The effect of water mobility on horseradish peroxidase (HRP) activity in solutions was investigated by measuring water activity (aw), freezable water content, (1)H proton transverse relaxation time and water self-diffusivity determined by nuclear magnetic resonance. The effect of system mobility as described by viscosity and glass transition temperature (T'g) was also studied. The aw and viscosity of aqueous solutions were modulated using ligands (glucose, sorbitol and trehalose) and a thickener (maltodextrin). The effectiveness of a solute in the inhibition of HRP activity was better related to its ability to reduce the mobility of the system than to its water mobility depleting effect. The relationship among viscosity and peroxidase activity was influenced by the type of enzyme but not by the substrate. Bovine lactoperoxidase activity was hindered by viscosity changes more than HRP activity (tested in the same system) due to the higher molecular weight of the former enzyme.
Keywords: Freezable water; Glass transition temperature; Nuclear magnetic resonance; Peroxidase activity; Viscosity; Water activity; Water mobility.
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