Enzymatic synthesis of [beta-32P]ADP using adenylate kinase and [gamma-32P]ATP

Use of helper enzymes for ADP removal in infrared spectroscopic experiments: application to Ca2+-ATPase

Adenylate kinase (AdK) and apyrase were employed as helper enzymes to remove ADP in infrared spectroscopic experiments that study the sarcoplasmic reticulum Ca(2+)-ATPase. The infrared absorbance changes of their enzymatic reactions were characterized and used to monitor enzyme activity. AdK transforms ADP to ATP and AMP, whereas apyrase consumes ATP and ADP to generate AMP and inorganic phosphate. The benefits of using them as helper enzymes are severalfold: i), both remove ADP generated after ATP hydrolysis by ATPase, which enables repeat of ATP-release experiments several times with the same sample without interference by ADP; ii), AdK helps maintain the presence of ATP for a longer time by regenerating 50% of the initial ATP; iii), apyrase generates free P(i), which can help stabilize the ADP-insensitive phosphoenzyme (E2P); and iv), apyrase can be used to monitor ADP dissociation from transient enzyme intermediates with relatively high affinity to ADP, as shown here for ADP dissociation from the ADP-sensitive phosphoenzyme intermediate (Ca(2)E1P). The respective infrared spectra indicate that ADP dissociation relaxes the closed conformation immediately after phosphorylation partially back toward the open conformation of Ca(2)E1 but does not trigger the transition to E2P. The helper enzyme approach can be extended to study other nucleotide-dependent proteins.

A simple, single-tube radioisotopic assay for the phosphorylation/inactivation activity of the pyruvate,orthophosphate dikinase regulatory protein

A simple, single-tube radiolsotopic method has been developed to assay the relative phosphorylation (inaetivation) activity of the bifunctional regulatory protein (RP) of C4-leaf pyruvate,orthophosphate dikinase (PPDK) in desalted leaf homogenates and partially purified preparations. RP catalyzes the inactivation of maize PPDK by phosphorylation of Thr-456, utilizing [β-P]ADP as the specific phosphoryl donor. Existing spectrophotometric and radioisotopic assays for the detection of RP activity are either relatively insensitive or labor-intensive and timeconsuming. We describe a modified radioisotopic assay that couples the synthesis of [β-(32)P]ADP by exogenous adenylate kinase with the subsequent RP-catalyzed [β-(32)P]ADP-dependent phosphorylation of exogenous maize PPDK. The incorporation of [β-(32)P] is dependent on the initial concentrations of ATP and PPDK, as well as the presence of active RP. Desalted leaf homogenates of C3 species fail to catalyze (32)P incorporation into exogenous maize PPDK. Conversely, heterologous systems containing the maize target enzyme and leaf homogenats of other C4 species result in PPDK-specific (32)P-incorporation. This simple radioisotopic assay is at least 40-times more sensitive than the routine spectrophotometric assay, and qualitatively exhibits comparable sensitivity and requires significantly less time than the currently available radioisotopic RP assay. The present assay reliably generates [β-(32)P]ADP and as such may be useful for studies of other systems requiring β-labeled ADP, which is not commercially available.

Purification and properties of a novel nucleotide-hydrolysing enzyme (5'-nucleotidase) from Boophilus microplus

The tick Boophilus microplus contains a nucleoside phosphate-hydrolysing enzyme which, in many respects, resembles the well characterized 5'-nucleotidase from mammalian tissue. The tick enzyme has been purified to homogeneity. It is a membrane-bound glycoprotein with an apparent Mr of 67,000 and, although it fails to hydrolyse a range of nucleoside 2'- or 3'-monophosphates, it has broad specificity for the 5' derivatives. Further investigation of the enzyme's substrate specificity, however, shows some important differences from the mammalian nucleotidases. It hydrolyses both bis-p-nitrophenyl phosphate and p-nitrophenyl phenylphosphonate, typical substrates for phosphodiesterases. However, the tick enzyme is most strikingly different from the mammalian enzymes in that it hydrolyses not only AMP but ADP and ATP as well. Further, the products of the hydrolysis of ATP are adenosine and tripolyphosphate, a reaction which has not been reported previously. The products of ADP hydrolysis are adenosine and pyrophosphate.

Demonstration of plasma-membrane adenosine diphosphatase activity in rat lung

The adenosine diphosphatase (ADPase) activity of rat lung has been investigated. Subcellular fractionation of lung tissue homogenates by sucrose density gradient centrifugation has shown the ADPase activity to be associated with the plasma membrane. ADPase was solubilised from the membranes and fractionated by ammonium sulphate precipitation to separate a specific, low-Km ADPase from non-specific alkaline phosphatase activity. The solubilised ADPase has a Km of 50 microM at pH 7.5 and appears to be distinct from ATPase.