Ascorbate peroxidase isoforms localized in the stroma and thylakoid of higher plant chloroplasts are rapidly inactivated by hydrogen peroxide if the second substrate, ascorbate, is depleted. However, cytosolic and micro-body-localized isoforms from higher plants as well as ascorbate peroxidase B, an ascorbate peroxidase of a red algaGaldieria partita, are relatively tolerant.