EFFECT OF METAL-IONS ON PEROXIDASE-ACTIVITY FROM GRAPEVINE CELLS

Grapevine (Vitis vinifera cv. Monastrell) suspension cell cultures tre ated with 1.5 mM fosetyl-Al, a systemic fungicide for grapevine diseas es caused by oomycetes, showed a reduction in the level of peroxidase (EC 1.11.1.7) activity found in the culture medium with respect to non -inoculated cells. This effect of fosetyl-Al on peroxidase was due to Al3+ To determine if this effect was due to the binding of Al3+ to fun ctional groups of the plasma membrane/cell wall interface, the effect of other cations with similar or different binding preference sequence s was tested. The effectiveness of these other cations in lowering per oxidase activity followed the order La > Cd/Hg > Ni/Zn/Co > Li, which is in accordance with their valence (C3+ > C2+ > C+) and with ionic ra dius [La (1.15 Angstrom) > Cd/Hg (0.97-1.10 Angstrom) > Ni/Zn/Co (0.72 -0.74 Angstrom) > Li (0.60 Angstrom)]. However, although valence and i onic radius criteria are sufficient to explain the effects of metal io ns on total peroxidase activity, it is necessary to understand their l igand bonding preferences to explain the effects of metals on particul ar peroxidase isoenzymes. Of the ions tested, only La3+ reproduced at an isoenzyme level the effect observed with fosetyl-Al. This is not su rprising since La3+ is the only cation together with Al3+ that belongs to class A metals, with a preference sequence for cellular ligands, O > N > S. From these results it may be concluded that the effect of fo setyl-Al (Al3+) on peroxidase is caused by aluminum binding to critica l binding sites in the plasma membrane/cell wall interface, in which b ackbone carbonyls, side-chain carboxylates and alcohol functions appea r to be the main targets for fosetyl-Al (Al3+) action.