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The maize seed expression system is a powerful platform for recombinant protein production. It has been utilized for high-yield production of industrial cellulases such as endo-1,4-β-D-glucanase (E1) and 1,4-β-D-glucan cellobiohydrolases (CBHI & CBHII). However, inhibitory compounds that could significantly reduce the cellulolytic efficiency were detected even in purified E1 and CBHII samples. After several steps of purification and analysis, the inhibitory compound found in the purified E1 sample had characteristics consistent with a thermostable polysaccharide with a molecular size over 10 kDa. Further polysaccharide analysis confirmed the molecular size of the detected polysaccharide and suggested that it is an arabinoxylan with a high arabinose/xylose ratio. Even though the strict column purification process should have removed water-soluble non-protein molecules during protein purification, the inhibitor …