Ornithine transcarbamylase in liver mitochondria

Ornithine Transcarbamylase - From Structure to Metabolism: An Update

Ornithine transcarbamylase (OTC; EC 2.1.3.3) is a ubiquitous enzyme found in almost all organisms, including vertebrates, microorganisms, and plants. Anabolic, mostly trimeric OTCs catalyze the production of L-citrulline from L-ornithine which is a part of the urea cycle. In eukaryotes, such OTC localizes to the mitochondrial matrix, partially bound to the mitochondrial inner membrane and part of channeling multi-enzyme assemblies. In mammals, mainly two organs express OTC: the liver, where it is an integral part of the urea cycle, and the intestine, where it synthesizes citrulline for export and plays a major role in amino acid homeostasis, particularly of L-glutamine and L-arginine. Here, we give an overview on OTC genes and proteins, their tissue distribution, regulation, and physiological function, emphasizing the importance of OTC and urea cycle enzymes for metabolic regulation in human health and disease. Finally, we summarize the current knowledge of OTC deficiency, a rare X-linked human genetic disorder, and its emerging role in various chronic pathologies.

Ornithine transcarbamylase downregulation is associated with poor prognosis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-associated mortalities worldwide. The role of ornithine transcarbamylase (OTC) in HCC remains unclear. In the present study, the expression of OTC in HCC was analyzed based on datasets from the Gene Expression Omnibus database of the National Center for Biotechnology Information and further confirmed by immunohistochemistry, western blotting analysis and reverse transcription-quantitative polymerase chain reaction assays on clinical samples and cell lines. Furthermore, the associations between OTC expression and clinicopathological parameters as well as clinical outcome, including the overall and disease-free survival rates were analyzed. Finally, the effect of OTC on HCC cells was measured using proliferation, bromodeoxyuridine and colony-formation assays. Lower OTC expression was observed in HCC cells and tissues compared with primary human hepatocytes. Further investigation demonstrated that low expression of OTC in HCC was associated with larger tumor size and advanced grade. A Kaplan-Meier analysis revealed that patients with lower levels of OTC exhibited shorter overall and disease-free survival times. Notably, OTC silencing with RNA interference facilitated cell proliferation in HCC SK-Hep-1 and Huh-7 cells. However, overexpression of OTC led to inhibition of cell proliferation. In conclusion, the present study identified a novel role of OTC in HCC development, providing a potential novel therapeutic target for this disease.

Experimental nonalcoholic steatohepatitis compromises ureagenesis, an essential hepatic metabolic function

Nonalcoholic steatohepatitis (NASH) is increasing in prevalence, yet its consequences for liver function are unknown. We studied ureagenesis, an essential metabolic liver function of importance for whole body nitrogen homeostasis, in a rodent model of diet-induced NASH. Rats were fed a high-fat, high-cholesterol diet for 4 and 16 wk, resulting in early and advanced experimental NASH, respectively. We examined the urea cycle enzyme mRNAs in liver tissue, the hepatocyte urea cycle enzyme proteins, and the in vivo capacity of urea-nitrogen synthesis (CUNS). Early NASH decreased all of the urea cycle mRNAs to an average of 60% and the ornithine transcarbamylase protein to 10%, whereas the CUNS remained unchanged. Advanced NASH further decreased the carbamoyl phosphate synthetase protein to 63% and, in addition, decreased the CUNS by 20% [from 5.65 ± 0.23 to 4.58 ± 0.30 μmol × (min × 100 g)(-1); P = 0.01]. Early NASH compromised the genes and enzyme proteins involved in ureagenesis, whereas advanced NASH resulted in a functional reduction in the capacity for ureagenesis. The pattern of urea cycle perturbations suggests a prevailing mitochondrial impairment by NASH. The decrease in CUNS has consequences for the ability of the body to adjust to changes in the requirements for nitrogen homeostasis e.g., at stressful events. NASH, thus, in terms of metabolic consequences, is not an innocuous lesion, and the manifestations of the damage seem to be a continuum with increasing disease severity.

A highly basic N-terminal extension of the mitochondrial matrix enzyme ornithine transcarbamylase from rat liver

We have deduced the amino acid sequence of the N-terminal leader peptide of the mitochondrial enzyme ornithine transcarbamylase from a cDNA clone obtained from a rat liver cDNA library. The sequence is remarkable in being highly basic, having 4 arginine, 3 lysine and 1 histidine with no acidic residues in a total of 32 residues. The leader sequence has no extensive hydrophobic stretches, has 72% homology with the leader peptide of human ornithine transcarbamylase [1], and in terms of its basic character resembles the N-terminal extensions on a number of fungal mitochondrial [2-5] and pea chloroplast [6] proteins. Thus the basic nature of these leader peptides may constitute the signal for mitochondrial import.

Expression of carbamoyl phosphate synthetase I and ornithine transcarbamoylase genes in Chinese hamster ovary dhfr-cells decreases accumulation of ammonium ion in culture media

Ammonium ion accumulation in mammalian cell culture media causes toxicity which inhibits cell growth and productivity. To reduce the level of the accumulated ammonium ion, carbamoyl phosphate synthetase I (CPS I) and ornithine transcarbamoylase (OTC) were used, which catalyze the first and second steps of the urea cycle in the liver. To examine the effects of overexpressed CPS I and OTC genes on the concentration of the ammonium ion in culture media, the two genes were introduced into Chinese hamster ovary (CHO) dhfr-cells. The CPS I expressing cell lines (CPS I-CHO) and both CPS I and OTC expressing cell lines (CPS I/OTC-CHO) were confirmed at the mRNA level and analyzed in terms of the cell growth and the accumulation of ammonium ion in culture media. The accumulation of ammonium ion was approximately 25-33% less in CPS I/OTC-CHO than in either CPS I-CHO or the vector-control cell lines. Interestingly however, the cell growth was approximately 15-30% faster in both CPS I-CHO and CPS I/OTC-CHO than in the control cell lines. Forced expression of urea cycle enzymes in the CHO cells revealed that both the expression of CPS I and OTC can reduce the accumulation of ammonium ion in the culture media.

Processing of artificial peptide-DNA-conjugates by the mitochondrial intermediate peptidase (MIP)

Import of DNA from the cytoplasm into the mitochondrial matrix is an obligatory step for an in organello site-directed mutagenesis or gene therapy approach on mitochondrial DNA diseases. In this context, we have developed an artificial DNA translocation vector that is composed of the mitochondrial signal peptide of the ornithine transcarbamylase (OTC) and a DNA moiety. While this vector is capable of directing attached passenger molecules to the mitochondrial matrix, the recognition of this artificial molecule by the endogenous mitochondrial signal peptide processing machinery as well as the cleavage of the peptide plays a pivotal role in the release of the attached DNA. To study the proteolytic processing of the artificial vector, various signal peptide-DNA-conjugates were treated with purified mitochondrial intermediate peptidase. When the leader peptide is directly linked to the DNA moiety without an intervening spacer, MIP processing is prevented. Cleavage of the peptide can be restored, however, when the first ten amino acid residues of the mature part of OTC are appended at the carboxy-terminal end of the signal peptide. Our results show that artificial peptide-DNA-conjugates are recognized by the mitochondrial proteolytic machinery, and therefore an interference of the peptide with the DNA function can be excluded.

Cell transplantation in liver-directed gene therapy

Somatic cell gene therapy is a new field of biomedical research that encompasses a variety of traditional basic research and clinical disciplines. This new approach to therapeutics has the potential to prevent, treat, or cure a variety of inherited and acquired diseases. Two divergent strategies of hepatocyte transplantation are being employed in animal models and clinical trials in an attempt to correct genetic deficiencies. Allogeneic hepatocyte transplantation has two main advantages over autologous cell transplantation. First, invasive surgical procedures are not required in the recipient. Second, allogeneic cells can be administered repetitively, so that multiple harvests are not necessary. The major drawbacks to allogeneic hepatocyte transplants are rejection and the risks of immunosuppression. Although there is no clinical experience with the treatment of genetic disease by allogeneic hepatocyte transplantation, a variety of animal models have been characterized, including the Gunn rat (UDP-glucuronosyl transferase deficient), the Nagase analbuminemic rat, and the Watanabe heritable hyperlipidemic rabbit (LDL receptor deficient). The use of genetically corrected autologous cells represents a different and more elegant approach to the correction of inherited disease. A segment of liver is harvested from the affected individual. Recombinant retroviruses are used to transduce normal genes--with a variety of promoter/enhancer constructs--into the patients own hepatocytes. The genetically corrected hepatocytes are then transplanted back into the patient. This approach, known as ex vivo gene therapy, eliminates the risk of rejection and the need for immunosuppression. The safety and efficacy of this approach has been proven in a variety of preclinical animals models, including Watanabe rabbits, dogs, and Papio spp. A clinical trial for the treatment of familial hypercholesterolemia is currently in progress. A number of approaches for the reintroduction of hepatocytes into the recipient have been proposed, including catheter-mediated delivery into the inferior mesenteric vein, the umbilical vein, or into the spleen. Candidate diseases, which are likely to result in the first clinical trials include familial hypercholesterolemia, ornithine transcarbamylase deficiency, Crigler-Najjar syndrome, alpha 1-antitrypsin deficiency, and phenylketonuria.

Evolutionary aspects of urea cycle enzyme genes

The functions and expression pattern of urea cycle enzymes have undergone considerable changes during the course of evolution. Sequence analyses shows that urea cycle enzymes from mammals are homologous to microbial enzymes of the arginine-metabolic pathway. Recently, an unexpected relationship was found between argininosuccinate lyase (EC 4.3.2.1), the fourth enzyme of the cycle, and delta-crystallin, a lens structural protein of birds and reptiles.

Tissue- and developmental stage-specific expression of the rat ornithine carbamoyltransferase gene in transgenic mice

Rat ornithine carbamoyltransferase (OCT; EC 2.1.3.3) is encoded by a large gene of 75 kilobases. Expression of this gene is restricted to the liver and small intestine, and there is an increase in expression late in gestation. The recombinant gene carrying 1.3 kilobases of the 5' flanking region of the gene fused to the rat OCT cDNA was microinjected into fertilized eggs, and 17 transgenic mice were produced. Expression in the liver of the transgene was detected in three mice. In these mice, the transgene expression was observed exclusively in the liver and small intestine. Expression of the transgene in the intestine was comparable to that of the endogenous mouse OCT gene, whereas expression in the liver was much lower than that of the endogenous gene. The developmental pattern of expression of the transgene was similar to that of the endogenous gene. Therefore, the 5' flanking sequence of the rat OCT gene seems to be sufficient for the developmental and tissue-specific expression of the gene. An explanation for low expression in the liver remains the subject of ongoing study.

Different structures in the amino-terminal domain of the ornithine transcarbamylase leader peptide are involved in mitochondrial import and carboxyl-terminal cleavage

The cytoplasmic precursor of mitochondrial ornithine transcarbamylase (carbamoyl-phosphate:L-ornithine carbamoyltransferase, EC 2.1.3.2) contains an amino-terminal leader peptide of 32 amino acids. Secondary structure and helical-wheel analyses predict that the extreme amino-terminal domain (residues 1-15) forms an alpha-helix. To test this thesis, leucine residues at positions 2, 5, 8, and 9 were systematically replaced by either helix-breaking glycine residues or by helix-preserving alanine residues. Triple substitutions of glycine for leucine in positions 2, 5, and 9 or 5, 8, and 9 abolished the uptake of the rat precursor by intact mitochondria, whereas similar alanine substitutions had much less effect. Theoretical computations predicted that the decreased helical stability of the Gly-5,8,9 substitution could be significantly increased by replacing a serine in position with phenylalanine. The introduction of Phe-3, indeed, restored the mitochondrial uptake of the mutant precursor. These results lend strong support to the hypothesis that an alpha-helix is present at the leader's amino terminus during the import of the precursor by mitochondria. Although the precursors with the triply-substituted leaders were impaired with respect to import, they were still cleaved readily by a protease found in a mitochondrial matrix fraction. Substitution of glycine or alanine for all four leucine residues, however, rendered the leader uncleavable at the carboxyl-terminal cleavage site. These results suggest that the structure of the amino-terminal domain is important for recognition of the carboxyl-terminal cleavage sites by the matrix proteases.

Compared expression levels of ornithine transcarbamylase and carbamylphosphate synthetase in liver and small intestine of normal and mutant mice

Enzymatic assay, electrophoretic immunoblotting and RNA dot-blot techniques were employed to investigate the expression of the ornithine transcarbamylase (OTC) gene in liver and small intestine of Sparse fur mice with abnormal skin and hair (Spf-ash) and Sparse fur mice (Spf) which exhibit an X-linked OTC deficiency. We found a reduced OTC activity in these two tissues. We now show that this reduction is less pronounced in the intestine than in the liver of the Spf-ash strain. During the first 2 weeks of life, the deficiency appears to be less severe than in the adult mice. The enzymatic activity of carbamylphosphate synthetase I (CPS), another enzyme of the urea cycle, is significantly modified in the Spf mutant strain only.

The genetic structure of mouse ornithine transcarbamylase

The gene encoding the mouse urea cycle enzyme, ornithine transcarbamylase has been isolated on five partially overlapping bacteriophage lambda clones. We have characterized the gene and found that it is split between ten exons distributed over approximately 70 kb of the X chromosome. The introns range in size from 88 bases to the relatively unusual size of approximately 26 kilobases, while the splice donor/splice acceptor sequences conform to the consensus established for other eukaryotic genes.

Structure of the rat ornithine carbamoyltransferase gene, a large, X chromosome-linked gene with an atypical promoter

Rat mitochondrial ornithine carbamoyltransferase (EC 2.1.3.3) is encoded by a gene located on the X chromosome and expressed specifically in the liver and small intestine; we have cloned this gene and determined its structure. The gene is 75 kilobases long and is split into 10 exons. The introns range in length from 85 bases to 26 kilobases. The sum of the total exons is 1.5 kilobases and occupies only 2% of the gene; this value being one of the lowest among genes heretofore reported. The first exon encodes most of the NH2-terminal presequence that functions as a mitochondrial targeting signal. Putative binding sites for the two substrates of the enzyme, carbamoyl phosphate and ornithine, are encoded by exons 3 and 9, respectively. A set of "CAAT box"- and "ATA box"-like sequences is present about 200 bases upstream from the 5' end of the mRNA. About 35 bases downstream from this set of putative promoter elements, an 11-nucleotide sequence around the 5' end of the mRNA reappears, as a direct repeat. This pair of direct repeats may play a role in pulling the cap site and the promoter elements together. Upstream and downstream from the 5' end of the mRNA there are several sequences that resemble the transcription factor Sp1 binding site, the enhancer core sequence, the consensus sequence for the glucocorticoid receptor binding sites, and the putative enhancer element of the antithrombin III gene, another gene that is expressed specifically in the liver.

Ornithine transcarbamylase deficiency in spf and spf-ash mice: genes, mRNAs and mRNA precursors

Two ornithine transcarbamylase-deficient mice are available, the spf with a variant enzyme and spf-ash with a markedly decreased enzyme protein. Genomic DNA, mRNA and the nuclear precursors for the enzyme in these mutants were analyzed. Southern blot analysis of genomic DNA showed no abnormality in the mutant mice. Blot analysis of hepatic mRNA revealed a slight decrease (67% of control) in spf and a marked decrease (12% of control) in spf-ash; no difference in size was found among the control and the mutant mice (about 1.8 kb). Blot analysis of nuclear mRNA precursors (greater than 25, approximately 9.0 and 4.0 kb) showed no significant difference in size and amount among the control, spf and spf-ash. These results suggest that ornithine transcarbamylase deficiency in the spf-ash results from a mutation, which to some extent affects mRNA processing.

The ornithine transcarbamylase leader peptide directs mitochondrial import through both its midportion structure and net positive charge

The cytoplasmically synthesized precursor of the mitochondrial matrix enzyme, ornithine transcarbamylase (OTC), is targeted to mitochondria by its NH2-terminal leader peptide. We previously established through mutational analysis that the midportion of the OTC leader peptide is functionally required. In this article, we report that study of additional OTC precursors, altered in either a site-directed or random manner, reveals that (a) the midportion, but not the NH2-terminal half, is sufficient by itself to direct import, (b) the functional structure in the midportion is unlikely to be an amphiphilic alpha-helix, (c) the four arginines in the leader peptide contribute collectively to import function by conferring net positive charge, and (d) surprisingly, proteolytic processing of the leader peptide does not require the presence of a specific primary structure at the site of cleavage, in order to produce the mature OTC subunit.

Chicken ornithine transcarbamylase: its unexpected expression

Ornithine transcarbamylase (OTC), one of the enzymes of the urea cycle, is detectable in some strains of chickens, although they have no functional urea cycle. The enzyme consists of three identical subunits of 36 kd and is present in mitochondria of the kidney. Using immunoabsorbent column chromatography, we found further evidence that the enzyme is detectable as a precursor form (40 kd) in chicken brain, heart, liver, pancreas, gizzard, small intestine, and breast muscle. When an extract of small intestine containing only precursor OTC was treated with a kidney extract, the precursor was converted into OTC. This suggests that there is a tissue-specific processing protease in the kidney which splits a peptide off the precursor, causing the expression of OTC activity in this organ. However, the reason why the enzyme or its precursor is expressed in these organs is not known. The results of this study suggest that, unlike mammals, chickens are more organ specific with regard to the ability to incorporate precursor OTC into mitochondria.

In situ staining procedure for the detection of ornithine transcarbamylase activity in polyacrylamide gels

Ornithine transcarbamylase deficiency is a human genetic disease potentially susceptible to gene therapy. A murine model system exists for the disease in the sparse-fur (spf) mouse. Before gene therapy studies can be performed it is necessary to have practical methods which could detect successful gene transfer. Therefore we have developed an in situ staining procedure for the detection of ornithine transcarbamylase activity in polyacrylamide gels. Following electrophoretic separation under nondenaturing conditions inorganic phosphate cleaved from carbamyl phosphate in gels as a result of enzymatic activity was precipitated as phosphomolybdic acid and visualized by reduction with ascorbic acid. Results from the procedure correlated with ornithine transcarbamylase activity as measured by solution assay for citrulline, the other product of the reaction. This procedure readily distinguished mutant forms of ornithine transcarbamylase as exemplified by the murine spf mutation and resolved ornithine transcarbamylases of all animals tested into multiple forms. The procedure further distinguished ornithine transcarbamylases of animals of several different genera while yielding virtually identical patterns of the enzyme from species within the same genus. This procedure also suggested that the human enzyme was more labile than murine ornithine transcarbamylase; direct thermolability studies confirmed this finding.

Turnover of rat liver ornithine transcarbamylase

The relative half-life of ornithine transcarbamylase from rat liver has been determined using the double isotope technique and affinity chromatography. The calculated half-life (6-9 days) is similar to that of mitochondria and of the other mitochondrial enzyme of the urea cycle, carbamoyl-phosphate synthase. Therefore, both mitochondrial urea cycle enzymes are most probably degraded mainly via the lysosomal (autophagic) pathway of mitochondrial protein degradation.

Synthesis, intracellular transport and processing of mitochondrial urea cycle enzymes

Carbamyl phosphate synthetase I and ornithine transcarbamylase are matrix enzymes synthesized outside the mitochondria in the form of larger precursors and are transported rapidly into mitochondria, in association with post-translational proteolytic processing to the mature enzymes. Treatment of isolated rat hepatocytes with 40 micrograms/ml of rhodamine 123 resulted both in a potent inhibition of the processing of the enzyme precursors and in accumulation of the precursors. In pulse-chase experiments, the labeled precursor disappeared much more slowly in the presence of the dye. Rhodamine 123 strongly inhibited the uptake and processing of the ornithine transcarbamylase precursor by isolated rat liver mitochondria. Other positively charged rhodamines such as rhodamines 6G and 6GX were also strongly inhibitory. On the other hand, rhodamine B which has no net charge was much less inhibitory. These results suggest that the positively charged rhodamines inhibit the binding of the positively charged enzyme precursors to a negatively charged protein(s) or to phospholipids of the mitochondrial outer membrane. Potassium and magnesium ions, and probably a cytosolic protein(s), were required for the maximal uptake and processing of the ornithine transcarbamylase precursor by the isolated mitochondria. The concentrations of potassium and magnesium ions required for the maximal transport and processing were about 120 mM and 0.8-1.6 mM, respectively. Dialyzed postribosomal supernatant of rabbit reticulocyte lysate (36-72 mg protein/ml), in combination with potassium and magnesium ions, stimulated the precursor transport and processing 3- to 4-fold. The stimulatory activity of the dialyzed lysate was inactivated by trypsin treatment or heat treatment. No significant amount of the enzyme precursor was associated with the mitochondria when incubation was performed in the absence of these compounds. All these results indicate that potassium and magnesium ions, and probably a cytosolic protein(s), are required for the binding of the ornithine transcarbamylase precursor to the mitochondria or its transport into the organelle.

Ornithine transcarbamylase deficiency: a case with a truncated enzyme precursor and a case with undetectable mRNA activity

The cell-free translation of ornithine transcarbamylase (OTC) mRNA from the livers of two heterozygous patients (from different families) with OTC deficiency was performed. The enzyme activities and the immunoreactive proteins in both patients were about 5% of those in controls. Immunoblotting assay of liver extracts from both patients showed decreased amounts of the OTC protein. The mRNA from the liver of patient 1 directed the synthesis of a very small amount of OTC precursor of normal subunit size (40,000 Da), whereas that from patient 2 directed the synthesis of small amounts of two distinct in vitro products; one was 40,000 Da and the other was about 30,000 Da. The in vitro product of normal precursor synthesized with mRNA from patient 2 was converted to mature-sized OTC by isolated rat liver mitochondria, whereas the smaller product was degraded during the incubation with the mitochondria. These results indicate that in both patients the translatable level of mRNA for active OTC from liver cells was much lower than that in the controls. The results also suggest that in patient 2, the smaller product presumably derived from an abnormal gene could not be transferred to the mitochondria.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunological evidence for a carbamylphosphate synthetase lesion resulting in the formation of enzyme with altered sub-unit size

A partial carbamylphosphate synthetase (CPS: EC 6.3.4.16) deficiency (McKusick 23730) was found in a male child who presented with generalized convulsions, rickets and apnoeic attacks at six months of age. By his second year he showed serious developmental delay and a gut biopsy revealed an absence of CPS activity with an elevated ornithine transcarbamylase activity. Analysis of the gut biopsy sample on SDS-polyacrylamide gels, followed by electrophoretic transfer to a nitrocellulose filter probed with monospecific antibodies to CPS showed that the child had normal levels of immunoreactive enzyme, but instead of one band corresponding to normal CPS with a subunit size of 165,000 u, the patient had three immunoreactive bands, one larger and two smaller than that found in normal controls. The genetic defect in this child therefore results in an unusual form of CPS being made which has markedly reduced enzyme activity.

The precursor to ornithine carbamyl transferase is transported to mitochondria as a 5S complex containing an import factor

The precursor to ornithine carbamyl transferase (Mr = 40,000) was synthesized in a rabbit reticulocyte lysate system and purified by immunoaffinity chromatography. Import of purified precursor by isolated mitochondria depended upon the presence of import factor(s) in fresh reticulocyte lysate. Velocity sedimentation analyses indicated that import factor binds to precursor to form a 5S complex (approximately 90 kDa); in this form, precursor was efficiently imported by isolated mitochondria. The ability of the 5S complex to deliver precursor into mitochondria was not affected by pretreatment with high concentrations of RNase. Import factor did not bind to mitochondria in the absence of precursor; upon binding of precursor to mitochondria in the presence of import factor, subsequent transmembrane uptake of precursor did not require the continued presence of additional lysate components.

Transport of proteins into mitochondrial matrix. Evidence suggesting a common pathway for 3-ketoacyl-CoA thiolase and enzymes having presequences

Rat liver 3-ketoacyl-CoA thiolase, a mitochondrial matrix enzyme which catalyzes a step of fatty acid beta-oxidation, was synthesized in a rabbit reticulocyte lysate cell-free system. The in vitro product was apparently the same in molecular size and charge as the subunit of the mature enzyme. The enzyme synthesized in vitro was transported into isolated rat liver mitochondria in an energy-dependent manner. In pulse experiments with isolated rat hepatocytes at 37 degrees C, the radioactivity of the newly synthesized enzyme in the cytosolic fraction remained essentially unchanged during 5-20 min of incubation, whereas that of the enzyme in the particulate fraction increased with time during the incubation. The pulse-labeled enzyme disappeared with an apparent half-life of less than 3 min from the cytosolic fraction, in pulse-chase experiments. Purified 3-ketoacyl-CoA thiolase inhibited the mitochondrial uptake and processing of the precursors of the other matrix enzymes, ornithine carbamoyltransferase, medium-chain acyl-CoA dehydrogenase and acetoacetyl-CoA thiolase. These results indicate that 3-ketoacyl-CoA thiolase has an internal signal which is recognized by the mitochondria and suggest that this enzyme and the three others are transported into the mitochondria by a common pathway.

Purine nucleotides stimulate while carbamoyl phosphate protects inactivation of ornithine transcarbamoylase by disrupted lysosomes

Ornithine transcarbamoylase (OTC) is inactivated by liver lysosomes. Carbamoyl phosphate prevents the inactivation of OTC by lysosomes, while ATP, ADP, GTP, GDP 1,N6-ethenoadenosine 5'-triphosphate and particularly epsilon-ATP stimulate it. Both stimulation and protection occur at concentrations within the physiological range of ATP and carbamoyl phosphate. Inactivation of OTC is followed by extensive proteolysis. Since the inactivation is prevented by leupeptin, antipain and L-(tosylamido-2-phenyl)ethylchloromethyl ketone, the proteolytic susceptibility of OTC to lysosomes could be due to thiol endopeptidase(s). 1,N6-Ethenoadenosine 5'-triphosphate also markedly increases OTC susceptibility to trypsin and elastase. ATP analogs had no stimulatory effect on OTC inactivation by lysosomes; none of the inhibitors of ATPases tested inhibited the ATP effect. The ATP stimulation does not require Mg2+. These findings indicate a new role for ATP, GTP and related nucleotides in protein breakdown. The ATP, ADP, GTP, GDP stimulation, together with the carbamoyl protection of OTC, agree well with the molecular plasticity hypothesis model.

The primary structure of the imported mitochondrial protein, ornithine transcarbamylase from rat liver: mRNA levels during ontogeny

Ornithine transcarbamylase, one of the enzymes of the urea cycle in ureotelic organisms, is synthesized in the cytoplasm of hepatocytes as a precursor larger than the mature form found in the mitochondrial matrix. We deduced the amino acid sequence of the precursor of ornithine transcarbamylase from rat liver from the nucleotide sequence of overlapping cDNA clones spanning the complete coding region, 3' untranslated region, and most of the 5' untranslated region of the mRNA. The mature enzyme consists of 322 amino acids and is derived from the larger precursor by proteolytic removal of 32 amino acids from the amino-terminus. The amino-terminal extension contains eight basic and no acidic residues. This highly basic character appears to be a feature of presequences on cytoplasmically synthesized mitochondrial proteins. Comparison of the amino acid sequence determined for the enzyme from rat with that from human liver (Horwich et al., 1984) shows that there is a high degree of homology between the sequences of the mature protein (93%) and relatively less homology between the sequences of the amino-terminal extension (72%). The ornithine transcarbamylase from rat liver also shows a considerable degree of amino acid homology (44%) with the enzyme from Escherichia coli (Van Vliet et al., 1984) and leads to suggestions about residues involved in substrate binding and catalysis. An analysis of levels of RNA in fetal and neonatal liver shows that ornithine transcarbamylase mRNA levels increase from about 40% of adult levels at day 14 of gestation to a peak at day 20 of gestation, and, after a drop around the time of birth, rises to adult levels during the second week after birth.

A cDNA clone for the precursor of rat mitochondrial ornithine transcarbamylase: comparison of rat and human leader sequences and conservation of catalytic sites

We have cloned a DNA complementary to the messenger RNA encoding the precursor of ornithine transcarbamylase from rat liver. This complementary DNA contains the entire protein coding region of 1062 nucleotides and 86 nucleotides of 5'- and 298 nucleotides of 3'-untranslated sequences. The predicted amino acid sequence has been confirmed by extensive protein sequence data. The mature rat enzyme contains the same number of amino acid residues (322) as the human enzyme and their amino acid sequences are 93% homologous. The rat and human amino-terminal leader sequences of 32 amino acids, on the other hand, are only 69% homologous. The rat leader contains no acidic and seven basic residues compared to four basic residues found in the human leader. There is complete sequence homology (residues 58-62) among the ornithine and aspartate transcarbamylases from E. coli and the rat and human ornithine transcarbamylases at the carbamyl phosphate binding site. Finally, a cysteine containing hexapeptide (residues 268-273), the putative ornithine binding site in Streptococcus faecalis, Streptococcus faecium, and bovine transcarbamylases, is completely conserved among the two E. coli and the two mammalian transcarbamylases.

Gene for OTC: characterisation and linkage to Duchenne muscular dystrophy

Cloned coding sequences for rat and human ornithine transcarbamylase (OTC) were obtained by screening a rat and a human cDNA library respectively with a synthetic oligonucleotide corresponding to 27 bases of the rat sequence. These clones, 1100 bp long for the rat clone and 1300 bp for the human, contain approximately 80% of the human OTC coding sequence. The OTC mRNA length determined by Northern blot analysis is 1700bp. The human OTC sequence was shown to be localised Xp11.4-Xp21 using somatic cell hybrids. There is a frequent RFLP revealed with the restriction enzyme MspI. OTC is located more closely to the Duchenne muscular dystrophy mutation than previously reported markers such as RC8 and L1.28, and therefore should prove useful in carrier detection and haplotype analysis of families carrying the mutation causing the disease.

Molecular cloning and nucleotide sequence of cDNA for rat ornithine carbamoyltransferase precursor

Messenger RNA of rat ornithine carbamoyltransferase (EC 2.1.3.3), a mitochondrial matrix enzyme, was enriched by immunoprecipitation of rat liver free polysomes, and recombinant plasmids were prepared from the enriched mRNA by a vector-primer method. The cDNA clones for ornithine carbamoyltransferase were identified by hybrid-arrested translation and hybrid-selected translation. One of the clones, designated pOTC-1, contained a 1.6-kilobase insert and hybridized to a mRNA of approximately equal to 1.8 kilobases in rat liver. The cDNA clone was subjected to nucleotide sequence analysis. The deduced amino acid sequence indicates that the ornithine carbamoyltransferase precursor consists of the mature enzyme of 322 amino acid residues and an NH2-terminal peptide extension (presequence) of 32 amino acid residues. The presequence contains 8 basic amino acid residues, no acidic residues, and no hydrophobic amino acid stretch. The amino acid sequence of the rat ornithine carbamoyltransferase was compared with the recently reported sequence of the human enzyme [Horwich, A. L., Fenton, W. A., Williams, K. R., Kalousek, F., Kraus, J. P., Doolittle, R. F., Konigsberg, W. & Rosenberg, L. E. (1984) Science 224, 1068-1074]. The sequences of the mature enzyme portion are 93% identical, whereas those of the presequences are 69% identical. There are two highly conserved segments in the presequences of the rat and human enzymes. One of the two conserved segments is significantly similar to a segment of the presequence of yeast mitochondrial elongation factor EF-Tu. These results suggest that the homologous segments are important for the proteins that are synthesized in the cytosol to be transported into the mitochondrial matrix.

Biosynthesis of enzymes of rat-liver mitochondrial beta-oxidation

The biogenesis of seven enzymes involved in the mitochondrial fatty acid beta-oxidation of rat liver was studied. Hepatic RNA was translated in vitro in a rabbit reticulocyte lysate cell-free system and the translation products were immunoprecipitated, subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by fluorography. The translation products obtained in vitro of medium-chain and/or long-chain acyl-CoA dehydrogenase (these enzymes were immunochemically cross-reactive), enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and acetoacetyl-CoA thiolase and probably also short-chain acyl-CoA dehydrogenase were larger than the subunits of the corresponding mature enzymes by 2-4.5 kDa, whereas the 3-oxoacyl-CoA thiolase obtained in vitro was approximately the same size as the mature subunit. The free polysome fraction of rat liver was 4.3-9.0-times more active than the membrane-bound polysome fraction in the synthesis of these seven enzymes. The enzyme activities were increased after administration of di(2-ethylhexyl)phthalate; the extent of the increase varied from one enzyme to another. The increase in the cell-free translation activity of total hepatic RNA for these enzymes after administration of the chemical was markedly different among individual enzymes and higher than that in the rates of synthesis of the corresponding enzymes which were determined by the experiment in vivo.

Selection of a cDNA clone which contains the complete coding sequence for the mature form of ornithine transcarbamylase from rat liver: expression of the cloned protein in Escherichia coli. Molecular cloning of rat ornithine transcarbamylase

A cDNA clone corresponding to the mature form of ornithine transcarbamylase (OTCase) was selected from a rat liver cDNA library constructed in bacteriophage lambda gt10. OTCase clones were selected using a synthetic DNA probe of 15 bases corresponding to the 3' end of the OTCase mRNA [Horwich, A. L., Kraus, J.P., Williams, K., Kalousek, F., Königsberg, W. & Rosenberg, L.E. (1983) Proc. Natl Acad. Sci. USA, 80, 4258-4262]. Putative OTCase clones were subcloned into the expression vector, pUC9, and the identity of inserts confirmed by colony immunoassay and by electrophoretic transfer of cloned proteins from sodium dodecyl sulphate/polyacrylamide gels to nitrocellulose filters followed by probing with monospecific anti-OTCase antibodies and 125I-labelled protein A. A clone corresponding to the full-length mature form of rat liver OTCase (plus 15 amino acids from Escherichia coli beta-galactosidase) was obtained and the identity of the clone was confirmed by comparison of the 5' sequence with a limited N-terminal amino acid sequence [Lusty, C., Jilka, R. L. & Nietsch, E. H. (1979) J. Biol. Chem. 254, 10030-10036]. A sequence discrepancy between the published sequence (Lusty et al.) and the sequence predicted from the cDNA structure is noted.

Transport of proteins into mitochondria

There is still much that is obscure concerning the transport of proteins into or through the mitochondrial membrane systems. In addition, as pointed out previously, it is unlikely that the details of the process are the same for proteins destined for different compartments of the organelle. A brief summary of the process for matrix proteins might be as follows: The proteins are synthesized on free polysomes as precursors of higher molecular weight than the native forms. These precursors are liberated into the cell cytosol and subsequently translocated into the mitochondria. This timing might be different in yeast under some circumstances, synthesis being completed in association with the mitochondria. The precursors interact with a receptor in the outer mitochondrial membrane interaction being mediated by the presequences of the precursors. The presequences therefore act as addressing signals as well as possibly playing a role in one or all of (a) solubilization of precursors, (b) prevention of premature assembly into multimeric structures, or (c) maintenance of nonnative configurations required for transport. Interaction occurs with a second receptor, this time in the inner membrane of the mitochondria, interaction being with multiple sites in the polypeptide chain. Transport across the inner membrane then occurs, this transport depending on a transmembrane electrochemical gradient of which the proton component is the essential part. Transport is accompanied or followed by proteolysis of the prepiece, and formation of the native structure. While steps 1 and 2 of this sequence can be considered well established, the remaining steps are still poorly understood or purely hypothetical. Nevertheless, this sequence of events is consistent with known facts about the process and provides a framework for future investigations.

Cell-free synthesis and transport of precursors of mutant ornithine carbamoyltransferases into mitochondria

Synthesis, mitochondrial transport and processing of ornithine carbamoyltransferase (EC 2.1.3.3) were studied in mutant mice strains (sparse-fur, spf, and sparse-fur with abnormal skin and hair, spf-ash) which exhibit a deficiency in this enzyme. Spf mice have an increased amount (about 150% of control) of the enzyme with abnormal kinetic properties, whereas spf-ash mice have a decreased amount (about 10% of control) of the enzyme with apparently normal kinetic properties. Precursors of the mutant enzymes were synthesized in a reticulocyte lysate cell-free system. The hepatic level of translatable mRNA coding for the enzyme and the rate of the enzyme synthesis in liver slices of spf mice were 58 and 60% of the controls, respectively. In the case of spf-ash mice the activity of translatable mRNA for the enzyme was 10% of the controls. These results indicate that the decreased amount of ornithine carbamoyltransferase protein in spf-ash mice is due mainly to a decreased level of translatable mRNA for the enzyme, whereas the increase in the enzyme amount in spf mice is presumably the result of a decreased rate of enzyme degradation. The subunit molecular weight of the spf enzyme precursor was practically the same as that of the normal enzyme precursor (Mr 40 000). Both precursors synthesized in vitro could be taken up and processed similarly to an apparently mature form (Mr 37 000). In the case of spf-ash enzyme, two discrete in vitro products were observed on sodium dodecyl sulfate polyacrylamide gel; one comigrated with the normal enzyme precursor and the other moved slightly slower. Both products appeared to be taken up and processed to the mature form of the enzyme.

Molecular cloning of the cDNA coding for rat ornithine transcarbamoylase

Ornithine transcarbamoylase is a mitochondrial matrix enzyme composed of three identical subunits encoded on the X chromosome. The subunit is synthesized on cytoplasmic polysomes as a precursor that is cleaved during transport into mitochondria. We report here the isolation and characterization of cDNA clones containing sequences corresponding to the mRNA encoding the ornithine transcarbamoylase subunit. cDNA was synthesized using rat liver mRNA enriched by polysome immunoadsorption for the low-abundance messenger species encoding the enzyme subunit. After insertion of cDNA into plasmid pBR322 and cloning in Escherichia coli, identification of the desired plasmids was accomplished by (i) differential colony hybridization using cDNA probes synthesized from mRNA of various tissues; (ii) differential blot hybridization using cDNA probes synthesized from mRNA enriched for or depleted of the ornithine transcarbamoylase message; (iii) hybrid-selected translation assays; and (iv) most definitively, structural analysis, which matched 25 consecutive amino acid residues determined by sequential Edman analysis of the carboxyl-terminal portion of the purified enzyme subunit with coding sequence present in the insert of one of the plasmids.