Phosphoryl transfer from o-carboxyphenyl phosphate to tri(hydroxymethyl)-aminomethane catalysed by alkaline phosphatase from E. coli

Alkyl- and aryl-substituted salicyl phosphates as detection reagents in enzyme-amplified fluorescence DNA hybridization assays on solid support

Nine salicyl phosphate esters with hydrophobic substituents (5-phenyl, 5-(2,4-difluorophenyl), 5-tert-octyl, 5-cumyl, 5-(4-tert-butylphenyl, 5-(1-adamantyl), 5-(n-dodecyl), 5-(1,1-diphenylethyl, and 5-trityl) were synthesized and found to be good substrates for calf intestinal alkaline phosphatase. The enzymatic hydrolysis produced the corresponding salicylates, which were strongly fluorescent when excited by ultraviolet light around 300 nm with maximum emission at 420-435 nm. The salicylates were less soluble and/or more adhesive than the nonfluorescent salicyl phosphate substrates, resulting in localization of fluorescence signal, which is a requirement for membrane-based assays. The salicyl phosphates bearing 8-14 carbon substitutents were found to be suitable detection reagents for dot-blot DNA hybridization assays on nylon membrane using a biotinylated probe, allowing the detection of 125 pg of target pBR322 plasmid DNA using a simple apparatus consisting of a transilluminator, a camera. and a 455-nm cutoff optical filter.

Endogenous phosphorylation and dephosphorylation of rat liver plasma membrane proteins, suggesting a 18 kDa phosphoprotein as a potential substrate for alkaline phosphatase

Purified rat liver plasma membranes were incubated for 0-60 min with [gamma-32P]ATP and analysis of 32P-labeled proteins by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed the presence of two shifted kinetic phenomena. The use of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), a potent inhibitor of protein kinases, allowed the identification of one as the endogenous protein phosphorylation. The other was shown to be the labeling of two phospho-intermediate forms of alkaline phosphatase (orthophosphoric monoester phosphohydrolase (alkaline optimum, EC 3.1.3.1.], which have apparent molecular masses of 151 and 135 kDa. Bromolevamisole, a potent inhibitor of the enzyme, stabilized these phospho-intermediates, and consequent on this inhibition the labelling of a 18 kDa phosphoprotein was augmented. So, when alkaline phosphatase was studied in its native plasma membrane environment, a specificity of this enzyme over the endogenous phosphoproteins was established.

Kinetic aspects of human placental alkaline phosphatase enzyme membrane

The crosslinking of alkaline phosphatase of human placenta with human serum albumin has been optimized. During the physico-chemical characterization of this immobilized biocatalyst, special attention was paid to attributes such as the irreversibility of the enzyme support bonding, the stability of the catalytic activity, and the effects of pH and temperature on this activity. Regarding stability, patterns of denaturation are proposed, to account for inactivation curves over time and under storage/operation conditions. These patterns, in some cases, indicate the existence of different populations of immobilized enzyme molecules, with a different degree of sensitivity to denaturation. The activity vs pH profiles are clearly modified by the immobilization process. This is because the pH of the free homogeneous solution, measurable with a pH-meter, differs from the real pH of the immediate microenvironment of the immobilized enzyme molecules due to the effects of proton accumulation in the microenvironment (in the reaction catalysed by alkaline phosphatase, protons are produced), to limitations to the free diffusion of H+ and to the possible partition effects of H+ due to polar interactions with residues or molecules of the enzyme membrane. In the experimental working conditions, the apparent optimum temperatures are centered at 40 degrees C, inactivation (thermal denaturation) occurring above this temperature. In the temperature range 10-40 degrees C, the kinetic control over the overall activity of the immobilized enzyme was observed, causing the Arrhenius profiles to be linear.

Alkaline phosphatase inactivation by mixed function oxidation systems

Alkaline phosphatase is inactivated by mixed function oxidation systems. OH. radicals, generated via an ascorbate-modified Haber-Weiss cycle or a Fenton-type reaction, seem to be responsible for the protein oxidative damage. Experiments with hydroxyl radical scavengers, enzyme substrates, products, and metal cofactors suggest that a "site-specific" radical attack takes place at or near the active center. Vitamin E fails to protect alkaline phosphatase; uric acid, instead, is particularly effective in shielding the protein against covalent modifications.

Evidence concerning a possible steady state rate equation for E. coli alkaline phosphatase

Steady state data was obtained for alkaline phosphatase over a wide range of experimental conditions using two substrates, four inhibitors, two modifiers and several pH, ionic strength and temperatures values. The data was fitted by rational functions of degree 1:1, 2:2 and 3:3 using a non-linear regression program and then the F-test was used to assess the goodness of fit. A proportion of the curves could only be fitted by 2:2 functions but many of them could be adequately fitted by 1:1 functions. No statistically significant improvement in fit occurred with 3:3 functions. Data was simulated using a computer program to see what sort of curves could be generated by a two sites mechanism proposed for alkaline phosphatase and this study showed it is difficult to detect cubic terms in this rate equation. It was concluded that alkaline phosphatase does not obey Michaelis-Menten kinetics. Rather, the steady state data require a mechanism of at least second degree but do not exclude a rate equation of third degree.

Transphosphorylation mechanism of hen's egg yolk alkaline phosphatase

1. Hen's egg yolk alkaline phosphatase can transfer the donor substrate phosphoryl to a hydroxyl containing acceptor. 2. This acceptor contains preferably an alpha-amino or alpha-imino group. 3. The optimal pH of the transfer reaction is at 9.45 with Tris and at 9.60 with diethanolamine as acceptor. 4. Transfer products were isolated and characterized. 5. The ratio of products is independent of the donor substrate. ROH and R'OP production increase linearly with the acceptor concentration, while Pi release and Km remain constant. 6. This points to a mechanism involving two phosphorylated intermediates. 7. The acceptor bypasses the isomerization step of the phosphoryl enzyme, which is necessary before hydrolysis can take place.

Negative cooperativity in alkaline phosphatase from E. col: new kinetic evidence from a steady-state study

1. A study has been carried out on the steady-state kinetics followed by the alkaline phosphatase from Escherichia coli at different pH, temperatures, ionic strengths, phosphate concentrations and in the presence of the effectors such as Tris, NH4+--NH3 and CH3OH; p-nitrophenyl phosphate was used as substrate. 2. Contrary to what has generally been accepted, in most cases the enzyme follows non-Michaelian kinetics for a wide substrate concentration range, giving concave-down Lineweaver-Burk plots. Only at high phosphate concentrations (5 . 10(-3) M) and at high ionic strengths (2.0 M) is a linear Lineweaver-Burk plot obtained (Michaelian kinetics). 3. In order to analyse the kind of kinetics obtained, a non-linear regression fitting method was used to obtain rate vs substrate concentration equations as polynomial quotients of minimum degree with positive coefficients. 4. Most of the data obtained follows 2:2 degree type equations. 5. These results tend to suggest an idea of cooperativity rather than one of independence between the sites of the dimeric enzyme. A model is discussed for cooperativity between the sites with a wide concentration range giving concave-down Lineweaver-Burk plots.

Kinetic studies of the transphosphorylation reactions catalyzed by alkaline phosphatase from E. coli: hydrolysis of p-nitrophenyl phosphate and o-carboxyphenyl phosphate in presence of Tris

1. Transphosphorylation of p-nitrophenyl phosphate and o-carboxyphenyl phosphate to Tris, has been studied at alkaline and acid pH. 2. The rate of release for all reactions products was Tris-dependent for both substrates, with a slight maximum for phenol at alkaline pH. These dependences have been analyzed from a mechanistic standpoint. 3. Individual constants of rate of a simple transphosphorylation mechanism have been determined. 4. At high Tris concentration (greater than 1.0 M) a slight competitive inhibition has been observed. 5. Inhibition in NH4+-NH3Cl buffer has been found at alkaline pH but not at acid pH. It would therefore seem that the non-protonated NH2 group of Tris is responsible for inhibition. 6. The results suggest the formation of complexes between Tris and the enzyme. Other possible alternatives are also analyzed.