Phomopsis cucurbitae is a latent infection fungus which
causes muskmelon fruit decay only after harvest. It can cause severe losses
during fruit storage and marketing in the U.S.A., Japan and some Central
American countries. Previous studies showed that P.
cucurbitae produced polygalacturonase (PG) in both culture and muskmelon
fruit tissue. In this study, a prominent PG isozyme (PG1)
produced by P. cucurbitae in decayed fruit was purified to homogeneity
by a procedure of extraction, ultrafiltration, preparative
isoelectric focusing, anion exchange, and gel filtration chromatography.
The isozyme had a primary endo-activity, a relative
molecular weight of 54 kD according to SDS-PAGE, and a pI of 4·2
based on IEF-PAGE. The isozyme was active in a low pH range
from 2 to 6, and was optimum at pH 5. Optimum temperature for the activity
of PG1 was 40–45°C. The Km and Vmax
on
polygalacturonic acid were 44·7 μg ml−1
and 0·14 U μg−1
enzyme, respectively. The purified PG1 effectively macerated mature fruit
tissue, suggesting that the isozyme may be involved in the pathogenesis
of Phomopsis fruit rot of muskmelon.