Purine nucleotides, effect

The association between RNase A and 3 -UMP or 3 -dUMP has been studied by P n.m.r. and kinetic methods, respectively. In both cases the participation of two dissociable groups at the active site of the enzyme was demonstrated, in agreement with n.m.r. and A -ray - studies on the binding of 3 -CMP to RNase. In the binding of Tj RNase to purine nucleotide monophosphates, the phosphate group appears to have an important effect while the ribose ring is relatively unimportant. ... 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

The answer is D. Impaired immune function in severe combined immunodeficiency (SCID) is the direct result of blocked DNA synthesis due to inadequate supplies of de-oxyribonucleotides in B and T cells. This effect arises by dATP-induced allosteric inhibition of ribonucleotide reductase, which catalyzes reduction of the 2 -hydroxyl groups on ADP and GDP to form dADP and dCDP. The ultimate cause of many cases of SCID is adenosine deaminase deficiency, which leads to accumulation of dATP and consequent inhibition of ribonucleotide reductase. Although the other enzymes mentioned are also involved in purine nucleotide metabolism, their deficiencies do not lead to SCID. 

Synthesis of purine nucleotides, showing the inhibitory effect of some structural analogs. 

A recent study (153a) of the effect of purine nucleotides on the Reactivated enzyme from muscle of several species showed that while... 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

A highly unusual feature of DHFR in Apicomplexa and Kinetoplastida is its association with thymidylate synthase in the same protein. DHFR activity is always located at the amino terminal portion, while the thymidylate synthase activity resides in the carboxyl terminal. The two enzyme functions do not appear to be interdependent eg, the DHFR portion of the P falciparum enzyme molecule was found to function normally in the absence of the thymidylate synthase portion. It is likely that since the protozoan parasites do not perform de novo synthesis of purine nucleotides, the primary function of the tetrahydrofolate produced by DHFR is to provide 5,10-methylenetetrahydrofolate only for the thymidylate synthase-catalyzed reaction. Physical association of the two enzymes may improve efficiency of TMP synthesis. If an effective means of disrupting the coordination between the two activities can be developed, this bifunctional protein may qualify as a target for antiparasitic therapy. 

The molecular basis for the therapeutic effects of the purine analogs is unknown. Intracellular 6-thioguanine causes inhibition of purine nucleotide metabolism and DNA synthesis and repair, resulting in inhibition of cell division and proliferation. 

A serious genetic disorder is associated with the salvage pathways, the Lesch-Nyhan syndrome. It is believed that it is caused by a failure to control the de novo purine biosynthetic pathway. In the Lesch-Nyhan syndrome, the enzyme HGPRTase is severely depressed. Because the de novo pathway is controlled largely via feedback effects of purine nucleotides, the pathway is derepressed and excessive quantities of purine nucleotides and their degradation product, uric acid, are accumulated. This results is neurologic effects, self-mutilation, and mental retardation. 

Okwuasaba FK, Cook MA (1980) The effect of theophylline and other methylxanthines on the presynaptic inhibition of the longitudinal smooth muscle of the guinea pig ileum induced by purine nucleotides. J Pharmacol Exp Ther 215 704-709... 

Fig. 10.2. The respiratory pattern of isolated brown fat mitochondria. A. When substrate (succinate) is added to brown fat mitochondria (here isolated from cold-acclimated or control guinea-pigs), they respire rapidly. Upon ADP addition the rate is initially increased (normal State 2-3 transition), but the ensuing State 4 rate is lower than State 2. Successive ADP additions result in a successively decreased State 4 rate. Numbers indicate respiratory rates in nmol oxygen-min -mg protein. (Adapted from Pedersen and Flatmark [93] for details see this paper.) B. The specific coupling effect of purine nucleotides (here ADP) can be demonstrated after addition of oligomycin so that the respiratory stimulation due to ATP synthesis is eliminated. Addition of the uncoupler FCCP results in a respiratory rate identical to that prior to ADP, indicating that the ADP effect is on coupling, and not due to inhibition of substrate oxidation. (Adapted from Cannon et al. [23] for details see this paper.)...

The detailed mechanism of myocardial protection via PC is not fully understood yet. Many pathways have been proposed and include myocardial stunning, synthesis of heat-shock proteins, involvement of G-proteins, and nitric oxide production [3-5]. The generally accepted model is that the ischemic phase leads to enhanced catabolism of purine nucleotides, resulting in a high level of adenosine. These activate PKC and a cascade of signaling steps leading to activation of MAP, MAPK and MAPKK, culminating in a marked effect on ATP-dependent channels [3,4,6, ]. 

---