Highly Efficient Bioconversion of trans-Resveratrol to δ-Viniferin Using Conditioned Medium of Grapevine Callus Suspension Cultures

δ-Viniferin is a resveratrol dimer that possesses potent antioxidant properties and has attracted attention as an ingredient for cosmetic and nutraceutical products. Enzymatic bioconversion and plant callus and cell suspension cultures can be used to produce stilbenes such as resveratrol and viniferin. Here, δ-viniferin was produced by bioconversion from trans-resveratrol using conditioned medium (CM) of grapevine (Vitis labruscana) callus suspension cultures. The CM converted trans-resveratrol to δ-viniferin immediately after addition of hydrogen peroxide (H₂O₂). Peroxidase activity and bioconversion efficiency in CM increased with increasing culture time. Optimized δ-viniferin production conditions were determined regarding H₂O₂ concentration, incubation time, temperature, and pH. Maximum bioconversion efficiency reached 64% under the optimized conditions (pH 6.0, 60 °C, 30 min incubation time, 6.8 mM H₂O₂). In addition, in vitro bioconversion of trans-resveratrol was investigated using CM of different callus suspension cultures, showing that addition of trans-resveratrol and H₂O₂ to the CM led to production of δ-viniferin via extracellular peroxidase-mediated oxidative coupling of two molecules of trans-resveratrol. We thus propose a simple and low-cost method of δ-viniferin production from trans-resveratrol using CM of plant callus suspension cultures, which may constitute an alternative approach for in vitro bioconversion of valuable molecules.