Synthesis and metabolic fate of purine nucleotides in cultured fibroblasts from normal subjects and from purine overproducing mutants

Crystal structure of E. coli PRPP synthetase

BACKGROUND

Ribose-phosphate pyrophosphokinase (EC 2.7.6.1) is an enzyme that catalyzes the ATP-dependent conversion of ribose-5-phosphate to phosphoribosyl pyrophosphate. The reaction product is a key precursor for the biosynthesis of purine and pyrimidine nucleotides.

RESULTS

We report the 2.2 Å crystal structure of the E. coli ribose-phosphate pyrophosphobinase (EcKPRS). The protein has two type I phosphoribosyltransferase folds, related by 2-fold pseudosymmetry. The propeller-shaped homohexameric structure of KPRS is composed of a trimer of dimers, with the C-terminal domains forming the dimeric blades of the propeller and the N-terminal domains forming the hexameric core. The key, conserved active site residues are well-defined in the structure and positioned appropriately to bind substrates, adenosine monophosphate and ribose-5-phosphate. The allosteric site is also relatively well conserved but, in the EcKPRS structure, several residues from a flexible loop occupy the site where the allosteric modulator, adenosine diphosphate, is predicted to bind. The presence of the loop in the allosteric site may be an additional level of regulation, whereby low affinity molecules are precluded from binding.

CONCLUSIONS

Overall, this study details key structural features of an enzyme that catalyzes a critical step in nucleotide metabolism. This work provides a framework for future studies of this important protein and, as nucleotides are critical for viability, may serve as a foundation for the development of novel anti-bacterial drugs.

Effect of testosterone on purine synthesis de novo in rat perineal muscle in vivo

We examined in vivo the influence of testosterone on purine synthesis de novo, in the levator ani and gastrocnemius muscles of the rat. The hypoxanthine, adenine and guanine contents and the rate of incorporation of [14C]formate into these purine bases were determined in castrated adult and prepubertal rats (groups 1 and 2) both before and after orchiectomy and, in the second case, at different times after testosterone treatment. Substantially similar behavior was found in both groups, with some specific differences. The results showed an increase in the basal levels after castration (except for a dramatic decrease in adenine and a rise in the Gua/Ade molar ratio in prepubertal rats) and a return to basal levels after hormone administration, which was also accompanied by variations in the Gua/Ade molar ratio. The kinetics of purine nucleotide synthesis de novo in vivo and, specifically, of the overall reactions: IMP formation from PRib-PP, IMP----AMP and IMP----GMP, were followed by evaluating the incorporation curves of [14C]formate into hypoxanthine, adenine and guanine. Our results show that testosterone administration enhanced the incorporation rate and gave characteristic patterns: a diphasic cyclic oscillation of the Ade values in adult castrated rats, and single peaks having a specific shape in the other cases. The Gua/Ade labeling ratio was unchanged in castrated rats and increased in both groups during the first 5 days after testosterone treatment, after which values even fell below normal; in most cases, values overlapped the pattern of the Gua/Ade molar ratio. The specific profile of the curves indicated that testosterone initially accelerated the turnover of guanylic acid and in the second phase re-established the normal behavior and ratio of AMP and GMP formation. These results indicate that the 'inosinic branch point' was subject to regulation by testosterone. The profiles of the incorporation curves and of the Gua/Ade ratio were indicative of a primary and secondary response to hormone action.

Characterization of purine nucleotide metabolism in primary rat cardiomyocyte cultures

Primary rat cardiomyocyte cultures were utilized as a model for the study of purine nucleotide metabolism in the heart muscle, especially in connection with the mechanisms operating for the conservation of adenine nucleotides. The cultures exhibited capacity to produce purine nucleotides from nonpurine molecules (de novo synthesis), as well as from preformed purines (salvage synthesis). The conversion of adenosine to AMP, catalyzed by adenosine kinase, appears to be the most important physiological salvage pathway of adenine nucleotide synthesis in the cardiomyocytes. The study of the metabolic fate of IMP formed from [14C]formate or [14C]hypoxanthine and that of AMP formed from [14C]adenine or [14C]adenosine revealed that in the cardiomyocyte the main flow in the nucleotide interconversion pathways is from IMP to AMP, whereas the flux from AMP to IMP appeared to be markedly slower. Following synthesis from labeled precursors by either de novo or salvage pathways, most of the radioactivity in purine nucleotides accumulated in adenine nucleotides, and only a small proportion of it resided in IMP. The results suggest that the main pathway of AMP degradation in the cardiomyocyte proceeds through adenosine rather than through IMP. About 90% of the total radioactivity in purines effluxed from the cells during de novo synthesis from [14C]formate or following prelabeling of adenine nucleotides with [14C]adenine were found to reside in hypoxanthine. The activities in cell extracts of AMP 5'-nucleotidase and IMP 5'-nucleotidase, which catalyze nucleotide degradation, and of AMP deaminase, a key enzyme in the purine nucleotide cycle, were low. The nucleotidase activity resembles, and that of the AMP deaminase contrasts the respective enzyme activities in extracts of cultured skeletal-muscle myotubes. The results indicate that in the cardiomyocyte, in contrast to the myotube, the main mechanism operating for conservation of nucleotides is prompt phosphorylation of AMP, rather than operation of the purine nucleotide cycle. The primary cardiomyocyte cultures are a plausible model for the study of purine nucleotide metabolism in the heart muscle.

Purine biosynthesis de novo in rat skeletal muscle

Purine biosynthesis by the 'de novo' pathway was demonstrated in isolated rat extensor digitorum longus muscle with [1-14C]glycine, [3-14C]serine and sodium [14C]formate as nucleotide precursors. Evidence is presented which suggests that the source of glycine and serine for purine biosynthesis is extracellular rather than intracellular. The relative incorporation rates of the three precursors were formate greater than glycine greater than serine. Over 85% of the label from formate and glycine was recovered in the adenine nucleotides, principally ATP. Azaserine markedly inhibited purine biosynthesis from both formate and glycine. Cycloserine inhibited synthesis from serine, but not from formate. Adenine, hypoxanthine and adenosine markedly inhibited purine synthesis from sodium [14C]formate.

Alterations in purine nucleotide metabolism during muscle differentiation in vitro

Pathways of purine nucleotide metabolism affecting the availability of ATP in the muscle tissue were studied in differentiating rat muscle cultures. The rate of de novo purine nucleotide synthesis and of AMP deamination were found to increase markedly with cell differentiation, but the rate of IMP dephosphorylation was similarly low in both myoblasts and contracting fibers. The above differentiation-associated alterations in purine nucleotide metabolism conform with the greater need for ATP as a source of energy in the contracting myotubes.

De novo purine synthesis, purine salvage, and DNA synthesis in normal and Lesch-Nyhan fibroblasts infected with Mycoplasma pneumoniae

The effects of Mycoplasma pneumoniae on host cell metabolism were studied by using two types of host cells, MRC-5 human lung fibroblasts, a normal cell line, and Lesch-Nyhan fibroblasts, a cell line deficient in hypoxanthine-guanine phosphoribosyl transferase (EC 2.4.2.8). The susceptibilities of the two cell types were determined by infecting the cells with M. pneumoniae at different multiplicities of infection (MOI). Our data indicate that the Lesch-Nyhan cells were four times more susceptible to damage by M. pneumoniae than the MRC-5 cells. The effects of different MOIs (10 and 50) on de novo purine synthesis. DNA synthesis, and the development of a cytopathic effect were determined. In both cell types, the higher MOI inhibited de novo purine synthesis to a greater extent than the lower MOI. This correlated closely with the cytopathic effect which developed in the monolayers (i.e., the more the inhibition of de novo purine synthesis, the greater the cytopathic effect which developed). In the Lesch-Nyhan cells, DNA synthesis was completely inhibited by the high MOI, whereas in the MRC-5 cells, DNA synthesis was stimulated by the high MOI. In the MRC-5 cells infected with M. pneumoniae, purine salvage activity increased, as indicated by an increase in adenosine deaminase (EC 3.5.4.4) activity. These data indicate that M. pneumoniae alters host cell metabolism, particularly the nucleic acid metabolic pathways. This may explain in part the mechanism of pathogenesis of M. pneumoniae infection.

Alterations in nucleotide content of human lung fibroblasts infected with Mycoplasma pneumoniae

The nucleotide content of normal MRC-5 human lung fibroblasts and fibroblasts infected with Mycoplasma pneumoniae PI 1428 was determined. Nucleotides from control and infected fibroblasts were extracted with 5% trichloracetic acid. After neutralization of the extracts, the nucleotides in the extracts were separated by anion-exchange chromatography. Significant differences were found between the nucleotide content of the control and infected cells. Nucleotide triphosphate levels were twofold higher in the control fibroblasts than in the infected fibroblasts 4 h after the initiation of infection. At the same time, nucleotide diphosphate and monophosphate levels were higher in the infected fibroblasts than in the control fibroblasts. Determination of the energy charge ratio for each set of nucleotides (adenosine, guanosine, cytidine, and uridine) demonstrated a shift of nucleotide content in the infected fibroblasts. Immediately after infection, the energy charge for each set of nucleotides was higher for the control fibroblasts than it was for the infected fibroblasts. This pattern continued throughout the infection period with only minor exceptions. The work presented here indicates a loss of energy charge in fibroblasts infected with M. pneumoniae and may help to explain some of the metabolic changes and cell damage which accompany infection.

Effects of fructose on synthesis and degradation of purine nucleotides in isolated rat hepatocytes

The effects of fructose on purine nucleotide synthesis and degradation were studied in isolated rat hepatocytes. Incubation of the hepatocytes with fructose resulted in deceleration of the rate of de novo purine synthesis, gauged by the rate of incorporation of precusor [14C]formate into total purines produced, and in acceleration of purine nucleotide degradation, as measured by the rate of conversion of prelabelled purine nucleotides into end-product allantoin. These effects were found to be associated with decreases in cellular content of ATP and Pi and in the metabolic availability of 5-phosphoribosyl 1-pyrophosphate. The results support the suggestion that the fructose-induced acceleration of purine degradation is mediated through activation of AMP deaminase. However, the results also suggest that decreased reutilization of hypoxanthine for IMP synthesis, due to the decreased PP-Rib-P availability, is an additional mechanism for the acceleration of purine degradation. The decreased PP-Rib-P availability is also suggested to be the main mechanism for the fructose-induced deceleration of purine synthesis.

Characterization of purine nucleotide metabolism in primary rat muscle cultures

The synthesis and metabolic fate of purine nucleotides were studied, employing labeled precursors, in primary rat muscle cultures. The cultures were found to produce purine nucleotides, by de novo and salvage pathways, both exhibiting dependence on cellular availability of substrate 5-phosphoribosyl-1-pyrophosphate (PPRibP). Depletion of cellular PPRibP decelerated the rate of purine synthesis, whereas increasing PPRibP generation by high Pi concentration in the incubation medium, accelerated purine synthesis. Ribose accelerated purine synthesis, indicating that ribose 5-phosphate availability in the cultured muscle is limiting for PPRibP synthesis. The study in the muscle cultures of the metabolic fate if IMP formed from [14C]formate and that of nucleotides formed from labeled purine bases, revealed that the main flow in the nucleotide interconversions pathways is from AMP to IMP. The flow from IMP to GMP and to AMP appeared to be of a lesser magnitude and virtually no flow could be detected from GMP to IMP. The greatest proportion of radioactivity of purine nucleotides following synthesis by either de novo or salvage pathways, accumulated in IMP, reflecting the relative rates of flows between the various nucleotides and probably also a relatively low, or inhibited activity of the IMP nucleotidase. The results suggest that primary muscle cultures are a plausible model for the study of the role of purine metabolism in muscle work.