Metabolism of nucleosides by isolated mouse liver mitochondria

Origin of guanine nucleotides in isolated heart mitochondria

Presence of guanine nucleotide within the matrix of mitochondria is uncontested; the mechanism by which GTP takes up residence in the matrix is unknown. In this report, we demonstrate for the first time that direct transport of guanine nucleotide across the inner membrane of heart mitochondria is possible. Transport of guanine nucleotides from the medium to the matrix was suggested by inhibition of translation in isolated rat heart mitochondria when GTP-gamma-S was added to the medium. This result suggested that GTP was one source of matrix GTP. Other sources were investigated by measuring matrix uptake and conversion to GTP of several purines, purine nucleosides, and purine nucleotides. Results demonstrated that [14C]-guanine and [3H]-guanosine were not taken up by isolated mitochondria and were not converted to any other compound. While [14C]-ATP and [3H]-AMP were taken up readily into the matrix, radioactivity was never associated with a guanine compound. [3H]-IMP was not taken up into the matrix and was never converted to another compound. Our data showed that label added as [3H]-GTP, [3H]-GDP, or [3H]-GMP was readily taken up and concentrated in the matrix of isolated mitochondria.

Identification of a novel membrane transporter associated with intracellular membranes by phenotypic complementation in the yeast Saccharomyces cerevisiae

A partial mouse cDNA was isolated by its ability to functionally complement a thymidine transport deficiency in plasma membranes of the yeast, Saccharomyces cerevisiae. The full-length cDNA encoded a previously unidentified 27-kDa protein (mouse transporter protein (MTP)) with four predicted transmembrane-spanning domains. MTP mRNA was detected in cells of several mammalian species, and its predicted protein sequence exhibited near identity (98%) with that of a human cDNA (HUMORF13). MTP and its homologs evidently reside in an intracellular membrane compartment because a protein (about 24 kDa) that was recognized by MTP-specific antibodies was observed in a subcellular fraction of rat hepatocytes enriched for Golgi membranes. Deletion of the hydrophilic C terminus of MTP, which encompassed two putative signal motifs for intracellular localization (Tyr-X-X-hydrophobic amino acid), allowed expression of recombinant protein (MTP deltaC) in plasma membranes of Xenopus laevis oocytes. MTP deltaC-expressing oocytes exhibited greater fragility than nonexpressing oocytes, and those that survived the experimental manipulations were capable of mediated uptake of thymidine, uridine, and adenosine. Thymidine uptake by MTP deltaC-expressing oocytes was inhibited by thymine and dTMP. MTP may function in the transport of nucleosides and/or nucleoside derivatives between the cytosol and the lumen of an intracellular membrane-bound compartment.

The partial purification and characterization of purine nucleoside phosphorylase from mammalian mitochondria

Cytosolic purine nucleoside phosphorylase (PNPase) is a well known, and described enzyme which exists in a variety of organisms, both procaryotic and eucaryotic. More recently this enzyme was found in bovine liver mitochondria. The mitochondrial purine nucleoside phosphorylase was purified 63 fold and has a molecular weight of 48-60 kD. From Lineweaver-Burk plots apparent KM's of 23 microM for inosine, 42 microM for deoxyinosine, 40 microM for phosphate, 2 microM for hypoxanthine, and 163 microM for ribose-1-phosphate were calculated. Both 8-aminoguanosine (Ki = 0.5 microM) and araG (Ki = 381 microM) are inhibitors of the enzyme. The protein's isoelectric point (pI) was calculated at a pH of 4.2. Preliminary immunological work showed no cross-reactivity between epitopes on the mitochondrial protein and those on PNPase from human erythrocytes. The apparent KM's calculated for the mitochondrial enzyme are, with the exception of that using hypoxanthine, within the range commonly associated with KM's from the cytosolic species. The mitochondrial enzyme's molecular weight and pI are less than normally described. The enzyme's isolation from mitochondria, together with several unique characteristics, suggest that it is a separate protein from that found in the cytosol.

Phosphorylation of deoxyguanosine in intact and fractured mitochondria

The phosphorylation of deoxyguanosine was measured in fractured and intact mitochondria and an apparent Km of 16 microM for deoxyguanosine was calculated using fractured mitochondria. The effects of various deoxynucleotides on the phosphorylating activity in fractured organelles was tested at both a high and low ratio of NXP/ATP and at two pH values, 7.0 and 5.5. Exogenous dGTP, dGDP or dITP were inhibitory under all conditions tested. With a NXP/ATP ratio of 0.08 at pH 7.0, TTP, TDP, dADP, ADP, UTP and UDP were stimulatory, but at pH 5.5 only TTP elicited that response. When the NXP/ATP ratio was 10 at pH 5.5, TTP and UTP increased the activity more than 10-fold, whereas, at pH 7.0 TTP, TDP, dADP, ADP, UTP, UDP caused stimulation, but to a much lesser extent. When exogenous Mg2+, Mn2+ or Ca2+ were added to intact mitochondria, the rates of phosphorylation were lowered. In fractured mitochondria in the absence of exogenous ATP, little phosphorylation occurs, hence these metal ions caused little change. ATP-Mg, ATP-Mn and ATP-Ca, each at 0.05 mM caused a small inhibition with intact mitochondria, whereas, these compounds supported phosphorylation with fractured organelles. ATP-Mn (10 mM) or ATP-Ca (10 mM) stimulated phosphorylation in both intact and fractured mitochondria. Intact mitochondria synthesized dGMP, dGDP and dGTP when metal ion or ATP-Me concentrations were low (0.05 mM) or when Mg2+ concentration was high (10 mM). Additions of ATP-Ca, ATP-Mn, ATP-Mg, Mn2+ or Ca2+ at 10 mM cause the loss of dGDP and dGTP formation and, in most cases, an increase in the synthesis of dGMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Human deoxycytidine kinase. Purification and characterization of the cytoplasmic and mitochondrial isozymes derived from blast cells of acute myelocytic leukemia patients

A procedure for purifying human cytoplasmic and mitochondrial deoxycytidine kinase (NTP:deoxycytidine 5'-phosphotransferase, EC 2.7.1.74) was developed. Both purified isozymes have a similar molecular weight, activation energy and catalyze the reaction by a sequential mechanism. These two isozymes differ with respect to their substrate specificities. With cytoplasmic deoxycytidine kinase, ATP, GTP and TTP have the highest reaction velocity. Pyrimidine nucleoside triphosphates have higher affinity but lower V than purine nucleoside triphosphates. Cytidine and arabinosylcytidine can serve as substrates. With mitochondrial isozyme only ATP gives the highest reaction velocity. ATP and dATP have the same Km but different V values. Besides deoxycytidine, also deoxythymidine but not cytidine or arabinosylcytidine can serve as substrates. There are also differences between these two isozymes with respect to their sensitivity to inhibition. For cytoplasmic enzyme, Br5dCyd and Iodo5dCyd are not inhibitory. Both dCTP and UTP are competitive inhibitors (Ki 0.25 and 0.5 micronM, respectively) with respect to ATP. For mitochondrial isozyme both Br5dCyd and Iodo5dCyd are inhibitory and dCTP and TTP are competitive inhibitors (Ki 2 and 10 micronM, respectively) with respect to ATP.