Comparison of the OPA Anhydrases

It is natural to wish to impose a classification scheme upon the OPA anhydrases that would imply a set of phylogenetic relationships. The classification scheme of squid-type and Mazur-type anhydrases has proven useful in that it was quickly possible to differentiate the squid-type OPA anhydrase from the other forms. As will be seen below, many of the OPA anhydrase activities lie somewhere in between. 

The multiple activities in T. thermophila share some of the characteristics of both the squid-type OPA anhydrase and classical Mazur-type OPA anhydrase found in hog kidney. In crude preparations, the OPA anhydrase activity has the characteristics of the hog kidney OPA anhydrase in that it hydrolyzes soman faster than DFP, is stimulated by Mn2+, and is inhibited by mipafox. Further purification has revealed that the hydrolysis of soman and the stimulation of this hydrolysis by Mn2+ is principally due to the Tt DFPase-4. The Tt DFPase-1, Tt DFPase-2, and Tt DFPase-3 hydrolyze soman and DFP at approximately the same rates and demonstrate only moderate stimulation of soman hydrolysis by Mn2+ and yet are inhibited by mipafox. The Tetrahymena OPA anhydrases fall within a narrow range from 96,000 Da to 67,000 Da. However, this range of molecular weights is larger than that typically ascribed to the Mazur-type enzymes. The Tetrahymena OPA anhydrases can be purified by ammonium sulfate precipitation, like the squid-type OPA anhydrase. 

Although possessing very similar kinetics and characteristics in homogenate form (Table 10.2), the OPA anhydrase activities of T. thermophila and R. cuneata 

The bacterial activities again point to the diversity of the OPA anhydrases. The OT strain JD100 is able to degrade soman, sarin, and dimebu, but not DFP. The bacterial activities reported to date all seem insensitive to ammonium sulfate inhibitions and have molecular weights above that of the hog kidney OPA anhydrase. 

The opd OPA anhydrase is smaller than the bacterial OPA anhydrase studied to date and has an apparent molecular weight of 60,000 to 65,000 Da with 35,000 Da subunits. To date, the other bacterial OPA anhydrases have not been tested using paraoxon as a substrate although JD6.5 hydrolyzes the related compound NPEPP. 

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