Isocitrate lyase, ICL (EC 4.1.3.1), an enzyme that cleaves isocitrate into succinate, and glyoxylate appears to play a pivotal role in the detoxification of aluminum (Al) in Pseudomonas fluorescens. Here, we present evidence that the 4-fold increase in ICL activity observed in Al-stressed cells is due to the overexpression of this enzyme. Blue-Native-PAGE, Western blotting, and spectrophotometric experiments revealed that ICL is optimally expressed at 35h of growth in Al-stressed cells. However, following the immobilization of Al, at 60 h of growth, the level of the enzyme decreases markedly. This enzyme that exists as a homotetramer with a molecular mass of similar to 133 kDa appears to be transcriptionally regulated. The overexpression of ICL may be a specific response to Al-stress as P. fluorescens grown in the presence of such metals as Ga3+, Pb2+, and Ca2+ does not undergo any significant increase in ICL activity. Thus, these findings support the notion that the overexpression of ICL plays a pivotal role in the survival and in the increased oxalogenesis observed in Al-stressed P. fluorescens. (C) 2004 Elsevier Inc. All rights reserved.