Arsenolysis and phosphorolysis of citrulline in mammalian liver

Inorganic phosphate, arsenate, and vanadate enhance exonuclease transcript cleavage by RNA polymerase by 2000-fold

Inorganic P_i is involved in all major biochemical pathways. Here we describe a previously unreported activity of P_i We show that P_i and its structural mimics, vanadate and arsenate, enhance nascent transcript cleavage by RNA polymerase (RNAP). They engage an Mg²⁺ ion in catalysis and activate an attacking water molecule. P_i, vanadate, and arsenate stimulate the intrinsic exonuclease activity of the enzyme nearly 2,000-fold at saturating concentrations of the reactant anions and Mg²⁺ This enhancement is comparable to that of specialized transcript cleavage protein factors Gre and TFIIS (3,000- to 4,000-fold). Unlike these protein factors, P_i and its analogs do not stimulate endonuclease transcript cleavage. Conversely, the protein factors only marginally enhance exonucleolytic cleavage. P_i thus complements cellular protein factors in assisting hydrolytic RNA cleavage by extending the repertoire of RNAP transcript degradation modes.

Arsenate and phosphate as nucleophiles at the transition states of human purine nucleoside phosphorylase

Purine nucleoside phosphorylase (PNP) catalyzes the reversible phosphorolysis of 6-oxypurine (2'-deoxy)ribonucleosides, generating (2-deoxy)ribose 1-phosphate and the purine base. Transition-state models for inosine cleavage have been proposed with bovine, human, and malarial PNPs using arsenate as the nucleophile, since kinetic isotope effects (KIEs) are obscured on phosphorolysis due to high commitment factors. The Phe200Gly mutant of human PNP has low forward and reverse commitment factors in the phosphorolytic reaction, permitting the measurement of competitive intrinsic KIEs on both arsenolysis and phosphorolysis of inosine. The intrinsic 1'-(14)C, 1'-(3)H, 2'-(2)H, 9-(15)N, and 5'-(3)H(2) KIEs for inosine were measured for arsenolysis and phosphorolysis. Except for the remote 5'-(3)H(2), and some slight difference between the 2'-(2)H KIEs, all isotope effects originating in the reaction coordinate are the same within experimental error. Hence, arsenolysis and phosphorolysis proceed through closely related transition states. Although electrostatically similar, the volume of arsenate is greater than phosphate and supports a steric influence to explain the differences in the 5'-(3)H(2) KIEs. Density functional theory calculations provide quantitative models of the transition states for Phe200Gly human PNP-catalyzed arsenolysis and phosphorolysis, selected upon matching calculated and experimental KIEs. The models confirm the striking resemblance between the transition states for the two reactions.

Glutathione-supported arsenate reduction coupled to arsenolysis catalyzed by ornithine carbamoyl transferase

Three cytosolic phosphorolytic/arsenolytic enzymes, (purine nucleoside phosphorylase [PNP], glycogen phosphorylase, glyceraldehyde-3-phosphate dehydrogenase) have been shown to mediate reduction of arsenate (AsV) to the more toxic arsenite (AsIII) in a thiol-dependent manner. With unknown mechanism, hepatic mitochondria also reduce AsV. Mitochondria possess ornithine carbamoyl transferase (OCT), which catalyzes phosphorolytic or arsenolytic citrulline cleavage; therefore, we examined if mitochondrial OCT facilitated AsV reduction in presence of glutathione. Isolated rat liver mitochondria were incubated with AsV, and AsIII formed was quantified. Glutathione-supplemented permeabilized or solubilized mitochondria reduced AsV. Citrulline (substrate for OCT-catalyzed arsenolysis) increased AsV reduction. The citrulline-stimulated AsV reduction was abolished by ornithine (OCT substrate inhibiting citrulline cleavage), phosphate (OCT substrate competing with AsV), and the OCT inhibitor norvaline or PALO, indicating that AsV reduction is coupled to OCT-catalyzed arsenolysis of citrulline. Corroborating this conclusion, purified bacterial OCT mediated AsV reduction in presence of citrulline and glutathione with similar responsiveness to these agents. In contrast, AsIII formation by intact mitochondria was unaffected by PALO and slightly stimulated by citrulline, ornithine, and norvaline, suggesting minimal role for OCT in AsV reduction in intact mitochondria. In addition to OCT, mitochondrial PNP can also mediate AsIII formation; however, its role in AsV reduction appears severely limited by purine nucleoside supply. Collectively, mitochondrial and bacterial OCT promote glutathione-dependent AsV reduction with coupled arsenolysis of citrulline, supporting the hypothesis that AsV reduction is mediated by phosphorolytic/arsenolytic enzymes. Nevertheless, because citrulline cleavage is disfavored physiologically, OCT may have little role in AsV reduction in vivo.

Plasma ornithine carbamyl transferase level as an indicator of ischaemic injury of rat liver

The activity of ornithine carbamyl transferase (OCT) and glutamate pyruvate transaminase (GPT) in serum has been correlated with the extent of necrosis 24 h after different periods of ischaemia in rat liver. The extent of necrosis has been quantified as the volume density of necrosis in the total ischaemic liver lobes using tetranitro BT. The GPT-activity in serum is maximal between 1 and 5 h after different periods of ischaemia, whereas OCT reaches its maximum between 5 and 12 h after ischaemia. The total amount of leaked enzyme-activity as well as the peak value give a linear correlation with the extent of necrosis for OCT and GPT. There is a difference between the character of these two enzymes in that a small leakage of GPT does not indicate liver cell necrosis later on. However, the appearance of OCT in the blood, an enzyme localized in the mitochondrial matrix, has a predictive value for the extent of necrosis, likely to occur later on. GPT, an enzyme from the cytoplasm, can also occur in the blood during the reversible stage of liver cell damage.

Plasma ammonia and liver ornithine transcarbamoylase activity in zinc-deficient rats

The effects of zinc deficiency on the activity of hepatic ornithine carbamoyltransferase (OCT) and plasma ammonia were studied in rats. One group received (ad libitum) zinc-deficient diet containing 2 ppm zinc and the other group received a diet containing 110 ppm zinc (group pair-fed control) equal to the amount consumed by zinc-deficient rats during the previous 24 h. Rats were killed at weekly intervals. Blood urea nitrogen (BUN), plasma ammonia, and hepatic OCT activity were determined. By end of the 1st wk on zinc-deficient diet, the plasma ammonia levels became significantly higher than those of the controls and remained elevated thoughout the study period. BUN increased initially for 2 wk in the deficient rats, but by the end of 4 wk the levels were lower than in the controls. The hepatic OCT activity in deficient animals was significantly lowered as compared to the controls by the 3rd wk. It is concluded that an increase in plasma ammonia may occur as a result of deficiency of zinc.

Structural and regulatory mutations allowing utilization of citrulline or carbamoylaspartate as a source of carbamoylphosphate in Escherichia coli K-12

Escherichia coli mutants lacking carbamoylphosphate synthase require arginine and uracil for growth. It is, however, possible to obtain mutants in which carbamoylphosphate is obtained by phosphorolysis of citrulline or carbamyolaspartate. Citrulline utilizers are argG bradytrophs or strains in which the synthesis of ornithine carbamoyltransferase (either of the F or I type) is specifically depressed by unstable chromosomal rearrangements or stable mutations that presumably affect the operators of those genes. Carbamoylaspartate utilization as a source of carbamoylphosphate appears to require more than one mutation; the best-understood strains are pyrD pyrH or pyrC pyrH mutants in which aspartate carbamoyltransferase activity is high and the pool of cytidine triphosphate (feedback inhibitor of aspartate carbamoyl-transferase) is presumably low and in which channeling of carbamoylaspartate towards pyrimidine biosynthesis is considerably reduced. Selection of enzyme overproducers based on a metabolic dependency for a reversed enzymatic reaction can be regarded as a means for isolating regulatory mutants.

A direct method for the estimation of ornithine carbamoyltransferase activity in serum

Ornithine carbamoyltransferase (EC 2.1.3.3) activity was estimated by determining the amount of citrulline produced. The citrulline was colorimetrically measured by using a diacetylmonoxime-thiosemicarbazide reaction without deproteinization. The color complex which had the maximum absorbance at 515 nm was stable at room temperature in daylight for at least several hours. The proposed method was superior to other methods already reported in sensitivity, stability and simplicity.