Lupin peroxidases. II. Binding of acidic isoperoxidases to cell walls

Abstract

Extracellular acidic isoperoxidases (EC 1.11.1.7), isolated from both the cell walls and intercellular spaces of lupin (Lupinus albus L. cv. multolupa) hypocotyls, bound to water‐insoluble pectins of wall fragments also isolated from the hypocotyls. The binding was sáturable by increasing the isoenzyme concentration in the assay medium and it was dependent on the pH; neutral pH (6.0–7.0) favoured release, while acidic pH (4.0–5.0) favoured the attachment to the cell wall. Binding of acidic isoperoxidases to wall fractions was correlated with the in vitro acid‐induced growth of hypocotyl segments, and both were modulated in the same direction by the Ca2+/H+ ratio in the incubation media, although the two responses were clearly separated when the Ca2+/H+ ratio varied. Binding of acidic isoperoxidases of cell walls could operate as a fine control of the activity of these cell wall enzymes, although its physiological role in the cell wall stiffening remains unclear. Some aspects of Ca2+ on the control of peroxidase activity at this level are also discussed.