Targeting of pre-ornithine transcarbamylase to mitochondria: definition of critical regions and residues in the leader peptide

The cytoplasmically synthesized precursor of the mitochondrial matrix enzyme, ornithine transcarbamylase (OTC), is directed to mitochondria by its amino-terminal leader peptide. To define the critical residues and/or regions in the OTC leader peptide, we have synthesized OTC precursors with alterations in the leader portion. Analysis of deletions reveals that the middle portion of the 32 residue leader peptide is absolutely required for both mitochondrial uptake and proteolytic processing, whereas NH2-terminal and penultimate COOH-terminal portions are not. Analysis of precursors with single substitutions revealed complete loss of function when arginine 23 was substituted with glycine. Additional substitutions suggested that the critical role of this arginine residue may be mediated by participation in a local secondary structure, very likely an alpha-helix, which is proposed to be an essential element in the midportion of the leader peptide.

Arginine metabolism in Saccharomyces cerevisiae: subcellular localization of the enzymes

Subcellular localization of enzymes of arginine metabolism in Saccharomyces cerevisiae was studied by partial fractionation and stepwise homogenization of spheroplast lysates. These enzymes could clearly be divided into two groups. The first group comprised the five enzymes of the acetylated compound cycle, i.e., acetylglutamate synthase, acetylglutamate kinase, acetylglutamyl-phosphate reductase, acetylornithine aminotransferase, and acetylornithine-glutamate acetyltransferase. These enzymes were exclusively particulate. Comparison with citrate synthase and cytochrome oxidase, and results from isopycnic gradient analysis, suggested that these enzymes were associated with the mitochondria. By contrast, enzymatic activities going from ornithine to arginine, i.e., arginine pathway-specific carbamoylphosphate synthetase, ornithine carbamoyltransferase, argininosuccinate synthetase, and argininosuccinate lyase, and the two first catabolic enzymes, arginase and ornithine aminotransferase, were in the "soluble" fraction of the cell.

Identification and characterization of two temperature-induced surface-associated proteins of Streptococcus suis with high homologies to members of the Arginine Deiminase system of Streptococcus pyogenes

The present study was performed to identify stress-induced putative virulence proteins of Streptococcus suis. For this, protein expression patterns of streptococci grown at 32, 37, and 42 degrees C were compared by one- and two-dimensional gel electrophoresis. Temperature shifts from 32 and 37 to 42 degrees C induced expression of two cell wall-associated proteins with apparent molecular masses of approximately 47 and 53 kDa. Amino-terminal sequence analysis of the two proteins indicated homologies of the 47-kDa protein with an ornithine carbamoyltransferase (OCT) from Streptococcus pyogenes and of the 53-kDa protein with the streptococcal acid glycoprotein (SAGP) from S. pyogenes, an arginine deiminase (AD) recently proposed as a putative virulence factor. Cloning and sequencing the genes encoding the putative OCT and AD of S. suis, octS and adiS, respectively, revealed that they had 81.2 (octS) and 80.2% (adiS) identity with the respective genes of S. pyogenes. Both genes belong to the AD system, also found in other bacteria. Southern hybridization analysis demonstrated the presence of the adiS gene in all 42 serotype 2 and 9 S. suis strains tested. In 9 of these 42 strains, selected randomly, we confirmed expression of the AdiS protein, homologous to SAGP, by immunoblot analysis using a specific antiserum against the SAGP of S. pyogenes. In all strains AD activity was detected. Furthermore, by immunoelectron microscopy using the anti-S. pyogenes SAGP antiserum we were able to demonstrate that the AdiS protein is expressed on the streptococcal surface in association with the capsular polysaccharides but is not coexpressed with them.

Cytochemical demonstration of ornithine carbamoyltransferase activity in liver mitochondria of rat and mouse

In order to demonstrate ornithine carbamoyltransferase activity cytochemically, thin slices of liver and kidney of rat and mouse were fixed in cold acetone or formol-calcium, and incubated in a medium containing l-ornithine, carbamoyl phosphate, Tris-maleate buffer (pH 7.2), lead nitrate and sucrose. The specific reaction product occurred in the mitochondria of the hepatocytes only, and not in other cells of the liver or kidney. The specificity of the reaction was supported by the following observations. (1) The mitochondrial reaction was not obtained in sections incubated in a medium from which ornithine was omitted. (2) Other amino acids gave no reaction. (3) p-Chloromercuribenzoate suppressed the reaction. (4) The hepatocytes of chick (uricotelic) did not give the reaction. A nonspecific reaction in lysosomes and brush borders is caused probably by acid and alkaline phosphatase activities.

Arginine catabolism in Lactobacillus sake isolated from meat

Lactobacillus sake isolated from meat can hydrolyze arginine via the arginine deiminase pathway. Two enzymes, arginine deiminase and ornithine transcarbamylase, have been revealed by detection of their reaction products, citrulline and ornithine, respectively. The production of citrulline depends on the concentration of glucose in a synthetic medium; it does not occur when the concentration of glucose is 27.5 mM or higher.

Adaptive changes in the capacity of systems used for the synthesis of citrulline in rat liver mitochondria in response to high- and-low-protein diets

1. Citrulline synthesis was measured in mitochondria from rats fed on a standard diet, a high-protein diet, or on glucose. 2. With NH(4)Cl as the nitrogen source the rate of citrulline synthesis was higher in mitochondria from rats fed on a high-protein diet than in those from rats fed on a standard diet. When rats were fed solely on glucose the rate of synthesis of citrulline from NH(4)Cl was very low. 3. With glutamate as the nitrogen source the relative rates of citrulline synthesis were much lower than when NH(4)Cl was present, but similar adaptive changes occurred. 4. The activity of the mitochondrial glutamate-transporting system increased two to three times on feeding rats on a high-protein diet, but the K(m) for glutamate was unchanged. 5. Adaptive changes in certain intramitochondrial enzymes were also measured. 6. The results were interpreted to indicate that when an excess of substrate was present, citrulline synthesis from NH(4)Cl was rate-limited by the intramitochondrial concentration of N-acetyl-glutamate, but citrulline synthesis from glutamate was rate-limited primarily by the activity of the glutamate-transporting system.

Carbamoyl phosphate compartmentation in Neurospora: histochemical localization of aspartate and ornithine transcarbamoylases

Carbamoyl phosphate is required for arginine and pyrimidine synthesis. In the arginine pathway, it is used in the ornithine transcarbamoylase (EC 2.1.2.1) reaction; in the pyrimidine pathway, it is used in the aspartate transcarbamoylase (EC 2.1.3.2) reaction. In Neurospora crassa, two pathway-specific enzymes catalyze the synthesis of carbamoyl phosphate, and two path-specific pools of carbamoyl phosphate are maintained. Histochemical studies show that ornithine transcarbamoylase is located in mitochondria, and, with less certainty, that aspartate transcarbamoylase is confined largely to nuclei. The enzymes that form carbamoyl phosphate are associated with the respective transcarbamoylases in the cell. Therefore, the segregation of carbamoyl phosphate pools could be accounted for by one or both organellar membranes, which demarcate two separate sites of carbamoyl phosphate metabolism in Neurospora. The alternative possibility that the enzyme complex that produces and consumes carbamoyl phosphate in the pyrimidine pathway could explain the channelling of carbamoyl phosphate, wholly or in part, is discussed.

Subcellular localization of the enzymes of the arginine dihydrolase pathway in Trichomonas vaginalis and Tritrichomonas foetus

The enzymes of the arginine dihydrolase pathway were demonstrated in Tritrichomonas foetus and their subcellular localization determined for both T. foetus and Trichomonas vaginalis. Ornithine carbamyltransferase (anabolic and catabolic activities), ornithine decarboxylase and carbamate kinase activity were localized predominately (56-80%) in the non sedimentable fraction of both species. A large proportion (35-40%) of the arginine deiminase was, however, recovered in the large granular fraction, and this distribution was unchanged by increasing the ionic strength of the buffer. Upon density gradient centrifugation the particles containing arginine deiminase activity had an isopycnic density of 1.09 g/ml in percoll, and separated from hydrogenosomes (1.18 g/ml) and lysosomes (1.12 g/ml). Arginine deiminase was also the only enzyme of the dihydrolase pathway which demonstrated latency upon treatment of the 1.09 g/ml fraction with non-ionic detergents. The results demonstrate the presence of the arginine dihydrolase pathway in T. foetus and indicate that at least a portion of the arginine deiminase in trichomonads is membrane associated.

Explanation for different types of regulation of arginine biosynthesis in Escherichia coli B and Escherichia coli K12 caused by a difference between their arginine repressors

In Escherichia coli K12, formation of the enzymes of arginine biosynthesis are controlled by arginine, with complete repression during growth with added arginine, severe repression (about 95%) during growth without added arginine and complete derepression during arginine-limited growth. In E. coli B, the degree of repression is not correlated with arginine concentrations. Under all conditions of growth enzyme formation is repressed, with repression being somewhat less in a medium with arginine than in a medium without arginine. These differences in repressibility between the two strains have been shown previously to be due to the presence of different alleles of argR, the gene for the arginine repressor. Here we have compared the binding of the two repressors to the operator sites of argF (ARG boxes). In DNase I footprinting and gel retardation experiments with argF ARG boxes we have shown that the arginine repressor of E. coli K12 bound to arginine (ArgRK-arg) has a greater affinity than the arginine repressor of E. coli B bound to arginine (ArgRB-arg), whereas free ArgRB (ArgRBf) has a much stronger affinity than free ArgRK (ArgRKf). The stronger binding of ArgRBf can explain the repression seen in E. coli B during arginine-limited growth and indicates that ArgRBf, but not ArgRKf, is able to repress enzyme synthesis under physiological conditions. The weaker repression of E. coli B than of E. coli K12 seen in the presence of arginine can be explained by the lower affinity of ArgRB-arg for operator sites as compared to ArgRK-arg. Another contributing cause for the weaker repression is the reduction of ArgRBf concentration due to autoregulation of the gene for the repressor. Thus the combined effects of repression by ArgRBf, but not ArgRKf, with the weaker repression by ArgRB-arg as compared to ArgRK-arg, convert the arginine dependent regulation in E. coli K12 to arginine independent regulation in E. coli B.

Effect of L-carnitine on cerebral and hepatic energy metabolites in congenitally hyperammonemic sparse-fur mice and its role during benzoate therapy

Sparse-fur (spf) mutant mice with X-linked ornithine transcarbamylase deficiency were used to study the effect of L-carnitine on energy metabolites in congenital hyperammonemia. L-Carnitine was used at doses of 2, 4, 8, or 16 mmol/kg body weight (BW), and levels of ammonia, glutamine, glutamate, and some intermediates of energy metabolism were measured in brain and liver of spf/Y mice. Cerebral and hepatic levels of ammonia were decreased with 4 mmol L-carnitine (P < .001), whereas other doses did not seem to have any effect on this metabolite. Cerebral levels of glutamine were decreased following administration of L-carnitine at doses of up to 4 mmol/kg BW, whereas hepatic glutamine levels remained unaltered at all doses of L-carnitine. Both cerebral and hepatic levels of pyruvate, lactate, and alpha-ketoglutarate were decreased at doses of up to 8 mmol L-carnitine/kg BW. L-Carnitine treatment elevated adenosine triphosphate (ATP), free coenzyme A (CoA), and acetyl CoA levels in both brain and liver of spf/Y mice. Cytosolic and mitochondrial redox ratios of spf/Y mice, which were altered by congenital chronic hyperammonemia, were partially corrected by L-carnitine administration. L-Carnitine supplementation to spf/Y mice during sodium benzoate therapy also restored the availability of free CoA and ATP, thus counteracting the adverse effects of higher doses of sodium benzoate. These changes in free CoA and acetyl CoA levels could be due to the deinhibition of pantothenate kinase and stimulation of fatty acid oxidation by L-carnitine.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of mRNA transcripts improves the success rate of molecular genetic testing in OTC deficiency

BACKGROUND

Ornithine transcarbamylase (OTC) deficiency is the most common inborn error of urea metabolism that can lead to hyperammonemic crises and orotic aciduria. To date, a total of 341 causative mutations within the OTC gene have been described. However, in about 20% of the patients with enzymatically confirmed OTC deficiency no mutation can be detected when sequencing of genomic DNA analyzing exons and adjacent intronic segments of the OTC gene is performed.

METHODS

Standard genomic DNA analysis of the OTC gene in five consecutive patients from five families revealed no mutation. Hence, liver tissue was obtained by needle sampling or open biopsy and RNA extracted from liver was analyzed.

RESULTS

Complex rearrangements of the OTC transcript (three insertions and two deletions) were found in all five patients.

CONCLUSION

In patients with a strong suspicion of OTC deficiency despite normal results of sequencing exonic regions of the OTC gene, characterization of liver OTC mRNA is highly effective in resolving the genotype. Liver tissue sampling by needle aspiration allows for both enzymatic analysis and RNA based diagnostics of OTC deficiency.

The effect of protein content of the diet on the rate of urea formation in sheep liver

1. In the livers of six sheep given a high-protein diet, the concentrations of certain urea-cycle enzymes [ornithine transcarbamoylase, arginine synthetase (combined activity of argininosuccinate synthetase and argininosuccinase) and arginase] were significantly greater than when the sheep were given a low-protein diet. Alkaline phosphatase activity/mg. of liver protein was not significantly affected by diet. 2. Three sheep previously given the high-protein diet showed no significant rise in the concentration of ammonia in the blood after the administration of urea (0.5g./kg. body wt.). The concentration of ammonia in the blood of the three sheep given the low-protein diet rose exponentially with time after dosing with urea and all sheep died. 3. It is suggested that tolerance to ammonia toxicity in the sheep is at least partly a function of the activity of the urea-cycle enzymes in the liver.

Cytological changes in the mouse anterior pituitary after neonatal thymectomy: a light ane electron microscopical study

A detailed light and electron microscope examination of the anterior pituitary gland of mice at different times after neonatal thymectomy was performed. Neonatal removal of the thymus resulted in a progressive increase in the number of somatotrophic hormone-producing cells showing characteristic changes represented by extremely distended cisternae of the endoplasmic reticulum and reduction in the number of hormone granules. These modifications were not observed in sham-operated controls or in neonatally splenectomized mice. The functional significance of these changes in relation to thymectomy and the significance of somatotrophic hormone for the development of the thymus and for immunological maturation is discussed. It is suggested that somatotrophic hormone and possibly other hormones determine certain specific steps of differentiation of precursor cells to immunocompetent cells and that the thymus is an endocrine target gland of somatotrophic hormone.

Isolation of Aleutian mink disease virus by affinity chromatography

Affinity chromatography was used to isolate the Aleutian disease virus of mink. Dissociation of the immunoadsorbent-virus complex with 0.75 molar sodium chloride and then with a glycine-hydrochloride gradient released infective particles resembling picornaviruses. The elution profile suggests that two different types of virus-antibody complexes are formed, one dissociated by sodium chloride and another that requires glycine-hydrochloride in addition to sodium chloride for release of virus.