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

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.

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.

Characterization of the metallocenter of rabbit skeletal muscle AMP deaminase. A new model for substrate interactions at a dinuclear cocatalytic Zn site

We have previously provided evidence for a dinuclear zinc site in rabbit skeletal muscle AMPD compatible with a (micro-aqua)(micro-carboxylato)dizinc(II) core with an average of two histidine residues at each metal site. XAS of the zinc binding site of the enzyme in the presence of PRN favors a model where PRN is added to the coordination sphere of one of the two zinc ions increasing its coordination number to five. The uncompetitive nature of the inhibition of AMPD by fluoride reveals that the anion probably displaces the nucleophile water molecule terminally coordinated to the catalytic Zn(1) ion at the enzyme C-terminus, following the binding of AMP at the Zn(2) ion located at N-terminus of the enzyme. Thus, the two Zn ions in the AMPD metallocenter operate together as a single catalytic unit, but have independent function, one of them (Zn(1)) acting to polarize the nucleophile water molecule, whilst the other (Zn(2)) acts transiently as a receptor for an activating substrate molecule. The addition of fluoride to AMPD also abolishes the cooperative behaviour induced in the enzyme by the inhibitory effect of ATP at acidic pH that probably resides in the competition with the substrate for an adenine nucleotide specific regulatory site located in the Zn(2) ion binding region and which is responsible for the positive homotropic cooperativity behaviour of AMPD.

Characterization of the metallocenter of rabbit skeletal muscle AMP deaminase. Evidence for a dinuclear zinc site

XAS of Zn-peptide binary and ternary complexes prepared using peptides mimicking the potential metal binding sites of rabbit skeletal muscle AMP deaminase (AMPD) strongly suggest that the region 48-61 of the enzyme contains a zinc binding site, whilst the region 360-372 of the enzyme is not able to form 1:1 complexes with zinc, in contrast with what has been suggested for the corresponding region of yeast AMPD. XAS performed on fresh preparations of rabbit skeletal muscle AMPD provides evidence for a dinuclear zinc site in the enzyme compatible with a (mu-aqua)(mu-carboxylato)dizinc(II) core with an average of two histidine residues at each metal site and a Zn-Zn distance of about 3.3 Angstrom. The data indicate that zinc is not required for HPRG/AMPD interaction, both zinc ions being bound to the catalytic subunit of the enzyme, one to the three conserved amino acid residues among those four assumed to be in contact with zinc in yeast AMPD, and the other at the N-terminal region, probably to His-52, Glu-53 and His-57. Tryptic digests of different enzyme preparations demonstrate the existence of two different protein conformations and of a zinc ion connecting the N-terminal and C-terminal regions of AMPD.

Immunohistochemical analysis of human skeletal muscle AMP deaminase deficiency. Evidence of a correlation between the muscle HPRG content and the level of the residual AMP deaminase activity

We have previously described that, in healthy human skeletal muscle, an anti-histidine-proline-rich-glycoprotein (HPRG) antibody selectively binds to type IIB fibers that are well known to contain the highest level of AMP deaminase (AMPD) activity, suggesting an association of the HPRG-like protein to the enzyme isoform M. The present paper reports an immunohistochemical study performed on human skeletal muscle biopsies from patients with AMPD deficiency and carried out utilizing both the anti-HPRG antibody and an anti-AMPD antibody specific for the isoform M. A correlation between the muscle content of the HPRG-like protein and the level of AMPD activity was demonstrated. In the specimens from patients with Acquired AMPD deficiency the HPRG-immunoreactivity was less intense than that shown by the control subjects and was related to the residual AMPD activity. The patients affected by Primary and Coincidental AMPD deficiency, which were characterized by an absence of enzyme activity and AMPD immunoreactivity, showed the lowest HPRG immunoreactivity that was clearly detectable by Western blot analysis, but not by immunohistochemistry. The interpretation of the significance of these observations suggests a physiological mutual dependence between skeletal muscle HPRG and AMPD polypeptides with regard to their stability.

A calpain-like proteolytic activity produces the limited cleavage at the N-terminal regulatory domain of rabbit skeletal muscle AMP deaminase: evidence of a protective molecular mechanism

On storage at 4 degrees C, rabbit skeletal muscle AMP deaminase undergoes limited proteolysis with the conversion of the native 85-kDa enzyme subunit to a 75-kDa core that is resistant to further proteolysis. Further studies have shown that limited proteolysis of AMP deaminase with trypsin, removing the 95-residue N-terminal fragment, converts the native enzyme to a species that exhibits hyperbolic kinetics even at low K+ concentration. The results of this report show that a 21-residue synthetic peptide, when incubated with the purified enzyme, is cleaved with a specificity identical to that reported for ubiquitous calpains. In addition, the cleavage of a specific fluorogenic peptide substrate by rabbit m-calpain is inhibited by a synthetic peptide that corresponds to residues 10-17 of rabbit skeletal muscle AMP deaminase; this peptide contains a sequence (K-E-L-D-D-A) that is present in the fourth subdomain A of rabbit calpastatin, suggesting that the N-terminus of AMP deaminase shares with calpastatin a regulatory sequence that might exert a protective role against the fragmentation-induced activation of AMP deaminase. These observations suggest that a calpain-like proteinase present in muscle removes from AMP deaminase a domain that holds the enzyme in an inactive conformation and which also contains a regulatory region that protects against unregulated proteolysis. We conclude that proteolysis of AMP deaminase is the basis of the large ammonia accumulation that occurs in skeletal muscle subjected to strong tetanic contraction or passing into rigor mortis.

Expression, purification, and inhibition of in vitro proteolysis of human AMPD2 (isoform L) recombinant enzymes

AMP deaminase (AMPD) is a multigene family in higher eukaryotes whose three members encode tetrameric isoforms that catalyze the deamination of AMP to IMP. AMPD polypeptides share conserved C-terminal catalytic domains of approximately 550 amino acids, whereas divergent N-terminal domains of approximately 200-330 amino acids may confer isoform-specific properties to each enzyme. However, AMPD polypeptides are subject to limited N-terminal proteolysis during purification and subsequent storage at 4 degrees C. This presents a technical challenge to studies aimed at determining the structural and functional significance of these divergent sequences. This study describes the recombinant overexpression of three naturally occurring human AMPD2 proteins, 1A/2, 1B/2, and 1B/3, that differ by N-terminal extensions of 47-128 amino acids, resulting from the use of multiple promoters and alternative splicing events. A survey of protease inhibitors reveals that E-64 and leupeptin are able to maintain the subunit structure of each AMPD2 protein when they are included in extraction and storage buffers. Gel filtration chromatography of these three purified AMPD2 enzymes comprised of intact subunits reveals that each migrates faster than expected, resulting in observed molecular masses significantly greater than those predicted for native tetrameric structures. However, chemical crosslinking analysis indicates four subunits per AMPD2 molecule, confirming that these enzymes have a native tetrameric structure. These combined results suggest that AMPD2 N-terminal extensions may exist as extended structures in solution.

Characterization of the zinc-binding site of the histidine-proline-rich glycoprotein associated with rabbit skeletal muscle AMP deaminase

The AMP deaminase-associated variant of histidine-proline-rich glycoprotein (HPRG) is isolated from rabbit skeletal muscle by a modification of the protocol previously used for the purification of AMP deaminase. This procedure yields highly pure HPRG suitable for investigation by x-ray absorption spectroscopy of the zinc-binding behavior of the protein. X-ray absorption spectroscopy analysis of a 2:1 zinc-HPRG complex shows that zinc is bound to the protein, most probably in a dinuclear cluster where each Zn(2+) ion is coordinated, on average, by three histidine ligands and one heavier ligand, likely a sulfur from a cysteine. 11 cysteines of HPRG from different species are totally conserved, suggesting that five disulfide bridges are essential for the proper folding of the protein. At least another cysteine is present at different positions in the histidine-proline-rich domain of HPRG in all species, suggesting that this cysteine is the candidate for zinc ligation in the muscle variant of HPRG. The same conclusion is likely to be true for the six histidines used by the protein as zinc ligands. The presence in muscle HPRG of a specific zinc-binding site permits us to envisage the addition of HPRG into the family of metallochaperones. In this view, HPRG may enhance the in vivo stability of metalloenzymes such as AMP deaminase.

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.

Localization of N-terminal sequences in human AMP deaminase isoforms that influence contractile protein binding

The reversible association of AMP deaminase (AMPD, EC 3.5.4.6) with elements of the contractile apparatus is an identified mechanism of enzyme regulation in mammalian skeletal muscle. All three members of the human AMPD multigene family contain coding information for polypeptides with divergent N-terminal and conserved C-terminal domains. In this study, serial N-terminal deletion mutants of up to 111 (AMPD1), 214 (AMPD2), and 126 (AMPD3) residues have been constructed without significant alteration of catalytic function or protein solubility. The entire sets of active enzymes are used to extend our understanding of the contractile protein binding of AMPD. Analysis of the most truncated active enzymes demonstrates that all three isoforms can associate with skeletal muscle actomyosin and suggests that a primary binding domain is located within the C-terminal 635-640 residues of each polypeptide. However, discrete stretches of N-terminal sequence alter this behavior. Residues 54-83 in the AMPD1 polypeptide contribute to a high actomyosin binding capacity of both isoform M spliceoforms, although the exon 2- enzyme exhibits significantly greater association compared to its exon 2+ counterpart. Conversely, residues 129-183 in the AMPD2 polypeptide reduce actomyosin binding of isoform L. In addition, residues 1-48 in the AMPD3 polypeptide dramatically suppress contractile protein binding of isoform E, thus allowing this enzyme to participate in other intracellular interactions.

Towards an understanding of the functional significance of N-terminal domain divergence in human AMP deaminase isoforms

Human AMP deaminase (AMPD; EC 3.5.4.6) isoforms are encoded by a multigene family and have conserved C-terminal domains that contain catalytic residues and an ATP-binding site. N-terminal domains diverge dramatically, yet are conserved when compared across mammalian species. Cross-species conservation of entire gene-specific polypeptides (e.g., rat versus human AMPD1) suggests that divergent N-terminal domains may play a role in isoform-specific properties of the enzyme. It now has become evident that the majority of published data used to characterize purified AMPD isoforms were likely derived from preparations lacking significant portions of their N-terminal domains (up to nearly 100 residues). Accumulating evidence indicates that divergent N-terminal sequences do influence catalytic behavior, protein-protein interactions, and intracellular distributions of this enzyme.

Novel aspects of tetramer assembly and N-terminal domain structure and function are revealed by recombinant expression of human AMP deaminase isoforms

AMP deaminase isoforms purified from endogenous sources display smaller than predicted subunit molecular masses, whereas baculoviral expression of human AMPD1 (isoform M) and AMPD3 (isoform E) cDNAs produces full-sized recombinant enzymes. However, nearly 100 N-terminal amino acid residues are cleaved from each recombinant polypeptide during storage at 4 degreesC. Expression of N-truncated cDNAs (DeltaL96AMPD1 and DeltaM90AMPD3) produces stable recombinant enzymes exhibiting subunit molecular masses and kinetic properties that are similar to those reported for purified isoforms M and E. Conversely, wild type recombinant isoforms display significantly higher Km(app) values in the absence of ATP. Gel filtration analysis demonstrates native tetrameric structures for all recombinant proteins, except the wild type AMPD1 enzyme, which forms aggregates of tetramers that disperse upon cleavage of N-terminal residues at 4 degreesC. These data: 1) confirm that available literature on AMP deaminase is likely derived from N-truncated enzymes and 2) are inconsistent with a new model proposing native trimeric structure of an N-truncated rabbit skeletal muscle AMP deaminase (Ranieri-Raggi, M., Montali, U., Ronca, F., Sabbatini, A., Brown, P. E., Moir, A. J. G., and Raggi, A. (1997) Biochem. J. 326, 641-648). N-terminal residues also influence actomyosin-binding properties of the enzyme, which are enhanced and suppressed by AMPD1 and AMPD3 sequences, respectively. Finally, co-expression of AMPD1 and AMPD3 recombinant polypeptides produces tetrameric enzymes with either isoform-specific or mixed subunits, and also reveals that tetramer assembly is driven by relative polypeptide abundance with no apparent preference for like subunits.

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-deaminases from chicken and rabbit muscle: partial primary sequences of homologous 17-kDa CNBr fragments: autorecognition by rabbit anti-[chicken AMPD]

The apparent size (87.5 kDa) of the major polypeptide in freshly isolated chicken muscle AMP deaminase (AMPD.M) was comparable with that predicted from the sequences of the genes for the major muscle isoforms from human and rat. The size of the subunit of AMP deaminase from chicken muscle is indistinguishable from that of the rabbit enzyme. The peptide profiles of cyanogen bromide digests of AMPD.M from chicken and rabbit share a 17-kDa fragment, representing approximately 20% of the intact subunits of these enzymes. The first 25 residues of these fragments are 88.5% identical; the rabbit and chicken segments are greater than 92% and 84% identical, respectively, to the sequences predicted for residues 310-335 for AMPD.M from human and rat. Polyclonal rabbit antisera directed against AMPD.M from chicken breast recognize the full-length AMPD.M polypeptides on immunoblots of extracts of both avian and rabbit muscle, including an antiserum from the rabbit in which the antibody was prepared. The 17-kDa fragments, derived by incomplete cleavage of highly conserved internal segments of the deaminase subunit, share epitopes involved in the autorecognition of rabbit AMPD.M by rabbit polyclonal antibodies directed against the avian AMPD.M.

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.

Mammalian-heart adenylate deaminase: cross-species immunoanalysis of tissue distribution with a cardiac-directed antibody

A sheep antiserum against purified rabbit-heart adenylate deaminase (EC 3.5.4.6) (AMPD) was developed and validated as an immunologic probe to assess the cross-species tissue distribution of the mammalian cardiac AMPD isoform. The antiserum and the antibodies purified therefrom recognized both native and denatured rabbit-heart AMPD in immunoprecipitation and immunoblot experiments, respectively, and antibody binding did not affect native enzyme activity. The immunoprecipitation experiments further demonstrated a high antiserum titer. Immunoblot analysis of either crude rabbit-heart extracts or purified rabbit-heart AMPD revealed a major immunoreactive band with the molecular mass (approximately 81 kDa) of the soluble rabbit-heart AMPD subunit. AMPD in heart extracts from mammalian species other than rabbit (including human) was equally immunoreactive with this antiserum by quantitative immunoblot criteria. Although generally held to be in the same isoform class as heart AMPD, erythrocyte AMPD was not immunoreactive either within or across species. Nor was AMPD from most other tissues [e.g., white (gastrocnemius) muscle, lung, kidney] immunoreactive with the cardiac-directed antibody. Limited immunoreactivity was evidenced by mammalian liver, red (soleus) muscle, and brain extracts across species, indicating the presence of a minor cardiac(-like) AMPD isoform in these tissues. The results of this study characterize the tissue distribution of the cardiac AMPD isoform using a molecular approach with the first polyclonal antibodies prepared against homogeneous cardiac AMPD. This immunologic probe should prove useful at the tissue level for AMPD immunohistochemistry.