Reaction Product Variability and Biological Activity of the Lactoperoxidase System Depending on Medium Ionic Strength and pH, and on Substrate Relative Concentration

Abstract

The potential of ions produced in water by the lactoperoxidase system against plant pests has shown promising results. We tested the bioactivity of ions produced by the lactoperoxidase oxidation of I and SCN in several buffers or in tap water and characterized the ions produced. In vitro biological activity was tested against Penicillium expansum, the causal agent of mold in fruits, and the major cause of patulin contamination of fruit juices and compotes. In buffers, the ionic concentration was increased 3-fold, and pathogen inhibition was obtained down to the 1:15 dilution. In tap water, the ionic concentration was weaker, and pathogen inhibition was obtained only down to the 1:3 dilution. Acidic buffer increased ion concentrations as compared to less acidic (pH 5.6 or 6.2) or neutral buffers, as do increased ionic strength. C-labelled SCN and MS showed that different ions were produced in water and in buffers. In specific conditions the ion solution turned yellow and a product was formed, probably diiodothiocyanate (I SCN ), giving an intense signal at 49.7 ppm in C-NMR. The formation of the signal was unambiguously favored in acidic media and disadvantaged or inhibited in neutral or basic conditions. It was enhanced at a specific SCN : I ratio of 1:4.5, but decreased when the ratio was 1:2, and was inhibited at ratio SCN >I . We demonstrated that the formation of the signal required the interaction between I and SCN , and MS showed the presence of I SCN .

Effect of the Nature and Relative Concentration of Substrate, Water Mineralization, and Storage Temperature on the Oxidants Produced by Lactoperoxidase and on Their Antifungal Activity against Penicillium expansum and Botrytis cinerea

“…[9,10]. While studying biological lactoperoxidase activity in the presence of an SCN − /I − mixture, a few articles have addressed the products generated under such and such experimental conditions [11,12,21,27]; several have showed that a more stable oxidative complex was formed in specific pH and ionic strength conditions [25,[27][28][29]. We found OSCN − after LPO oxidation of SCN − and I 2 after LPO oxidation of I − .…”

“…SO 4 2− was measured in high concentrations, demonstrating that SCN − was oxidized. 13 C NMR showed that no interhalogen such as ICN, I(SCN) 2 − , or I 2 SCN − was observable [25,38]. Anionic chromatography did not detect OI − .…”

“…The pathogens at a final concentration of 1.10 6 spores/mL were cultured for five days in threefold-diluted sterile potatodextrose broth in the presence or absence (control) of an inhibiting solution, in the dark at 22 • C in ELISA plate wells, following the protocol of [25]. The controls were prepared in four replicates.…”

“…Optical densities (ODs) at 650 nm were measured after 48 h and 120 h of incubation and mean ODs at 120 h were calculated to perform data analysis. The percentage of inhibition of a pathogen was calculated using the following formula [25]:…”

“…The effect of substrate alone (KI, KSCN and H 2 O 2 ) on the growth of P. expansum and B. cinerea have been performed in the reference [11,25] and no inhibitory effect of substrates alone have been observed.…”