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thiaminase An enzyme present in many species of micro‐organisms, plants, and fish which splits thiamin (vitamin B1), forming products that have anti‐vitamin activity. Non‐enzymic cleavage of thiamin, for example by polyphenols, is also sometimes called thiaminase action. Chastek paralysis in foxes and mink fed diets rich in raw fish, and blind staggers in horses and other animals eating bracken fern, are due to acute vitamin B1 deficiency caused by dietary thiaminase.