Exceptional thermal stability of therapeutical enzymes entrapped in alumina sol–gel matrices†
Alumina sol–gel matrices emerge in this study as very interesting biocompatible superior materials for entrapment and stabilization of enzymes. Three therapeutic enzymes were selected for this study: acid phosphatase (AcP) which is used for treatment of bone dysplasia; a peroxidase (HRP), used for release of toxic drugs in vivo from their pro-drugs; and asparaginase (ASP) used for starving cancerous leukemia cells. The thermal stability of enzyme@alumina is so high that by heating, for instance, AcP@alumina to 60 °C, the enzymatic activity is not only kept, but actually increases, whereas is solution at that temperature there is total denaturation; and cycles of heating–cooling AcP@alumina at that temperature leave it unaffected. An unprecedented increase of six-orders of magnitude in the Arrhenius pre-factor was observed! This indicates the beneficial role of the microporosity in this type of entrapment. This exceedingly high stability is seen also with the other two enzymes: thus, when HRP or ASP are heated in solution to 75 °C the activity drops by 65% and 72%, respectively, but when entrapped it drops only by 1.2% and 1.9%, respectively. The thermal stability was studied in detail, by the follow-up of kinetics, by differential scanning calorimetry and by circular dichroism. These techniques indicate that the entrapment shifts the temperatures of denaturation higher by 30–50 °C. A special protein-friendly synthetic procedure using ultrasound was developed for the synthesis of these biomaterials. As alumina is already approved for injections (as an adjuvant in vaccinations), these findings highlight the possibility of overcoming the hurdle of the use of sol–gel materials as injectable carriers of therapeutic components.