Immunological relationships of bacterial L-asparaginases.

Rabbit antisera against L-asparaginase preparations from Escherichia coli, Erwinia carotovora, Citrobacter sp. and Chromobacterium violaceum showed on immunoelectrophoresis that only the enzymes from E. coli and Citrobacter are immunologically related. Purified preparations had to be used to determine the immunological cross-reactions. Immunoelectrophoresis at different pH values yielded the zero mobility points of the enzymes. The activity of the Er. carotovora preparation was enhanced up to fourfold by homologous antiserum but not by normal sera. Heterologous antisera also enhanced, but only at a higher concentration. Less enhancement was observed for the other enzymes with antisera as well as with bovine serum albumin. Inhibition was not observed.     

Structural aspects of L-asparaginases, their friends and relations

Enzymes capable of converting L-asparagine to L-aspartate can be classified as bacterial-type or plant-type L-asparaginases. Bacterial-type L-asparaginases are further divided into subtypes I and II, defined by their intra-/extra-cellular localization, substrate affinity, and oligomeric form. Plant-type L-asparaginases are evolutionarily and structurally distinct from the bacterial-type enzymes. They function as potassium-dependent or -independent Ntn-hydrolases, similar to the well characterized aspartylglucosaminidases with (alphabeta)2 oligomeric structure. The review discusses the structural aspects of both types of L-asparaginases and highlights some peculiarities of their catalytic mechanisms. The bacterial-type enzymes are believed to have a disordered active site which gets properly organized on substrate binding. The plant-type enzymes, which are more active as isoaspartyl aminopeptidases, pose a chemical challenge common to other Ntn-hydrolases, which is how an N-terminal nucleophile can activate itself or cleave its own alpha-amide bond before the activation is even possible. The K+ -independent plant-type L-asparaginases show an unusual sodium coordination by main-chain carbonyl groups and have a key arginine residue which by sensing the arrangement at the oligomeric (alphabeta)-(alphabeta) interface is able to discriminate among substrates presented for hydrolysis.     

Differential medium for revelation of bacterial producer strains of L-asparaginases

A specific, fast, and easy method for revelation of active plate producers of L-asparaginase using differential medium on the basis of LB or M9 with 1.5% agar was developed. Each 100 ml of LB or M9 medium additionally contained 6–7 ml of glycerol, 4 g of L-asparagine, 0.2 g of CaCO₃, and diagnostic components: 3 ml of 0.2 M CuSO₄ · 5H₂O and 2.5 ml of 0.1 M K₃Fe(CN)₆, pH 7.6–7.8. The results were counted 12–20 or 24–48 h after strain growth at 37°C in corresponding mediums. Red color of colonies and colored zone around them showed the ability of the strain under study to destroy asparaginic complexes. The recommended method allows revealing bacterial strains producing L-asparaginase with specific activity of not less than 0.1–3.0 MU/mg of protein.     

Tumor inhibitory and non-tumor inhibitory L-asparaginases from Pseudomonas geniculata.

Two enzymes that catalyze the hydrolysis of l-asparagine have been isolated from extracts of Pseudomonas geniculata. After initial salt fractionation, the enzymes were separated by chromatography on diethylaminoethyl-Sephadex and purified to homogeneity by gel filtration, ion-exchange chromatography, and preparative polyacrylamide electrophoresis. The enzymes differ markedly in physicochemical properties. One enzyme, termed asparaginase A, has a molecular weight of approximately 96,000 whereas the other, termed asparaginase AG, has a molecular weight of approximately 135,000. Both enzymes are tetrameric. The asparaginase A shows activity only with l-asparagine as substrate, whereas the asparaginase AG hydrolyzes l-asparagine and l-glutamine at approximately equal rates and it is also active with d-asparagine and d-glutamine as substrates. The asparaginase A was found to be devoid of antitumor activity in mice, whereas the asparaginase AG was effective in increasing the mean survival times of both C3H mice carrying the asparagine-requiring Gardner 6C3HED tumor line and Swiss mice bearing the glutamine-requiring Ehrlich ascites tumor line. These differences in antitumor activity were related to differences in the K(m) values for l-asparagine for the two enzymes. The asparaginase A has a K(m) value of 1 x 10(-3) M for this substrate whereas the corresponding value for the AG enzyme is 1.5 x 10(-5) M. Thus the concentration of asparagine necessary for maximal activity of the asparaginase A is very high compared with that of the normal plasma level of asparagine, which is approximately 50 muM.     

The mechanism of autocatalytic activation of plant-type L-asparaginases

Plant l-asparaginases and their bacterial homologs, such as EcAIII found in Escherichia coli, form a subgroup of the N-terminal nucleophile (Ntn)-hydrolase family. In common with all Ntn-hydrolases, they are expressed as inactive precursors that undergo activation in an autocatalytic manner. The maturation process involves intramolecular hydrolysis of a single peptide bond, leading to the formation of two subunits (alpha and beta) folded as one structural domain, with the nucleophilic Thr residue located at the freed N terminus of subunit beta. The mechanism of the autocleavage reaction remains obscure. We have determined the crystal structure of an active site mutant of EcAIII, with the catalytic Thr residue substituted by Ala (T179A). The modification has led to a correctly folded but unprocessed molecule, revealing the geometry and molecular environment of the scissile peptide bond. The autocatalytic reaction is analyzed from the point of view of the Thr(179) side chain rotation, identification of a potential general base residue, and the architecture of the oxyanion hole.     

Beta-aspartylpeptides as substrates of L-asparaginases from Escherichia coli and Erwinia chrysanthemi

L-Asparaginase is known to catalyze the hydrolysis of L-asparagine to L-aspartic and ammonia, but little is known about its action on peptides. When we incubated L-asparaginases purified either from Escherichia coli or Erwinia chrysanthemi - commonly used as chemotherapeutic agents because of their antitumour activity - with eight small beta-aspartylpeptides such as beta-aspartylserineamide, beta-aspartylalanineamide, beta-aspartylglycineamide and beta-aspartylglycine, we found that both L-asparaginases could catalyze the hydrolysis of five of them yielding L-aspartic acid and amino acids or peptides. Our data show that L-asparaginases can hydrolyze beta-aspartylpeptides and suggest that L-asparaginase therapy may affect the metabolism of beta-aspartylpeptides present in human body.     

Specificity of molecular recognition in oligomerization of bacterial L-asparaginases

Bacterial L-asparaginases, which are widely used in the antitumor therapy, act only as homotetramers, because their active sites are located at the interface between the subunits of these enzymes. High stability of aspaginase tetramers is determined by ion pair formation between subunits, and this suggests high specificity of molecular recognition during oligomerization of bacterial L-asparaginases. In this study we have investigated specificity of molecular recognition in oligomerization of some bacterial asparaginases by a biosensor based on surface plasmon resonance. It was shown, that a stable tetrameric complex could be formed only by the subunits of the same L-asparaginase. Using two mutants of Helicobacter pylori L-asparaginase it was shown that even a single point mutation at the interface of highly homologous and closely related subunits significantly reduced specificity of molecular recognition.     

Determination of argininosuccinate in normal blood serum and liver

Argininosuccinate has been determined in normal serum and liver extract of rats by chromatographic analysis on Dowex-1-acetate after a brief period of in vivo labeling with l-[U-¹⁴C] citrulline. The amounts found were within the range of other amino acid intermediates of the ornithine cycle, determined in the same samples. Both ¹⁴C and ninhydrin color were measured. Chromatography on Amberlite columns, subsequent to fractionation on Dowex, allowed determination of the other amino acids. Fractionation on Dowex-1-acetate provides a reliable and highly selective method for the analysis of minute amounts of argininosuccinate in biological samples containing other amino acids.