Abstract <p><b>Objective:</b> An important aspect of the processing and pre-processing of cellulose raw materials (including bran) is to obtain a high content of reducing agents in the final product. Experimentally selected process parameters and optimization of pre-processing conditions of plant raw materials, tailored to increase the amount of biologically valuable substances, will reduce the cost of the final product. In this work, the bioconversion of wheat bran polymers was carried out with hydrolytic enzyme preparations (ЕР). <b>Methods:</b> The degree of biotransformation of plant polymers was evaluated using crushed wheat bran with enzyme preparations and their complexes by chemical analysis and HPLC. <b>Results and Discussion:</b> The feedstock (wheat bran) was characterized by a low content of lignin (7.55%) and high pentosan content (17.9%). The largest amount of reducing substances in hydrolysates was determined for ЕР Amylolux ATS (0.23 g/g of raw materials), and for its complexes Amylolux ATS and Cellolux A (0.29 g/g of raw materials). The inclusion of proteolytic enzymes in the ЕР complex increases the amount of amine nitrogen (39.5 mg/g), while reducing kinematic viscosity. A greater amount of mannose (56.0 mg/g of bran), but less pentose (4.1 mg/g of bran) of hydrolysates of enzymatically processed wheat bran was determined in comparison with chemical treatment. <b>Conclusions:</b> Optimal parameters of enzymatic pretreatment of wheat bran for their conversion into biologically valuable carbohydrates have been determined, which is a promising direction of research and their practical use in the production of mannose, biofuels, chemicals and food additives.</p>