Cloning, sequence, expression and characterization of human beta-mannosidase

Beta-mannosidase (EC 3.2.1.25, MANB) dissects the non-reducing end of N-linked mannose moieties of glycoproteins in eukaryotic cells. The human beta-mannosidase gene was amplified by RT-PCR, cloned and sequenced. The DNA sequence was compared with reported human beta-mannosidase DNA sequence and sixteen nucleotide differences were found. The deduced amino-acid sequence showed that seven codons coded the same amino acids and nine codons coded different amino acids with reference to nucleotide substitution positions but did not affect recombinant MANB enzyme activity. No splice mutation was observed after comparison with reported MANB DNA sequences. A 75% homology of deduced amino-acid sequence was observed with mouse, goat and bovine beta-mannosidase amino-acid sequences. The cloned beta-mannosidase gene was subcloned into pET22b+ and pET28a+ expression vectors to transform the BL21-codon plus cells for expression of recombinant MAN22 and MAN28 enzymes, respectively. The optimized conditions for overexpression of recombinant beta-mannosidase enzyme were induction with 1 mM IPTG for 12 h at 37 degrees C. The expressed beta-mannosidase enzyme was purified to homogeneity by a combination of DEAE-ion exchange and size exclusion chromatography. The molecular mass of MAN22 and MAN28 enzymes is 97 kDa by SDS/PAGE and is confirmed by western blot analysis. The recombinant enzymes are active at 37 degrees C and at pH 5.0 and showed activity with p-nitrophenyl-beta-d-mannopyranoside and not with p-nitrophenyl-alpha-d-mannopyranoside. The K(m) value of enzymes was 2.53 mM. The enzyme activity was inhibited by Zn(2+), Co(2+), Cu(2+), Pb(2+), Ag(1+), iodoacetate, SDS, DMF, DMSO and ethanol. Fe(3+), Ca(2+) Mg(2+), Mn(2+), Triton X-100 and PMSF did not inhibit the enzyme activity. Northern blot analysis showed a transcript of about 3.7 kb in all cells and tissues studied. This is the first report on the expression and characterization of recombinant human MANB enzyme.