The unrelated salicylate hydroxylases NahG and NahU of the strains Pseudomonasfluorescens 142 NF and P. Putida BS3701 were extracted and purified by ion-exchange and hydrophobic and gel permeation chromatography. The extracted enzymes differed in kinetic and catalyst performance during salicylate hydrolysis. For NahU salicylate hydroxylase, Km and Vmax were found to be higher (3.1 +/- 0.6 microM and 7.7 +/- 0.4 microM/min, respectively) than for NahG salicylate hydroxylase (1.3 +/- 0.1 microM and 4.7 +/- 0.1 microM/min, respectively). The activity of both enzymes toward substituted salicylates was higher in cases where the substituent groups were in para position than in cases with those in meta position. The activity toward substituted salicylates with substituent groups in meta position was different. The activity of salicylate hydroxylase NahG was higher toward salicylates with substituent groups in position 3; salicylate hydroxylase NahU activity was higher toward those with substituent groups in position 5. This suggests a difference in the spatial configuration of active sites in extracted unrelated salicylate hydroxylases.