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Separation and purification of the alkaline phosphatase and a phosphodiesterase from Halobacterium cutirubrum.

ABSTRACT: 1. Halobacterium cutirubrum alkaline phosphatase is associated in crude extracts with a phosphodiesterase. 2. The enzymes were stabilized in buffers containing both (NH4)2SO4 and 10 mM-Mn2+. 3. Adsorption chromatography on Sepharose 6B/agarose-gel columns in the presence of 1.4M-(NH4)2SO4 gave a phosphatase-free phosphodiesterase and the alkaline phosphatase associated with some phosphodiesterase activity. 4. Further chromatography of the separated enzymes gave a good recovery of greater than 600-fold purified phosphodiesterase and greater than 3000-fold purified alkaline phosphatase. 5. The requirements of these enzymes and their relationship to each other was examined. 6. A detailed study showed that the alkaline phosphatase was adsorbed at least partially to agarose and dextran columns at all (NH4)2SO4 concentrations from 0.25 to 2M. 7. In contrast, no adsorption of the enzyme or protein standards was evident in 2.5M-KCl/l M-NaCl or 0.25 M-KCl/0.1 M-NaCl, in agreement with previous studies by Louis, Peterkin & Fitt [(1971) Biochem. J. 121, 635-641], thus confirming the validity of gel filtration in 2.5 M-KCl/1 M-NaCl as a method for determining the approximate molecular weights of extremehalophile proteins.

SUBMITTER: Fitt PS

PROVIDER: S-EPMC1161167 | biostudies-other | 1979 Aug

REPOSITORIES: biostudies-other

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Project description:Phosphodiesterase activity is a novel property of the still-enigmatic alkaline phosphatase from osseous plate. Bis-(p-nitrophenyl) phosphate was hydrolysed at both pH 7.5 and 9.4 with an apparent dissociation constant (K0.5) of 1.9 mM and 3.9 mM respectively. The hydrolysis of p-nitrophenyl-5'-thymidine phosphate followed hyberbolic kinetics with a K0.5 of 500 microM. For p-nitrophenyl phenylphosphonate, site-site interactions [Hill coefficient (h) = 1.3] were observed in the range between 0.2 and 100 microM, and K0.5 was 32.8 mM. The hydrolysis of cyclic AMP by the enzyme followed more complex kinetics, showing site-site interactions (h = 1.7) and K0.5 = 300 microM for high-affinity sites. The low-affinity sites, representing 85% of total activity, also showed site-site interactions (h = 3.8) and a K0.5 of about 22 mM. ATP and cyclic AMP were competitive inhibitors of bis-(p-nitrophenyl) phosphatase activity of the enzyme and Ki values (25 mM and 0.6 mM for cyclic AMP and ATP respectively) very close to those of the K0.5 (22 mM and 0.7 mM for cyclic AMP and ATP respectively), determined by direct assay, indicated that a single catalytic site was responsible for the hydrolysis of both substrates. Non-denaturing PAGE of detergent-solubilized enzyme showed coincident bands on the gel for phosphomonohydrolase and phosphodiesterase activities. Additional evidence for a single catalytic site was the similar pKa values (8.5 and 9.7) found for the two ionizing groups participating in the hydrolysis of bis-(p-nitrophenyl) phosphate and p-nitrophenyl phosphate. The alkaline apparent pH optima, the requirement for bivalent metal ions and the inhibition by methylxanthines, amrinone and amiloride demonstrated that rat osseous-plate alkaline phosphatase was a type I phosphodiesterase. Considering that there is still confusion as to which is the physiological substrate for the enzyme, the present results describing a novel property for this enzyme could be of relevance in understanding the mineralization process.

Project description:Catalytic promiscuity, an evolutionary concept, also provides a powerful tool for gaining mechanistic insights into enzymatic reactions. Members of the alkaline phosphatase (AP) superfamily are highly amenable to such investigation, with several members having been shown to exhibit promiscuous activity for the cognate reactions of other superfamily members. Previous work has shown that nucleotide pyrophosphatase/phosphodiesterase (NPP) exhibits a >10?-fold preference for the hydrolysis of phosphate diesters over phosphate monoesters, and that the reaction specificity is reduced 10³-fold when the size of the substituent on the transferred phosphoryl group of phosphate diester substrates is reduced to a methyl group. Here we show additional specificity contributions from the binding pocket for this substituent (herein termed the R' substituent) that account for an additional ~250-fold differential specificity with the minimal methyl substituent. Removal of four hydrophobic side chains suggested on the basis of structural inspection to interact favorably with R' substituents decreases phosphate diester reactivity 10?-fold with an optimal diester substrate (R' = 5'-deoxythymidine) and 50-fold with a minimal diester substrate (R' = CH?). These mutations also enhance the enzyme's promiscuous phosphate monoesterase activity by nearly an order of magnitude, an effect that is traced by mutation to the reduction of unfavorable interactions with the two residues closest to the nonbridging phosphoryl oxygen atoms. The quadruple R' pocket mutant exhibits the same activity toward phosphate diester and phosphate monoester substrates that have identical leaving groups, with substantial rate enhancements of ~10¹¹-fold. This observation suggests that the Zn²? bimetallo core of AP superfamily enzymes, which is equipotent in phosphate monoester and diester catalysis, has the potential to become specialized for the hydrolysis of each class of phosphate esters via addition of side chains that interact with the substrate atoms and substituents that project away from the Zn²? bimetallo core.

Dataset Information

Separation and purification of the alkaline phosphatase and a phosphodiesterase from Halobacterium cutirubrum.

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