Résumé
The major limitation of lipase-catalyzed acylation of the polar bioactive dipeptide carnosine (β-Ala-His) is its very low solubility in solvents other than water. In the current work, two approaches were developed to overcome this constraint. The first acylation strategy consisted of the common use of a lipase in an anhydrous organic medium. However, the peptide is only weakly soluble in an anhydrous organic medium. A preliminary step of high pressure homogenisation was performed to improve the dispersion of carnosine particles and to reduce the particle size, thereby enhancing the accessible area of the substrate to the enzyme. The second approach used an aqueous medium, in which carnosine was totally solubilized, and aimed to evaluate the ability of an acyltransferase to catalyze acylation of the peptide. The enzymatic acylation of carnosine in anhydrous organic solvent was catalyzed by lipase B from Candida antarctica. The size of carnosine particles was decreased significantly in the organic solvent after high pressure pre-treatment and high temperature, leading to a substrate conversion yield of 39% after 120 h of reaction. The acyltransferase of Candida parapsilosis was demonstrated to be able to catalyze the acylation of carnosine in aqueous medium. Under these conditions, a substrate conversion yield of 48% was obtained when the reaction was run for 48 h, suggesting that this enzyme may be important to develop processes for acylation of polar peptides. In both reaction systems, the product was identified as N-acyl carnosine. The antioxidant activities of the purified amide were determined and compared to those of carnosine. Its xanthine oxidase inhibition activity was demonstrated to be similar to that of the natural dipeptide, and its superoxide radical scavenging activity was enhanced above that of the natural dipeptide, suggesting that acylation did not affect the antioxidant properties of the peptide.