Oaks emit large amounts of isoprene, a compound that plays an important role in tropospheric chemistry. Isopentenyl diphosphate isomerase (IDI, E.C. 126.96.36.199) catalyzes the isomerization of isopentenyl diphosphate (IDP) to dimethylallyl diphosphate (DMADP), and in isoprene-emitting plants, isoprene synthase (IS) converts the DMADP to isoprene. To study the role of IDI in isoprene biosynthesis of oak leaves, we compared IDI and IS activities in pedunculate oak (Quercus robur L.) and pubescent oak (Quercus pubescens Willd.) with the isoprene emission rates of these species. We developed a non-radioactive enzyme assay to detect IDI activity in crude leaf extracts of Q. robur. The substrate dependency of IDI activity showed biphasic kinetics with Michaelis constants (K(m)(IDP)) of 0.7 +/- 0.2 micro M for a high-affinity phase and 39.5 +/- 6.9 micro M for a low-affinity phase, potentially attributable to different IDI isoforms. Under standard assay conditions, the temperature optimum for IDI activity was about 42 degrees C, but IDI activity was detectable up to 60 degrees C. A sharp pH optimum appeared around pH 7, with 20 mM Mg(2+) also required for IDI activity. Neither IDI activity nor IS activity showed diurnal variation in Q. robur leaves. The sum of IDI activities showed a significant linear correlation with IS activity in both Q. robur and Q. pubescens leaves, and both enzyme activities showed a linear relationship to isoprene emission factors in leaves of these oak species, indicating the possible involvement of IDI in isoprene biosynthesis by oak leaves.