Inhibition of tomato pectin methylesterase by partially purified kiwi pectin methylesterase inhibitor protein

The inhibition of tomato pectin methylesterase (PME) by a recently discovered kiwi pectin methylesterase inhibitor (PMEI) is described. PME was consequently purified by CM Sephadex C‐50, Concanavalin A‐Sepharose 4 B and Mono S chromatography, and PMEI by Q‐Sepharose and Sephacryl S‐200 chromatography. Inhibition of tomato PME activity under optimal conditions (0.125 m NaCl, pH 7.5) by partially purified kiwi PMEI (MW of 27 kD, pI ≥ 3.67) was independent of the PMEI/PME ratio between 36 and 61% of the maximal uninhibited activity. the non‐competitive inhibition observed was optimal in the pH range of 5 to 7. PMEI was inactivated by heating to 120°C, and showed actinidin‐like activity towards N‐α‐benzyloxycarbonyl‐L‐lysine p‐nitrophenyl ester (CBZ‐lys‐ONp) and azocasein which was partially inhibited by the protease inhibitor leupeptin.