Specific modification of the GTP binding sites of rat 5'-adenylic acid aminohydrolase by periodate-oxidized GTP

Regulation of skeletal muscle AMP deaminase: lysine residues are critical for the pH-dependent positive homotropic cooperativity behaviour of the rabbit enzyme

Reaction of rabbit skeletal muscle AMP deaminase with a low molar excess of trinitrobenzene sulfonic acid (TNBS) results in conversion of the enzyme into a species with about six trinitrophenylated lysine residues per molecule which no longer manifests positive homotropic cooperativity at pH 7.1 or at the optimal pH value of 6.5 in the presence of low K+ concentrations. Substitution of the reactive thiol groups with 5,5'-dithiobis-(2-nitrobenzoic acid) does not protect the enzyme from the TNBS-induced changes of the catalytic properties, indicating that cysteine residues modification is not at the basis of the effects of TNBS treatment on AMP deaminase and strongly suggesting the obligatory participation of lysine residues to the constitution of a regulatory anionic site to which AMP must bind to stimulate the enzyme at alkaline pH. The TNBS-treated enzyme is also completely desensitized to inhibition by ATP, but not to inhibition by GTP and stimulation by ADP. This observation suggests a connection between the operation of the hypothesized anionic activating site, responsible for positive homotropic cooperativity, and the inhibition exerted by anionic compounds that compete for the same site, among them the most efficient metabolite being probably ATP.

Regulation of skeletal-muscle AMP deaminase: involvement of histidine residues in the pH-dependent inhibition of the rabbit enzyme by ATP

Reaction of rabbit skeletal-muscle AMP deaminase with a low molar excess of diethyl pyrocarbonate results in conversion of the enzyme into a species with one or two carbethoxylated histidine residues per subunit that retains sensitivity to ATP at pH 7.1 but, unlike the native enzyme, it is not sensitive to regulation by ATP at pH 6.5. This effect mimics that exerted on the enzyme by limited proteolysis with trypsin, which removes the 95-residue N-terminal region from the 80 kDa enzyme subunit. These observations suggest involvement of some histidine residues localized in the region HHEMQAHILH (residues 51-60) in the regulatory mechanism which stabilizes the binding of ATP to its inhibitory site at acidic pH. Carbethoxylation of two histidine residues per subunit abolishes the inhibition by ATP of the proteolysed enzyme at pH 7.1, suggesting the obligatory participation of a second class of histidine residues, localized in the 70 kDa subunit core, in the mechanism of the pH-dependent inhibition of the enzyme by ATP. At a slightly acidic pH, these histidine residues would be positively charged, resulting in a desensitized form of the enzyme similar to that obtained with the carbethoxylation reaction.

Regulation of skeletal-muscle AMP deaminase. Evidence for a highly pH-dependent inhibition by ATP of the homogeneous derivative of the rabbit enzyme yielded by limited proteolysis

Limited proteolysis of rabbit skeletal-muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) with trypsin results in conversion of the enzyme into a species which over the pH range 6.5-7.1 exhibits hyperbolic kinetics at low K+ concentration even in the absence of ADP, but shows a 20% decrease in activity at saturating substrate concentration. Analysis by sedimentation-equilibrium techniques reveals the proteolysed enzyme to be homogeneous and to have a molecular mass of 222,000 Da, indicative of a trimeric structure with a subunit molecular mass of 72,000 Da, in contrast with the tetrameric structure of the native enzyme, composed of four 79,000-Da subunits. These observations suggest a role of the 7,000-Da fragment which is removed by proteolysis in the maintenance of the three-dimensional structure of the subunit that causes the enzyme at low K+ concentration to show homotropic positive co-operativity. Study of the influence of pH, isolated from that of K+, on the kinetics of AMP deaminase reveals a highly pH-dependent inhibitory effect by ATP which is completely absent at acid pH values and abruptly manifests itself just above neutrality. This phenomenon may have significance in the metabolism of exercising muscle, in connection with the pH-dependent interaction of AMP deaminase with the thick filament.

Periodate-oxidized NADP+ is a powerful inhibitor of human placental estradiol-17 beta dehydrogenase

Periodate-oxidized NADP+ inhibits the NAD+-linked activity of human placental estradiol-17 beta dehydrogenase (EC 1.1.1.62). The inhibition appears to be competitive with respect to NAD+ and can be reversed by dialysis or gel filtration. The apparent inhibitor constant for the periodate-oxidized analogue is 0.047 microM. The presence in the incubation mixture of NAD+ protects the enzyme against inhibition. No inhibitory effects of the coenzyme analogue are observed on the NADP+-linked activity of the enzyme.

Interaction with troponin T from white skeletal muscle restores in white skeletal muscle AMP deaminase those allosteric properties removed by limited proteolysis

Limited proteolysis of rabbit skeletal muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) with trypsin results in conversion of the enzyme to a form which is no longer inhibited by ATP and exhibits hyperbolic kinetics even at low K+ concentration and in the absence of ADP. The interaction with troponin T from white skeletal muscle or with the phosphorylated 42-residue N-terminal peptide of troponin T restores in the trypsin-treated AMP deaminase the sensitivity to adenine nucleotides and increases the KA for K+ activation of the enzyme from 1 mM to 12 mM, this effect being diametrically opposite to that exerted by limited proteolysis on the native enzyme. Treatment of the N-terminal peptide of troponin T with alkaline phosphatase abolishes the modulating properties of the peptide, suggesting that phosphorylation-dephosphorylation processes may be involved in the regulation of the enzyme.

Regulation of porcine heart and skeletal muscle AMP-deaminase by adenylate energy charge

1. Cytosol from pig skeletal muscle, but not heart, contains an inhibitor of AMP-deaminase (AMP-D, EC 3.5.4.6) which reduces AMP-D activity 8-fold. 2. Heart and skeletal muscle AMP-D have been purified to apparent homogeneity by cellulose phosphate and DEAE-Sephacel chromatography. 3. AMP-D from skeletal muscle is inhibited more severely than the heart enzyme by an increase in adenylate energy charge to levels exceeding 0.4. Nevertheless both enzymes seem to be regulated by the energy charge, which contrasts with reports for rabbit heart AMP-D.

Involvement of lysine residue in the nucleotide binding of pigeon liver malic enzyme: modification with affinity label periodate-oxidized NADP

1. Periodate-oxidized NADP, a competitive inhibitor of malic enzyme with respect to NADP, inactivate the enzyme in mild conditions. 2. The inactivation is due to the modification of an essential lysine residue. 3. Two molecules of reagent were found to be incorporated into the enzyme tetramer after extensive modification. 4. Complete protection of malic enzyme from the oxidized NADP inactivation was afforded by NADP and its analogues. 5. The modified enzyme showed increased apparent Michaelis constant for the nucleotide coenzymes but the maximum velocity was decreased. 6. The binding between the modified enzyme and NADPH was impaired.

Periodate-oxidized AMP as a substrate, an inhibitor and an affinity label of human placental alkaline phosphatase

Human placental alkaline phosphatase (EC 3.1.3.1) was inactivated by periodate-oxidized AMP. The inactivation showed saturation kinetics and could be partially prevented by the substrate AMP or the product inhibitor inorganic phosphate. Oxidized AMP was itself a substrate for this enzyme, with an apparent Km of 0.67 mM. The hydrolytic products of oxidized AMP were identified as oxidized adenosine hemiacetals. Oxidized AMP was also found to be a non-competitive inhibitor with respect to p-nitrophenyl phosphate, with identical Kis and Kii values of 0.15 mM. Our results indicate that oxidized AMP could combine with the enzyme to form a binary complex, followed by reaction with the proximal lysyl amino group to yield a Schiff base. The latter was reduced with NaBH4 and identified by t.l.c. The incorporation of only 1.5 molecules of oxidized [14C]AMP per enzyme subunit resulted in a complete inactivation of the enzyme. The modified enzyme showed higher apparent Km for the substrates and higher Ki for inorganic phosphate, but lower [32P]phosphate incorporation, than the native enzyme. These results support the conclusion that a lysine residue is involved in the phosphate-binding site of human placental alkaline phosphatase.

Negative homotropic cooperativity in rat muscle AMP deaminase. A kinetic study on the inhibition of the enzyme by ATP

1. Rat skeletal muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) at optimal KCl concentrations shows a biphasic response to increasing levels of the allosteric inhibitor ATP. 2. Up to 10 micrometer, ATP appears to convert the enzyme to a form exhibiting sigmoidal kinetics while at higher concentrations its inhibitory effect is manifested by an alteration of AMP binding to AMP deaminase indicative of negative homotropic cooperativity at about 50% saturation. 3. AMP deaminase is inactivated by incubation with the periodate oxidation product of ATP. The (oxidized ATP)--AMP deaminase complex stabilized by NaBH4 reduction shows kinetic properties similar to those of the native enzyme in the presence of high ATP concentrations. 4. A plausible explanation of the observed cooperativity is that ATP induces different conformational state of AMP deaminase subunits, causing the substrate to follow a sequential mechanism of binding to enzyme. 5. Binding of the radioactive oxidized ATP shows that 3.2 mol of this reagent bind per mol AMP deaminase.

Aminoacyl-tRNA synthetases: affinity labeling of the ATP binding site by 2', 3' -ribose oxidized ATP

Homogeneous Escherichia coli methionyl-, isoleucyl-, tryptophanyl-, and phenylalanyl-tRNA synthetases and Bacillus stearothermophilus methionyl- and tyrosyl-tRNA synthetases are irreversibly inactivated by reaction of their active ATP sites with the 2',3'-dialdehyde derivative of ATP obtained by periodate oxidation. In each case, the amount of 14C-labeled dialdehyde derivative incorporated per molecule of inactivated enzyme appears consistent with the expected active stoichiometry of the synthetase. These results strongly support the presence, at the active site of the studied aminoacyl-tRNA synthetases, of a common residue, probably a lysine whose epsilon-NH2 group is known, from the work of others, to form a Schiff's base specifically with the 2',3'-dialdehyde derivatives of ribonucleotides.