Labial glands of Helicoverpa zea produced a glucose oxidase (GOX), which was present in the saliva and midgut lumen. We purified GOX 23-fold by isoelectric focusing of labial gland homogenates and investigated physical and kinetic properties of the enzyme. D-glucose and 6-deoxy-D-glucose were the optimal substrates of 22 carbohydrates tested with GOX. The enzyme was not inhibited by several inhibitors of fungal GOX but was sensitive to HgCl(2). Labial gland GOX activities varied daily during larval development with highest activities found when larvae were actively feeding. Based on pH optimum, pI, molecular weight estimate and K(m(glucose)), the insect enzyme is not derived from fungal GOXs but appears to have similar kinetic and physical attributes to other insect GOXs. Some possible functions are discussed, including antimicrobial properties, manipulating midgut O(2<B>) levels, altering host plant defense responses, and metabolizing carbohydrates. Arch. Copyright 1999 Wiley-Liss, Inc.