Regulation of plant cell-wall pectin methyl esterase by polyamines--interactions with the effects of metal ions.

The kinetic study of the de-esterification of natural pectin by soya bean or orange pectin methyl esterase shows that the rate of the reaction is highly controlled by the presence of polyamines. The reaction rate versus the polyamine concentration is a bell-shaped curve similar to that which is obtained when the concentration of salts is varied in the reaction mixture. However polyamines, in particular the largest ones, are more efficient than salts. The results may be interpreted by assuming that polyamines mainly interact with the negative charges of the pectic substrate which condition the binding of the pectin methyl esterase. Activating effects were observed at polyamine concentrations that have been shown to exist in the plant cell wall in vivo. Thus, polyamines may act as efficient regulators of the cell-wall pH via the control of the electrostatic cell-wall potential. If such is the case, they might have a role in all regulatory mechanisms in which cell-wall enzymes are involved.