Regulatory properties of AMP deaminase isoenzymes from rabbit red muscle

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.

Regulation of skeletal muscle AMP deaminase. Carbethoxylation of His-51 belonging to the zinc coordination sphere of the rabbit enzyme promotes its desensitization towards the inhibition by ATP

BACKGROUND

Skeletal muscle AMP deaminase (AMPD1) regulates the concentration of adenine nucleotides during muscle contraction. We previously provided evidence that rabbit AMPD1 is composed by two HPRG 73 kDa subunits and two 85 kDa catalytic subunits with a dinuclear zinc site with an average of two histidine residues at each metal site. AMPD1 is mainly expressed in fast twitching fibers and is inhibited by ATP. The limited trypsinization of the 95-residue N-terminal domain of rabbit AMPD1 desensitizes the enzyme towards ATP inhibition at the optimal pH 6.5, but not at pH 7.1.

METHODS

The modified residues of rabbit AMPD1 after incubation with radioactive diethyl pyrocarbonate ([¹⁴C]DEP) causing the desensitization to inhibition by ATP at pH 7.1 have been identified by sequence analysis and MS analysis of the radioactive peptides liberated from the carbethoxylated enzyme by limited proteolysis with trypsin.

RESULTS

The study confirms the presence of a dinuclear zinc site in rabbit AMPD1 and shows that carbethoxylation of His-51 at the N-terminus of the catalytic subunit removes the inhibition of the enzyme by ATP at pH 7.1.

CONCLUSIONS

The desensitization to ATP is due to the modification of His-51 of the Zn₂ coordination sphere which is transduced in a conformational change of the enzyme C-terminus, where an ATP-binding site has been localized.

GENERAL SIGNIFICANCE

The progress in the study of the complex regulation of rabbit AMPD1 that shares an identical amino acid sequence with the human enzyme is important in relation to the role of the enzyme during mammalian evolution.

Increased AMP deaminase activity decreases ATP content and slows protein degradation in cultured skeletal muscle

BACKGROUND

Protein degradation is an energy-dependent process, requiring ATP at multiple steps. However, reports conflict as to the relationship between intracellular energetics and the rate of proteasome-mediated protein degradation.

METHODS

To determine whether the concentration of the adenine nucleotide pool (ATP + ADP + AMP) affects protein degradation in muscle cells, we overexpressed an AMP degrading enzyme, AMP deaminase 3 (AMPD3), via adenovirus in C2C12 myotubes.

RESULTS

Overexpression of AMPD3 resulted in a dose- and time-dependent reduction of total adenine nucleotides (ATP, ADP and AMP) without increasing the ADP/ATP or AMP/ATP ratios. In agreement, the reduction of total adenine nucleotide concentration did not result in increased Thr172 phosphorylation of AMP-activated protein kinase (AMPK), a common indicator of intracellular energetic state. Furthermore, LC3 protein accumulation and ULK1 (Ser 555) phosphorylation were not induced. However, overall protein degradation and ubiquitin-dependent proteolysis were slowed by overexpression of AMPD3, despite unchanged content of several proteasome subunit proteins and proteasome activity in vitro under standard conditions.

CONCLUSIONS

Altogether, these findings indicate that a physiologically relevant decrease in ATP content, without a concomitant increase in ADP or AMP, is sufficient to decrease the rate of protein degradation and activity of the ubiquitin-proteasome system in muscle cells. This suggests that adenine nucleotide degrading enzymes, such as AMPD3, may be a viable target to control muscle protein degradation and perhaps muscle mass.

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.

Normal activities of AMP-deaminase and adenylate kinase in patients with McArdle disease

During physical activity in McArdle patients, little or no lactate is released in the skeletal muscle. However, excessive ammonia production has frequently been reported in these patients. Production of ammonia is catalysed by AMP deaminase (AMPD) and adenylate kinase (AK). The activities of AMPD and AK along with housekeeping enzyme phosphoglucoisomerase (PGI) were measured in 11 genetically confirmed McArdle patients and compared with 27 healthy controls. The AMPD and AK activities were not significantly different in patients and controls. The activity of PGI was significantly higher in patients than in controls suggesting compensation of the impaired glycogenolysis in McArdle. The ratios of activities of AMPD and AK over PGI were significantly lower in patients than in controls. High ammonia production in McArdle patients is not based on enzyme induction of AMPD and AK but possibly due to kinetic activation of the enzyme AMPD by increased concentration of the substrate AMP.

In silico analysis of exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome

Post-exertional malaise is commonly observed in patients with myalgic encephalomyelitis/chronic fatigue syndrome, but its mechanism is not yet well understood. A reduced capacity for mitochondrial ATP synthesis is associated with the pathogenesis of CFS and is suspected to be a major contribution to exercise intolerance in CFS patients. To demonstrate the connection between a reduced mitochondrial capacity and exercise intolerance, we present a model which simulates metabolite dynamics in skeletal muscles during exercise and recovery. CFS simulations exhibit critically low levels of ATP, where an increased rate of cell death would be expected. To stabilize the energy supply at low ATP concentrations the total adenine nucleotide pool is reduced substantially causing a prolonged recovery time even without consideration of other factors, such as immunological dysregulations and oxidative stress. Repeated exercises worsen this situation considerably. Furthermore, CFS simulations exhibited an increased acidosis and lactate accumulation consistent with experimental observations.

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.

Sprint training reduces urinary purine loss following intense exercise in humans

The influence of sprint training on endogenous urinary purine loss was examined in 7 active male subjects (age, 23.1 +/- 1.8 y; body mass, 76.1 +/- 3.1 kg; VO2 peak, 56.3 +/- 4.0 mL.kg-1.min-1). Each subject performed a 30 s sprint performance test (PT), before and after 7 d of sprint training. Training consisted of 15 sprints, each lasting 10 s, on an air-braked cycle ergometer performed twice each day. A rest period of 50 s separated each sprint during training. Sprint training resulted in a 20% higher muscle ATP immediately after PT, a lower IMP (57% and 89%, immediately after and 10 min after PT, respectively), and inosine accumulation (53% and 56%, immediately after and 10 min after the PT, respectively). Sprint training also attenuated the exercise-induced increases in plasma inosine, hypoxanthine (Hx), and uric acid during the first 120 min of recovery and reduced the total urinary excretion of purines (inosine + Hx + uric acid) in the 24 h recovery period following intense exercise. These results show that intermittent sprint training reduces the total urinary purine excretion after a 30 s sprint bout.

Presence in human skeletal muscle of an AMP deaminase-associated protein that reacts with an antibody to human plasma histidine-proline-rich glycoprotein

Histidine-proline-rich glycoprotein (HPRG) is a protein that is synthesized by parenchimal liver cells. The protein has been implicated in a number of plasma-specific processes, including blood coagulation and fibrinolysis. We have recently reported the association of an HPRG-like protein with rabbit skeletal muscle AMP deaminase (AMPD). The results of the immunological analysis reported here demonstrate that an antibody against human plasma HPRG reacts with an AMPD preparation from human skeletal muscle. To probe the localization of the putative HPRG-like protein in human skeletal muscle, serial sections from frozen biopsy specimens were processed for immunohistochemical and histoenzymatic stains. A selective binding of the anti-HPRG antibody to Type IIB muscle fibers was detected, suggesting a preferential association of the novel protein to the AMPD isoenzyme contained in the fast-twitch glycolytic fibers.

Genetic and other determinants of AMP deaminase activity in healthy adult skeletal muscle

AMPD1 genotype, relative fiber type composition, training status, and gender were evaluated as contributing factors to the reported variation in AMP deaminase enzyme activity in healthy skeletal muscle. Multifactorial correlative analyses demonstrate that AMPD1 genotype has the greatest effect on enzyme activity. An AMPD1 mutant allele frequency of 13.7 and a 1.7% incidence of enzyme deficiency was found across 175 healthy subjects. Homozygotes for the AMPD1 normal allele have high enzyme activities, and heterozygotes display intermediate activities. When examined according to genotype, other factors were found to affect variability as follows: AMP deaminase activity in homozygotes for the normal allele exhibits a negative correlation with the relative percentage of type I fibers and training status. Conversely, residual AMP deaminase activity in homozygotes for the mutant allele displays a positive correlation with the relative percentage of type I fibers. Opposing correlations in different homozygous AMPD1 genotypes are likely due to relative fiber-type differences in the expression of AMPD1 and AMPD3 isoforms. Gender also contributes to variation in total skeletal muscle AMP deaminase activity, with normal homozygous and heterozygous women showing only 85-88% of the levels observed in genotype-matched men.

Purine nucleotide cycle in rat renal cortex and medulla under conditions that mimic normal and low oxygen supply

The distinctive feature of the renal function and metabolism implicate a possibility of excessive ATP degradation during insufficient oxygen supply. Protection of the purine ring against degradation is one among other functions of the purine nucleotide cycle (PNC). The purpose of this study was to estimate the activity of PNC in cytosol of rat renal cortex and medulla under conditions that mimic normal and low oxygen supply in vivo. In normoxic-like condition the rate of AMP deamination was 1.7 and 2.0 nmol/mg protein/min in the cytosol of cortex and medulla, respectively. Under this condition, the rate of IMP reamination was similar to that of AMP deamination. In a hypoxia-like condition the rate of AMP deamination increased by 41% in cytosol from both parts of the kidney, while the rate of IMP reamination remained unchanged in the cytosol of medulla and decreased by 46% in the cortex cytosol. Distribution of the other enzymes of the PNC, that is, adenylosuccinate synthetase and adenylosuccinate lyase, in the cytosol of cortex and medulla correlated with that observed for AMP deamination and IMP reamination potentials. At 150 microM IMP, the activity of adenylosuccinate synthetase in the cortex and medulla was 0.34 and 1.24 nmol/mg protein/min, respectively. Activity of the adenylosuccinate lyase was severalfold greater than the respective activity of the adenylosuccinate synthetase. These results show that the efficiency of PNC is about twice as high in the medulla cytosol as in the cortex cytosol, and that the activity of PNC in kidney is mainly limited by the activity of adenylosuccinate synthetase and supply of AMP.

Evidence of a species-differentiated regulatory domain within the N-terminal region of skeletal muscle AMP deaminase

Rabbit skeletal muscle AMP deaminase was submitted to limited proteolysis by trypsin that converts the native 80 kDa enzyme subunit to a stable product of approx. 70 kDa, which, in contrast to the native enzyme, is not sensitive to regulation by ATP at pH 6.5. Tryptic peptide mapping indicates that proteolysis is confined to the N-terminal region of the molecule, identifying in this region of AMP deaminase a non-catalytic, 95 residue regulatory domain that stabilises the binding of ATP to a distant site in the molecule. Protein sequence analysis reveals a marked degree of divergence between rat and rabbit skeletal muscle AMP deaminases in the regions containing residues 7-12 and 51-52, giving molecular basis to the hypothesis of the existence of isoenzymes of AMP deaminase in the mature skeletal muscle of the mammals.

Altered kinetics of AMP deaminase by myosin binding

AMP deaminase catalyzes the deamination of AMP to inosine 5'-monophosphate (IMP) and ammonia. Factors controlling the enzyme in muscle can rapidly promote high rates of IMP formation when ATP utilization exceeds supply. We evaluated whether binding of AMP deaminase to myosin, which occurs during intense contraction conditions, alters the kinetic behavior of the enzyme. Reaction kinetics of myosin-bound and free AMP deaminase were evaluated. Reaction kinetics of the free enzyme yielded a near-linear double-reciprocal plot with an expected Km of approximately 1 mM AMP concentration (AMP). In contrast, reaction kinetics of AMP deaminase became bimodal when bound to myosin. At [AMP] less than 0.15 mM, a high-affinity Km (0.05-0.10 mM) with maximal velocity approximately 20% that of free enzyme was evident. At [AMP] greater than 0.15 mM, the Km and maximal velocity values were similar to that of the free enzyme. The 10- to 20-fold higher affinity Km would allow for a higher rate of AMP deamination at the low [AMP] found physiologically. AMP deaminase binding to myosin also induced a marked resistance to orthophosphate inhibition (10 mM) in the presence of 50 microM ADP. Results were similar for purified preparations of AMP deaminase bound to myosin subfragment 2 and crude extracts obtained from contracting muscle. Our results add further support to the hypothesis that AMP deaminase binding to myosin serves an important role in control of enzyme activity in contracting muscle.

AMP deaminase binding in contracting rat skeletal muscle

AMP deaminase, which hydrolyses AMP to inosine 5'-monophosphate (IMP) and NH3 at high rates during excessive energy demands in skeletal muscle, is activated when bound to myosin in vitro. We evaluated AMP deaminase binding in vivo during muscle contractions to assess whether binding 1) is inherent to deamination and found only with high rates of IMP production or simply coincident with the contractile process and 2) requires cellular acidosis. AMP deaminase activity (mumol.min-1.g-1) was measured in the supernatant (free) and 10(4)-g pellet (bound) homogenate fractions of muscle of anesthetized rats after in situ contractions to determine the percent bound. In resting muscle, nearly all (approximately 90%) AMP deaminase is free (cytosolic). During contractions when energy balance was well maintained, binding did not significantly differ from resting values. However, during intense contraction conditions that lead to increased IMP concentration, binding increased to approximately 60% (P less than 0.001) in fast-twitch and approximately 50% in slow-twitch muscle. Binding increased in an apparent first-order manner and preceded initiation of IMP formation. Further, binding rapidly declined within 1 min after cessation of intense stimulation, even though the cell remained extremely acidotic. Extensive binding during contractions was also evident without cellular acidosis (iodoacetic acid-treated muscle). Thus the in vivo AMP deaminase-myosin complex association/dissociation is not coupled to changes in cellular acidosis. Interestingly, binding remained elevated after contractions, if energy recovery was limited by ischemia. Our results are consistent with myosin binding having a role in AMP deaminase activation and subsequent IMP formation in contracting muscle.

Adenine nucleotide degradation in slow-twitch red muscle

The catabolism of adenine nucleotides (AdN) in rat soleus muscle (predominantly slow twitch) is very different from that in fast-twitch muscle. AMP deaminase is highly inhibited during brief (3 min) intense (120 tetani/min) in situ stimulation, resulting in little inosine 5'-monophosphate (IMP) accumulation (0.21 mumol/g). Even with ligation of the femoral artery during the same brief intense contraction conditions there is surprisingly little increase in IMP (0.37 mumol/g), although AdN depletion is evident (-1.30 mumol/g). We have tested the hypothesis that accumulation of purine nucleosides and bases accounts for the AdN depletion by measuring purine degradation products using high-performance liquid chromatography. There was no stoichiometric accumulation of purine degradation products to account for the observed AdN depletion even though metabolite recovery was essentially quantitative. We hypothesis that under these conditions AdN are converted to a form different from purine nucleoside and base degradation products. In contrast to the inhibition of AMP deamination seen during brief ischemia, slow-twitch muscle depletes a substantial fraction (28%) of muscle AdN (1.75 mumol/g) that can be accounted for stoichiometrically as purine degradation products during an extended 10-min ischemic period of mild (12 tetani/min) contraction conditions. IMP accumulation (1 mumol/g) is most prominent with inosine, accounting for 23% (0.4 mumol/g) of the depleted AdN, showing that slow-twitch red muscle is capable of both AMP deamination and the subsequent production of purine nucleosides during an extended period of ischemic contractions. The present results indicate that AdN metabolism in the soleus muscle is complex, yielding expected degradation products or a loss of total purines, depending on contraction conditions.(ABSTRACT TRUNCATED AT 250 WORDS)