Properties of a polygalacturonic acid-synthesizing enzyme system from Phaseolus aureus seedlings

A cell-free enzyme system from Phaseolus aureus seedlings, which is capable of catalyzing the synthesis of polygalacturonic acid, was found to utilize UDP- d-galacturonic acid in preference to other nucleoside diphosphate d-galacturonic acid derivatives. It did not incorporate d-galacturonic acid residues from UDP-methyl- d-galacturonate into polygalacturonic acid. Maximum rate of polymerization occurs where preparations from seedlings germinated for 3 days are reacted with substrate at 30 °, pH 6, in the presence of 1.7 m m MnCl 2 and 0.4 m sucrose. The enzyme system has an apparent Michaelis constant of 1.7 × 10 −6 m, and at a substrate concentration of 3.7 × 10 −5 m will catalyze the polymerization of d-galacturonic acid residues at the rate of 4.7 mμmoles per milligram protein per minute. The enzyme is inactivated spontaneously, the rate of which is a function of temperature and pH. The addition of MnCl 2 delays inactivation.