuccinate lyase activity

delta-Crystallin is a major lens protein present in the avian and reptilian lenses. To facilitate the cloning of the delta-crystallin gene, cDNA was constructed from the poly(A)+ RNA of pigeon lenses, amplified by the polymerase chain reaction (PCR). The PCR product was then subcloned into pUC19 vector and transformed into E. coli strain JM109. Plasmids purified from the positive clones were prepared for nucleotide sequencing by the dideoxynucleotide chain-termination method. Sequencing two clones, containing 1.4 kb DNA inserts coding for delta-crystallin allowed the construction of a complete, full-length reading frame of 1,417 bp covering a deduced protein sequence of 466 amino acids, including the universal translation-initiating methionine. The pigeon delta-crystallin shows 88, 83 and 69% sequence identity to duck delta 2, chicken delta 1 crystallins and human argininosuccinate lyase respectively. It is also shown that, in contrast to duck delta 2 crystallin which has a high argininosuccinate lyase activity, pigeon delta-crystallin appears to contain very low activity of this enzyme, despite the fact that they share a highly homologous structure. A structural comparison of delta-crystallins with or without enzymatic activity suggested several amino acid replacements which may account for the loss of argininosuccinate lyase activity in the lenses of certain avian species.

Kinetic analysis of duck epsilon-crystallin with L-lactate dehydrogenase activity: determination of kinetic constants and comparison of substrate specificity

A systematic analysis of the kinetic properties of duck lens epsilon-crystallin with lactate dehydrogenase [LDH, (E.C. 1.1.1.27)] activity was carried out by employing some 19 different alpha-keto acids as substrates for this NADH-dependent LDH-catalyzed reaction. The steady-state Michaelis and catalytic constants (Km, kcat) were determined for a broad range of organic compounds. The results provide important insights regarding the binding and affinity of substrates to active sites of this enzyme crystallin and indicate a great potential for the application of the stable epsilon-crystallin as a catalyst to the synthesis of some important chiral alpha-hydroxyacids in a convenient and efficient way. It is also demonstrated for the first time that in addition to the enzymatic activity of lactate dehydrogenase, duck epsilon-crystallin also possesses the enzymatic activity of malate dehydrogenase.

Staining of argininosuccinate lyase activity in polyacrylamide gel

A procedure for the direct staining of argininosuccinate lyase activity in polyacrylamide gel is described. The method was based on coupling one of the enzymatic products fumarate with fumarase and malic enzyme catalyzed reactions. Fumarate was first converted to L-malate by fumarase. Malic enzyme then catalyzed the oxidative decarboxylation of L-malate to give CO2 and pyruvate with concomitant reduction of NADP+ to NADPH. Finally the reducing power of NADPH was coupled to phenazine methosulfate and in turn to nitroblue tetrazolium yielding a deeply colored insoluble formazan which may be quantitized or semiquantitized by densitometer.

Facile synthesis of chiral 2-hydroxy acids catalyzed by a stable duck epsilon-crystallin with endogenous L-lactate dehydrogenase activity

Duck epsilon-crystallin, an abundant structural protein in lenses of some avian species, was shown to possess a genuine and stable L-lactate dehydrogenase (L-LDH; EC 1.1.1.27) activity suitable for the application to enzyme technology as a catalyst for the synthesis of chiral alpha-hydroxy acids. Two pharmaceutically important intermediates, 2-hydroxy acids (S)-2-hydroxybutanoic acid (S)-2-hydroxypentanoic acid have been synthesized in high yields and optical purity utilizing an in situ NADH regeneration system of duck epsilon-crystallin coupled with formate/formate dehydrogenase. This enzyme system is also shown to offer some advantages over the conventional L-LDH sources from several mammalian species.

Biochemical characterization and kinetic analysis of duck delta-crystallin with endogenous argininosuccinate lyase activity

Delta-Crystallin, the most abundant crystallin in the avian species, was isolated and purified from duck lenses. It was shown to possess endogenous argininosuccinate lyase activity catalysing the reversible cleavage of argininosuccinate to give fumarate and arginine with an equilibrium constant of 1.8 +/- 0.23 mM. In contrast, chicken lens delta-crystallin showed only 0.4-0.8% of the enzyme activity of duck delta-crystallin under identical assay conditions. Biochemical comparison of delta-crystallins from these two species revealed distinct differences in their structural and kinetic properties. An activity-staining method was developed for the easy detection of endogenous enzyme activity of delta-crystallin from crude lens extracts of different avian species. Two-dimensional gel electrophoresis of lens homogenates indicated that, in the chicken lens, delta-crystallin is composed mainly of a subunit with a pI of 5.9 and a subunit mass of 50 kDa, whereas that of duck lens possesses various 50 kDa subunits in a pI range of 5.9-6.8. Activity staining corroborated the fact that all charge isoenzymes of duck delta-crystallin possess enzyme activity whereas that of chicken delta-crystallin is devoid of activity. For duck delta-crystallin, variation of the enzyme activity with argininosuccinate concentration in the forward reaction followed saturation kinetics with an apparent Michaelis constant for the substrate of 17 +/- 5 microM. In the reverse reaction, initial-velocity studies showed intersecting patterns. Inhibitions of the forward reaction by products (fumarate and arginine) were both non-competitive with respect to argininosuccinate. Citrulline, an analogue of arginine, inhibited the enzyme activity in both directions and was competitive with respect to arginine but non-competitive with respect to fumarate or argininosuccinate. Succinate, which inhibited the bovine argininosuccinate lyase, did not affect the delta-crystallin enzyme activity in a concentration range between 1 and 300 mM. These results suggest a random Uni Bi kinetic mechanism for the argininosuccinate lyase activity of duck delta-crystallin with the formation of various abortive delta-crystallin-argininosuccinate-arginine, delta-crystallin-argininosuccinate-fumarate and delta-crystallin-argininosuccinate-citrulline ternary complexes.

Purification and characterization of the cytosolic NADP(+)-dependent malic enzyme from human breast cancer cell line

Cytosolic NADP(+)-dependent malic enzyme from a cultured human breast cancer cell line was purified to near homogeneity by two highly efficient chromatography systems: Pharmacia-LKB Q-Sepharose anion-exchange chromatography and adenosine-2',5'-bisphosphate-agarose affinity chromatography. The overall yield was 27%. The enzyme is presumably a tetramer composed of four probably identical subunits of Mr 65,000, which is similar to the enzyme from other sources. The pI and optimum reaction pH values for the tumor malic enzyme are 5.5 and 7.2, respectively. At pH 6.9, most of the enzyme exists as monomers. Activation energy for the enzyme-catalyzed oxidative-decarboxylation reaction is 57.4 kJ/mol. The enzyme is strictly NADP+ dependent, as NAD+ cannot support the oxidative-decarboxylation reaction. ATP at low concentration inhibits the enzyme activity. Fumarate at concentrations up to 5 mM does not affect the enzymatic reaction rate. Therefore the tumor cytosolic malic enzyme, unlike the mitochondrial malic enzyme, is not an allosteric regulatory enzyme.

Kinetic comparison of caiman epsilon-crystallin and authentic lactate dehydrogenases of vertebrates

Kinetic comparison of epsilon-crystallins isolated from the avian and reptilian species and the authentic lactate dehydrogenases (LDHs) was undertaken in order to clarify the identities of these structural lens proteins in relation to their enzymatic activity. Caiman epsilon-crystallin similar to the previously characterized duck epsilon-crystallin appeared to possess a genuine and stable LDH activity as detected by nitro blue tetrazolium staining on polyacrylamide gels and conventional kinetic assays. Kinetic parameters for pyruvate, L-lactate, NAD+, and three structural analogues of the coenzyme in this epsilon-crystallin catalyzed reaction were also determined and compared. Despite the structural similarities between epsilon-crystallins and chicken heart LDH, differences in charge and kinetic properties have been revealed by native isozyme electrophoresis and kinetic analysis as examined by initial velocity and substrate inhibition studies. It is found that the kinetic data analyzed for caiman epsilon-crystallin were more fitted with a compulsory ordered Bi-Bi sequential mechanism similar to those for the authentic LDHs and duck epsilon-crystallin. Caiman epsilon-crystallin has for the first time been established as a heart-type LDH based on the kinetic analysis and comparison with the authentic heart- and muscle-type LDHs from pig and chicken.

Kinetic mechanism of the endogenous lactate dehydrogenase activity of duck epsilon-crystallin

Initial velocity, product inhibition, and substrate inhibition studies suggest that the endogenous lactate dehydrogenase activity of duck epsilon-crystallin follows an order Bi-Bi sequential mechanism. In the forward reaction (pyruvate reduction), substrate inhibition by pyruvate was uncompetitive with inhibition constant of 6.7 +/- 1.7 mM. In the reverse reaction (lactate oxidation), substrate inhibition by L-lactate was uncompetitive with inhibition constant of 158 +/- 25 mM. The cause of these inhibitions may be due to epsilon-crystallin-NAD(+)-pyruvate and epsilon-crystallin-NADH-L-lactate abortive ternary complex formation as suggested by the multiple inhibition studies. Pyruvate binds to free enzyme very poorly, with a very large dissociation constant. Bromopyruvate, fluoropyruvate, pyruvate methyl ester, and pyruvate ethyl ester are alternative substrates for pyruvate. 3-Acetylpyridine adenine dinucleotide, nicotinamide 1,N6-ethenoadenine dinucleotide, and nicotinamide hypoxanthine dinucleotide serve as alternative coenzymes for epsilon-crystallin. All the above alternative substrates or coenzymes showed an intersecting initial-velocity pattern conforming to the order Bi--Bi kinetic mechanism. Nicotinic acid adenine dinucleotide, thionicotinamide adenine dinucleotide, and 3-aminopyridine adenine dinucleotide acted as inhibitors for this enzymatic crystallin. The inhibitors were competitive versus NAD+ and noncompetitive versus L-lactate. alpha-NAD+ was a noncompetitive inhibitor with respect to the usual beta-NAD+. D-Lactate, tartronate, and oxamate were strong dead-end inhibitors for the lactate dehydrogenase activity of epsilon-crystallin. Both D-lactate and tartronate were competitive inhibitors versus L-lactate while oxamate was a competitive inhibitor versus pyruvate. We conclude that the structural requirements for the substrate and coenzyme of epsilon-crystallin are similar to those of other dehydrogenases and that the carboxamide carbonyl group of the nicotinamide moiety is important for the coenzyme activity.

Ostrich crystallins. Structural characterization of delta-crystallin with enzymic activity

Lens crystallins from the African ostrich (Struthio camelus) were isolated and characterized. Four crystallin fractions corresponding to alpha-, delta/beta- and beta-crystallins similar to those of duck crystallins were isolated, but epsilon-crystallin was found to be absent. The native molecular masses and subunit structures of the purified fractions were analysed by gel filtration. SDS/PAGE and isoelectric focusing, revealing various extents of heterogeneity in each orthologous crystallin class. An ion-exchange chromatographic method was used for the large-scale preparation of delta-crystallin suitable for structural and enzymic studies. It was unexpectedly found that the purified native delta-crystallin of ostrich lens possessed high argininosuccinate lyase activity, in contrast with chicken delta-crystallin. The c.d. spectra indicated a predominant beta-sheet structure in alpha- and beta-crystallins, and a significant contribution of alpha-helical structure in the delta-crystallin fraction. The estimate of secondary structures from c.d. spectroscopy for each crystallin class bears a resemblance to that of duck crystallins, except that ostrich delta-crystallin possesses much less helical content than duck delta-crystallin. Comparison of crystallin compositions and structures from aquatic and terrestrial birds revealed distinct differences.