Purification and characterization of X-prolyl-dipeptidyl-aminopeptidase from Lactobacillus lactis and from Streptococcus thermophilus

Purification and partial characterization of an intracellular aminopeptidase from Streptococcus salivarius subsp. thermophilus strain ACA-DC 114

An intracellular aminopeptidase from Streptococcus salivarius subsp. thermophilus strain ACA-DC 114, isolated from traditional Greek yoghurt, was purified by chromatography on DEAE-cellulose and Sephadex G-100. The enzyme had a molecular weight of 89,000. It was active over a pH range 4.5-9.5 and had optimum activity on L-lysyl-4-nitroanilide at pH 6.5 and 35 degrees C with Km = 1.80 mmol/l; above 55 degrees C the enzyme activity declined rapidly. The aminopeptidase was capable of degrading substrates by hydrolysis of the N-terminal amino acid; it had very low endopeptidase and no carboxypeptidase activity. The enzyme was strongly inactivated by EDTA. Serine and sulphydryl group reagents had no effect on enzyme activity.

Purification of X-prolyl dipeptidyl aminopeptidase from Lactobacillus casei subspecies

Prolyl dipeptidylaminopeptidases from two subspecies of Lactobacillus casei were purified and biochemically characterized. L. casei ssp. casei UL21 (a debittering strain) and L. casei ssp. rhamnosus UL26 (a non-debittering strain) were the source bacteria for this study. Purification of the enzymes from both the sources was effected by a gel filtration step through Sephacryl S-300 followed by ion-exchange chromatography through DEAE Sephacel. This rendered an electrophoretically homogeneous enzyme preparation. The purified enzymes from both the sources showed similar temperature optimum (45 degrees C) and pH optimum (7.0). Their activity profiles on various substrates and the nature of inhibition by different inhibitors were also found to be similar, indicating that this enzyme is perhaps not significantly involved in the debittering process during the maturation of cheese.

Molecular cloning and sequence analysis of the X-prolyl dipeptidyl aminopeptidase gene from Lactococcus lactis subsp. cremoris

Lactococcus lactis subsp. cremoris P8-2-47 contains an X-prolyl dipeptidyl aminopeptidase (X-PDAP; EC 3.4.14.5). A mixed-oligonucleotide probe prepared on the basis of the N-terminal amino acid sequence of the purified protein was made and used to screen a partial chromosomal DNA bank in Escherichia coli. A partial XbaI fragment cloned in pUC18 specified X-PDAP activity in E. coli clones. The fragment was also able to confer X-PDAP activity on Bacillus subtilis. The fact that none of these organisms contain this enzymatic activity indicated that the structural gene for X-PDAP had been cloned. The cloned fragment fully restored X-PDAP activity in X-PDAP-deficient mutants of L. lactis. We have sequenced a 3.8-kb fragment that includes the X-PDAP gene and its expression signals. The X-PDAP gene, designated pepXP, comprises 2,289 nucleotide residues encoding a protein of 763 amino acids with a predicted molecular weight of 87,787. No homology was detected between pepXP and genes that had been previously sequenced. A second open reading frame, divergently transcribed, was present in the sequenced fragment; the function or relationship to pepXP of this open reading frame is unknown.

Genetics of the proteolytic system of lactic acid bacteria

The proteolytic system of lactic acid bacteria is of eminent importance for the rapid growth of these organisms in protein-rich media. The combined action of proteinases and peptidases provides the cell with small peptides and essential amino acids. The amino acids and peptides thus liberated have to be translocated across the cytoplasmic membrane. To that purpose, the cell contains specific transport proteins. The internalized peptides are further degraded to amino acids by intracellular peptidases. The world-wide economic importance of the lactic acid bacteria and their proteolytic system has led to an intensive research effort in this area and a considerable amount of biochemical data has been collected during the last two decades. Since the development of systems to genetically manipulate lactic acid bacteria, data on the genetics of enzymes and processes involved in proteolysis are rapidly being generated. In this review an overview of the latest genetic data on the proteolytic system of lactic acid bacteria will be presented. As most of the work in this field has been done with lactococci, the emphasis will, inevitably, be on this group of organisms. Where possible, links will be made with other species of lactic acid bacteria.

X-Prolyl-Dipeptidyl Aminopeptidase of Lactobacillus delbrueckii subsp. bulgaricus : Characterization of the Enzyme and Isolation of Deficient Mutants

Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397 is able to hydrolyze X-proline- para -nitroanilides and X-proline-β-naphthylamides (X for alanyl- or glycyl-). A single metal-independent cytoplasmic enzyme with a molecular weight estimated to be 82,000 is responsible for these activities and was named X-prolyl-dipeptidyl aminopeptidase (X-Pro-DPAP). Isolation and analysis of mutants totally deficient for X-Pro-DPAP activity showed that a total lack of this enzyme induces (i) a decrease in the growth rate; (ii) an increase in cell wall proteinase activity; (iii) the loss of three cell wall proteins with respective molecular masses of 16, 40, and 52 kilodaltons; and (iv) enhancement of a cell wall protein with a molecular mass of 150 kilodaltons. The involvement of X-Pro-DPAP in casein catabolism is discussed.

Purification and Partial Characterization of a Prolyl-Dipeptidyl Aminopeptidase from Lactobacillus helveticus CNRZ 32

X-prolyl-dipeptidyl aminopeptidase, which hydrolyzed Gly-Pro- p -nitroanilide (relative activity [RA] = 100%) and Arg-Pro- p -nitroanilide (RA, 130%), was purified to homogeneity from the cell extract of Lactobacillus helveticus CNRZ 32. The enzyme also hydrolyzed Ala-Pro-Gly (RA, 11%) and Ala-Ala- p -nitroanilide (RA, 2%) but was not active on Ala-Leu-Ala, dipeptides, and endopeptidase and carboxypeptidase substrates. The enzyme was purified 145-fold by streptomycin sulfate precipitation, ammonium sulfate fractionation, and a series of column chromatographies on DEAE-cellulose, arginine-Sepharose 4B, and glycyl-prolyl-AH-Sepharose 4B. The purified enzyme appeared as a single band on native polyacrylamide gel and sodium dodecyl sulfate-polyacrylamide gel electrophoreses and had a molecular weight of 72,000. Optima for activity by the purified enzyme were pH 7.0 and 40�C. The enzyme was incubated at 40�C for 15 min with various metal ions. It was activated by Mg 2+ (2.5 mM), Ca 2+ (0.1 to 2.5 mM), Na + (10 to 50 mM), and K + (10 to 50 mM) and was inhibited by Hg 2+ (0.1 to 2.5 mM), Cu 2+ (0.1 to 2.5 mM), and Zn 2+ (0.1 to 2.5 mM). Enzyme activity was partially inhibited by EDTA (1.0 mM, 20 h at 40�C), 1,10-phenanthroline (1.0 mM, 15 min at 40�C), phenylmethylsulfonyl fluoride (1.0 mM), N -ethylmaleimide (1.0 mM), and iodoacetate (1.0 mM). It was completely inhibited by diisopropyl fluorophosphate (1.0 mM, 2 h at 40�C) and p -chloromercuribenzoate (1.0 mM, 15 min at 40�C). The enzyme was not affected by dithioerythritol (1.0 to 10 mM).

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris AM2

An aminopeptidase was purified from cell extracts of Lactococcus lactis subsp. cremoris AM2 by ion-exchange chromatography. After electrophoresis of the purified enzyme in the presence or absence of sodium dodecyl sulfate, one protein band was detected. The enzyme was a 300-kilodalton hexamer composed of identical subunits not linked by disulfide bridges. Activity was optimal at 40°C and pH 7 and was inhibited by classical thiol group inhibitors. The aminopeptidase hydrolyzed naphthylamide-substituted amino acids, as well as dipeptides and tripeptides. Longer protein chains such as the B chain of insulin were hydrolyzed, but at a much slower rate. The Michaelis constant ( K m ) and the maximal rate of hydrolysis ( V max ) were, respectively, 4.5 mM and 3,600 pkat/mg for the substrate l -histidyl-β-naphthylamide. Amino acid analysis showed that the enzyme contained low levels of hydrophobic residues. The partial N-terminal sequence of the first 19 residues of the mature enzyme was determined. Polyclonal antibodies were obtained from the purified enzyme, and after immunoblotting, there was no cross-reaction between these antibodies and other proteins in the crude extract.