Requirement of Ca2+ for the gelatinization of pectin solution by a proteinous cell wall-bound factor and solubilization of the gel by acid treatment

The cell wall-bound pectin-gelatinizing factor, whose existence was reported in a previous paper (Yamaoka and Sato, 1981), was found to be composed of a proteinous factor (PGF) and calcium ion. Magnesium ion could not replace Ca²⁺ whereas Sr²⁺ could replace Ca²⁺ and Ba²⁺ could cause gelation without the action of PGF. In the hypocotyl of soybean seedlings, the basal region contained more PGF than did the apical region. The gel of apple pectin formed by the action of PGF and Ca²⁺ was solubilized by acid treatment for some time after the gelation. It was also found that the gel became more and more resistant to acidity with time. The hardened gel was almost completely dissolved by boiling in 0.2 M NaCl solution. The idea that the gelation of apple pectinin vitro may simulate the gelation of pectin in the primary cell wall was discussed in view of the results and the phenomenon of acid growth.