1 mg mL−1EfEndo18A (~3 μM) at 37 °C in 50 mM ammonium acetate buffer pH 6 for 16 h. A 5-μL aliquot of the supernatant was mixed with 7 μL loading buffer (NuPAGE; Invitrogen) and 3 μL reducing agent (NuPAGE; Invitrogen). The protein solutions were boiled for 10 min and analyzed by SDS-PAGE. The rate of hydrolytic activity was determined by incubating
50 μg RNaseB with 25 nM EfEndo18A or 25 nM EndoH from Streptomyces plicatus (NEB) in 50 mM ammonium acetate buffer pH 6 at 37 °C. Samples were then taken every fifth minute over a period of 30 min and analyzed by SDS-PAGE. Carbohydrates in the supernatants of the reactions were analyzed by mass spectrometry (MS) using an Ultraflex MALDI-TOF/TOF instrument (Bruker Daltonics GmbH, Bremen, Germany) controlled by flexcontrol v.3.3. For analysis with click here MS, 1 μL supernatant (diluted 5× in dH2O) was mixed with 2 μL of a 9 mg mL−1 solution of 2.5-dihydroxybenzoic acid in 30% acetonitrile and applied as a droplet to a MTP 384 target plate ground steel TF (Bruker Daltonics). After drying under a stream of air, mass spectra were recorded in the range from m/z 0–3000, and from an average of 300 laser shots with the lowest laser energy necessary to obtain sufficient signal-to-noise ratios.
The following settings were used: reflectron mode with an acceleration voltage of 25 kV, reflector voltage of 26 kV and pulsed ion extraction of 40 ns in the positive ion mode. Peak lists were generated using Bruker flexanalysis software v.3.3. Possible hydrolytic activity of EfEndo18A towards oligosaccharides was tested using the selleck compound chito-oligomer analogues, 4-methylumbelliferyl-β-d-N,N′-diacetylchitobioside Dichloromethane dehalogenase [4-MU-(GlcNAc)2] and 4-methylumbelliferyl-β-d-N-acetylglucosamine (4MU-NAG) as substrates. A 50-μL reaction mixture contained: 0.1 mg mL−1 bovine serum albumin (BSA), 50 μM 4-MU(GlcNAc)2 or 4MU-NAG, 0–50 nM
EfEndo18A in 50 mM citrate phosphate buffer pH 6. After incubation at 37 °C for 10 min, the reaction was stopped by adding 1.95 mL 0.2 M Na2CO3. The amount of released 4-MU was measured using a DyNA 200 Fluorimeter (Hoefer Pharmacia Biotech, San Francisco, CA). The hydrolysis of GlcNAc oligomers was analyzed in a reaction volume of 200 μL containing 0.1 mg mL−1 BSA, 200 μM (GlcNAc)4 or (GlcNAc)6 and 50 nM EfEndo18A in 50 mM ammonium acetate buffer pH 6. After incubation at 37 °C overnight, the reaction was stopped by adding 1 : 1 of 20 mM H2SO4 and reaction products were analyzed using a Dionex Ultimate 3000 HPLC system set up with a Rezex column (Phenomenex, Torrance, CA). The conditions used for the HPLC analysis were: mobile phase, 5 mM H2SO4; flow rate 1 mL min−1, detection of eluted oligosaccharides by recording absorption at 195 nm. The only detectable product, (GlcNAc)2, was quantified using external standards and the chromeleon 7.0 chromatography software (Dionex). Figure 1 shows an alignment of EfEndo18A with the commercial endoglycosidase EndoH from S.