Purification and characterization of polygalacturonase-inhibiting protein from Asian pear varieties

Polygalacturonase inhibitory protein (PGIP) was extracted from 'Shinli' pear tissue, purified and partially characterized. Extraction was carried out at 4 degrees C with a high ionic strength extraction buffer. After dialysis and concentration by ultrafiltration, the extract was chromatographed on size-exclusion chromatography (S-100), and its active fractions were applied on concanavalin A-Sepharose. The PGIP activity was bound by the lectin, and then eluted using I M a-methyl mannopyranoside, resulting in an 18-fold purification of the PGIP and demonstrating its glycoprotein nature. The following ion-exchange chromatography gave a PGIP that was 360-fold purified relative to the initial tissue extract, and having a 45 kDa molecular weight, as estimated by SDS-PAGE electrophoresis. PGIP inhibitory activity was tested against A. niger, C. acutatum and B. cinerea. The radial diffusion and reducing sugar assays showed that PGIP inhibition of three PGs was affected by pH. In vivo tests revealed that PGTP inhibited polygalacturonase from all three fungi. Heated for 20 min at 85 degrees C, the inhibitory activity of PGIP was reduced by 85-90%, and it was completely suppressed after being heated at 100 degrees C for 20 min.