Preparation of 5'-adenylic acid deaminase based on phosphate-induced dissociation of rat actomyosin-deaminase complexes

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Role of the HPRG Component of Striated Muscle AMP Deaminase in the Stability and Cellular Behaviour of the Enzyme

Multiple muscle-specific isoforms of the Zn²⁺ metalloenzyme AMP deaminase (AMPD) have been identified based on their biochemical and genetic differences. Our previous observations suggested that the metal binding protein histidine-proline-rich glycoprotein (HPRG) participates in the assembly and maintenance of skeletal muscle AMP deaminase (AMPD1) by acting as a zinc chaperone. The evidence of a role of millimolar-strength phosphate in stabilizing the AMPD-HPRG complex of both AMPD1 and cardiac AMP deaminase (AMPD3) is suggestive of a physiological mutual dependence between the two subunit components with regard to the stability of the two isoforms of striated muscle AMPD. The observed influence of the HPRG content on the catalytic behavior of the two enzymes further strengthens this hypothesis. Based on the preferential localization of HPRG at the sarcomeric I-band and on the presence of a Zn²⁺ binding motif in the N-terminal regions of fast TnT and of the AMPD1 catalytic subunit, we advance the hypothesis that the Zn binding properties of HPRG could promote the association of AMPD1 to the thin filament.

The role of histidine-proline-rich glycoprotein as zinc chaperone for skeletal muscle AMP deaminase

Metallochaperones function as intracellular shuttles for metal ions. At present, no evidence for the existence of any eukaryotic zinc-chaperone has been provided although metallochaperones could be critical for the physiological functions of Zn²⁺ metalloenzymes. We propose that the complex formed in skeletal muscle by the Zn²⁺ metalloenzyme AMP deaminase (AMPD) and the metal binding protein histidine-proline-rich glycoprotein (HPRG) acts in this manner. HPRG is a major plasma protein. Recent investigations have reported that skeletal muscle cells do not synthesize HPRG but instead actively internalize plasma HPRG. X-ray absorption spectroscopy (XAS) performed on fresh preparations of rabbit skeletal muscle AMPD provided evidence for a dinuclear zinc site in the enzyme compatible with a (μ-aqua)(μ-carboxylato)dizinc(II) core with two histidine residues at each metal site. XAS on HPRG isolated from the AMPD complex showed that zinc is bound to the protein in a dinuclear cluster where each Zn²⁺ ion is coordinated by three histidine and one heavier ligand, likely sulfur from cysteine. We describe the existence in mammalian HPRG of a specific zinc binding site distinct from the His-Pro-rich region. The participation of HPRG in the assembly and maintenance of skeletal muscle AMPD by acting as a zinc chaperone is also demonstrated.

Isolation by zinc-affinity chromatography of the histidine-proline-rich-glycoprotein molecule associated with rabbit skeletal muscle AMP deaminase. Evidence that the formation of a protein-protein complex between the catalytic subunit and the novel component is critical for the stability of the enzyme

The histidine-proline-rich glycoprotein (HPRG) component of rabbit skeletal muscle AMP deaminase under denaturing and reducing conditions specifically binds to a Zn(2+)-charged affinity column and is only eluted with an EDTA-containing buffer that strips Zn(2+) from the gel. The isolated protein is homogeneous showing an apparent molecular weight (MW) of 95000 and the N-terminal sequence L-T-P-T-D-X-K-T-T-K-P-L-A-E-K-A-L-D-L-I, corresponding to that of rabbit plasma HPRG. The incubation with peptide-N-glycosidase F promotes the reduction of the apparent MW of isolated HPRG to 70000, characterizing it as a N-glycosylated protein. The separation from AMP deaminase of an 85-kDa component with a blocked N terminus is observed when the enzyme is applied to the Zn-charged column under nondenaturing conditions. On storage under reducing conditions, this component undergoes an 85- to 95-kDa transition yielding a L-T-P-T-D-X-K-T-T-K-P-L N-terminal sequence, suggesting that the shift in the migration on SDS/PAGE as well as the truncation of the protein at its N terminus are promoted by the reduction of a disulfide bond present in freshly isolated HPRG. The separation of HPRG induces a marked reduction in the solubility of AMP deaminase, strongly suggesting a role of HPRG in assuring the molecular integrity of the enzyme.

Association of purified skeletal-muscle AMP deaminase with a histidine-proline-rich-glycoprotein-like molecule

Denaturation of rabbit skeletal-muscle AMP deaminase in acidic medium followed by chromatography on DEAE-cellulose in 8 M urea atpH 8.0 allows separation of two main peptide components of similar apparent molecular mass (75-80 kDa) that we tentatively assume correspond to two different enzyme subunits. Whereas the amino acid composition of one of the two peptides is in good agreement with that derived from the nucleotide sequence of the known rat and human AMPD1 cDNAs, the second component shows much higher contents of proline, glycine and histidine. N-Terminal sequence analysis of the fragments liberated by limited proteolysis with trypsin of the novel peptide reveals a striking similarity to the fragments produced by plasmin cleavage of the rabbit plasma protein called histidine-proline-rich glycoprotein (HPRG). However, some divergence is observed between the sequence of one of the fragments liberated from AMP deaminase by a more extensive trypsinization and rabbit plasma HPRG in the region containing residues 472-477. A fragment with a blocked N-terminus, which was found among those liberated by proteolysis with pepsin of either whole AMP deaminase or the novel component of the enzyme, shows an amino acid composition quite different from that of the N-terminus of the known subunit of AMP deaminase. By coupling this observation with the detection in freshly prepared AMP deaminase of a low yield of the sequence (LTPTDX) corresponding to that of HPRG N-terminus, it can be deduced that in comparison with HPRG, the putative HPRG-like component of AMP deaminase contains an additional fragment with a blocked N-terminus, which is liberated by a proteolytic process during purification of the enzyme. The implications of the association to rabbit skeletal-muscle AMP deaminase of a HPRG-like protein species are discussed.

AMP deaminase from equine muscle: purification and determination of regulatory properties

1. AMP deaminase from thoroughbred horse muscle was purified to apparent homogeneity and its regulatory properties were determined at pH 6.5 and 7.4. 2. The results are discussed in relation to the potential role of muscle AMP deaminase during exercise and the existence of two molecular forms depending on the pH.

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.

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.

The effects of S-adenosylhomocysteine and S-adenosylmethionine on some purine- and pyrimidine-metabolizing systems

The effects of S-adenosylhomocysteine and S-adenosylmethionine on some purine- and pyrimidine-metabolizing systems have been examined. Both compounds were capable of acting as relatively good inhibitors of adenosine deaminase, nucleoside phosphorylase, and adenylate deaminase activities but as relatively poor inhibitors of myokinase and nucleoside monophosphate kinase. The inhibitory effects were freely reversible. 5'-Nucleotidase, orotidine 5'- phosphate, and phosphodiesterase were unaffected. Nucleoside phosphorylase was competitively inhibited by both compounds, whereas mixed inhibitory effects occurred with adenosine deaminase.

Modification by liposomes of the adenosine triphosphate-activating effect on adenylate deaminase from pig heart

Adenylate deaminase (AMP deaminase, EC 3.5.4.6) of a high substrate specificity was purified from pig heart by chromatography on cellulose phosphate. The enzyme shows a co-operative binding of AMP [h (Hill coefficient) 2.35, with SO.5 (half-saturating substrate concentration) 5mM]. ATP and ADP act as positive effectors, lowering h to 1.55 and SO.5 to 1 mM. The addition of liposomes (phospholipid bilayers) to ATP-activated or ADP-activated enzyme causes a further shift of the h value to 1.04 and SO.5 to 0.5 mM. For ATP-activated enzyme the addition of liposomes increases Vmax. by about 100%, and for ADP-activated enzyme by 50%. Liposomes have no effect on the kinetics of AMP deaminase in the absence of ATP and ADP, and neither do they influence the inhibitory effect of orthophosphate on heart muscle AMP deaminase. Metabolic implications of these findings are discussed.

Effects of various ligands on interaction of AMP deaminase with myosin

Purified rat muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) binds tightly to rat myosin. The binding is abolished in the presence of low concentrations of various ligands. Pyrophosphate and GTP at concentrations as low as 0.1 micrometer were effective in abolishing the interaction between two proteins. Other nucleoside triphosphates were less effective than GTP and the concentrations required for 50% inhibition were approximately 0.3 to 0.7 micrometer. ADP and AMP are effective in inhibiting the interaction between two proteins, but they are less effective than the nucleoside triphosphates; 50% inhibition occurred at 34 micrometer with ADP and at 1 mM with AMP. Creatine phosphate and inorganic phosphate showed 50% inhibition at 5 to 6 mM. All of the compounds, which affected AMP deaminase activity, were effective in abolishing the interaction of the enzyme with myosin; however, the interaction-abolishing effects of the compounds are not parallel with their inhibitory effects on the deaminase activity. Although there exist three parental isozymes of AMP deaminase in the rat, all three enzymes interacted with myosin.

Changes in the activity of rat muscle AMP deaminase in relation to the proportion of dietary protein

1. The purine nucleotide cycle has been proposed (Lowenstein, 1972) as an alternative scheme for amino acid deamination in tissues, such as skeletal muscle, having low concentrations of glutamate dehydrogenase (EC1.4.1.2).

2. Activities of AMP deaminase (EC3.5.4.6), one of the enzymes of the cycle, have been measured in soleus, plantaris and extensor digitorum longus muscles of rats maintained for 18 d on diets providing 0, 0·035 or 0·10 net dietary protein energy (energy supplied by utilizable protein: total metabolizable energy, NDp:E), and in rats given the 0·10 NDp:E diet for 3 d after the 0 or 0·035 NDp:E regimens.

3. Concentration of AMP deaminase in the different muscles from the control (0·10 NDp:E diet) rats appeared to bear an inverse relationship to the proportion of mitochondria-rich fibres (i.e. rich in glutamate dehydrogenase) in each muscle.

4. Dietary protein deprivation (0 or 0·035 NDp:E) led to adaptive reductions in AMP-deaminase activity in the soleus and plantaris muscles, but in the extensor muscle the 0·035 NDp:E diet produced no change, while the 0 NDp:E diet caused an increase in activity.

5. Refeeding the 0·10 NDp:E diet to the protein-deprived rats caused reductions of AMP-deaminase activity to lower levels in all three muscles, except in the instance of soleus in rats refed after the 0·035 NPp:E diet.

6. In view of the different responses shown by the three muscles to the dietary treatments, the importance of specifying the particular muscles used in future nutritional studies is emphasized.

7. The adaptive changes in AMP deaminase are discussed in terms of operation of the purine nucleotide cycle for amino acid deamination responding to the changes in amino acid catabolism known to be caused in muscle by these protein-deficient diets.

Extraction and Some Properties of Adenosine 5′-Monophosphate Aminohydrolase from Prerigor and Postrigor Muscle of Cod

Adenosine 5′-monophosphate aminohydrolase (EC 3.5.4.6) activity of prerigor cod muscle could be extracted with water or 0.02 M succinate buffer in about 90% yield, but 0.02 M KCl gave a low yield. With postrigor muscle, the enzyme tended to be associated with the fibrillar protein fractions. The activity of the unpurified enzyme, measured by a pH-stat, showed a maximum at pH 7.0 in 0.02 M succinate, and about pH 6.6 in 0.1 M KCl. At pH 7.0, activities were maximal and approximately equal in 0.1 M KCl, 0.1 M NaCl, and 0.04 M potassium succinate. Phosphate buffer inhibited the reaction under certain conditions. The enzyme was sensitive to glass and other surfaces, and this seemed to be responsible for loss of activity during dialysis. The activation energy was 10 kcal per mole and the Michaelis–Menten constant was 1.4 × 10−3 moles per litre.