Interactions of isothiocyanates with egg white proteins

Abstract

SummaryGlucosinolates and their very highly reactive breakdown products (mainly isothiocyanates = ITC) belong to one of the most important natural toxicants. This paper deals with the interactions of allyl-, butyl-, phenyl-and benzyl ITC with egg white proteins resulting in the formation of ITC-proteinconjugates. The ITC's react with amino groups by forming thiourea derivatives. Such reactions are well described by changes in the amounts of free amino groups and available lysine. Further reaction sites are sulphhydryl side chains of proteins leading to dithiocarbamate esters. The investigations further show that such reactions are accompanied with a decrease in the solubility of the prepared conjugates. Simultaneously a shift of the isoelectric range to a lower value of pH can also be observed. Changes in electrophoretic patterns and mobility can also be seen due to the strong electrophilic attack of ITC to proteins. Comparison of the 4 investigated preparations shows that benzyl-ITC is the most reactive partner for egg white proteins. ZusammenfassungGlucosinolate und ihre reaktiven Spaltprodukte (hauptsachlich Isothiocyanate = ITC) zahlen zu den wichtigsten natiirlichen Schadstoffen. Wie gezeigt wird, fiihren Wechselwirkungen von Allyl-, Butyl-, Benzyl-und Phenyl-ITC mit Eiklarproteinen (frisches Eiklar) zur Bildung von ITC-ProteinKonjugaten. Die ITC reagierenmit Aminogruppen unter Bildung von Thioharnstoff-Derivaten. Diese Reaktionen lassen sich iiber Anderungen (Abnahme) im Gehalt an freien Aminogruppen und an verfiigbarem Lysin gut beschreiben. Die Sulfhydryl-Gruppen der Proteine reagieren ebenfalls mit den ITC unter Bildung von Dithiocarbamidsaureester (Nachweis iiber die Schwefelkohlenstoff-Freisetzung aus diesen Estern). Infolge dieser Reaktionen wird in Abhangigkeit von Art und Menge des eingesetzten Isothiocyanates die pH-abhangige Proteinloslichkeit herabgesetzt, wobei gleichzeitig eine Verschiebung des isoelektrischen Bereiches in Richtung niedrigerer pH-Werte erfolgt. Die elektrophile Reaktion der ITC mit den Proteingruppen fiihrt weiterhin zu einer Veranderung im elektrophoretischen Verhalten (Harnstoff-und SDS-PAGE) der Proteine. Von den untersuchten Isothiocyanaten zeigt sich das Benzyl-ITC als der reaktivste Partner gegenuber Eiklarproteinen.

Reactivity and Stability of Glucosinolates and Their Breakdown Products in Foods

“…A second effect was a reduced tryptic degradability, and the authors hypothesized that some of the OH functions of tyrosine may be modified 79. The reduced tryptic digestibility of proteins caused by isothiocyanate modification at pH values between 7.5 and 9 was also reported by other groups and has been attributed to a modification of lysine and probably arginine side chains 166b. 170 Additionally, the nitrogen atom of the secondary amine tryptophan can also be a target for modification by isothiocyanates.…”

“…Kroll et al. (1994) demonstrated that a similar isothiocyanate concentration (15.2 μmol benzyl isothiocyanate per gram of protein) reduced the available lysine content of the egg white by 6.6 % 166b. Therefore, these reactions will occur in sulfur‐ or lysine‐rich foods in a combination with ingredients/side dishes from Brassica plants and for example meat or eggs, and it is expected that modified proteins will be of lower nutritional quality.…”

Glucosinolates inBrassicavegetables: The influence of the food supply chain on intake, bioavailability and human health

“…The basis for the distribution of ITCs throughout the body is the reversibility of their binding to amino-and thiol-groups and especially the high degree of binding to cysteine and GSH [162,163,181,194]. The resulting thioldisulphide exchange give rise to an enormous range of possible intermediates and transport forms of ITCs, e.g.…”

Bioavailability of Isothiocyanates From Broccoli Sprouts in Protein, Lipid, and Fiber Gels

“…A less explored area is the role of other food components on bioavailability of ITCs during consumption of vegetables, as it happens in a regular meal. ITCs can react with free amino groups and sulfhydryl side chains of proteins, potentially reducing ITC bioaccessibility. On the contrary, an in vivo study showed that after consumption of a whole meal that included meat, the absorption of ITC was between one‐ and fivefold higher compared to ingestion without meat depending on the type of ITC .…”