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Biomedical subjects

W A Fenton

Publications and source records attributed to W A Fenton.

At least 55 records · Page 3Linked to original sources
PubMed1987

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.

Amino Acid Sequence↗
PubMed1986

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↗
PubMed1986

DNA analysis for ornithine transcarbamylase deficiency.

We have utilized the Southern blotting technique to analyse genomic DNA from males with ornithine transcarbamylase (OTC) deficiency and their families. Using a nearly full-length human cDNA probe, we have identified 3 patients with deletions at this locus and have characterized 4 different restriction fragment length polymorphisms that can be used as linkage markers for the OTC mutation. These polymorphisms occur at sufficiently high frequencies so as to enable us to distinguish the two X-chromosomes in approximately 80% of OTC carriers. As a direct consequence of these findings, prenatal diagnosis and carrier assessment can be offered to a large fraction of families at risk for OTC deficiency.

Chromosome Deletion↗
PubMed1986

Identification and application of additional restriction fragment length polymorphisms at the human ornithine transcarbamylase locus.

Two additional restriction fragment length polymorphisms (RFLPs) have been identified at the human ornithine transcarbamylase (OTC) locus. Approximately 11% of women are heterozygous for an RFLP characterized by polymorphic bands at 3.7 and 3.6 kilobasepairs (kbp) observed after DNA digestion with TaqI. Twenty-nine percent of women are heterozygous for an RFLP characterized by polymorphic bands at 18.0 and 5.2 kbp observed after digestion with BamHI. Thus, in combination with the previously reported RFLPs identified using MspI, the X chromosomes in approximately 80% of women at risk for having a son with OTC deficiency are distinguishable by RFLPs at the OTC locus. Furthermore, we show that these RFLPs will be useful in families for prenatal diagnosis of OTC deficiency, carrier detection, and carrier exclusion.

Chromosome Mapping↗
PubMed1985

Prenatal diagnosis and carrier detection of Duchenne muscular dystrophy with closely linked RFLPs.

By the use of a series of closely linked DNA probes detecting restriction fragment length polymorphisms (RFLPs) distributed over the short arm of the X chromosome, a double crossover was detected in a Duchenne muscular dystrophy carrier and an affected male fetus was diagnosed at 12 weeks of gestation, with a probable accuracy of more than 99.0%. A new mutation was identified in another family with the same degree of reliability; three females in this family were thus deemed not to be DMD carriers. The eleven RFLP-markers presently available on the short arm of the X chromosome are useful in the diagnosis of DMD since they bridge the Duchenne locus at genetic distances varying between 3 and 20 cmo. Moreover, recombination within the set of markers provides an independent way of regionally mapping these probes relative to each other along the short arm of the X chromosome.

Chromosome Mapping↗
PubMed1985

Expression of amplified DNA sequences for ornithine transcarbamylase in HeLa cells: arginine residues may be required for mitochondrial import of enzyme precursor.

Expression of ornithine transcarbamylase (OTC), a nuclear-coded mitochondrial enzyme, was programmed in HeLa cells by the use of a strategy of gene co-amplification. HeLa cells, ordinarily devoid of OTC activity, were transfected with a plasmid containing viral regulatory elements joined with two cDNA sequences, one encoding the human OTC precursor and a second encoding a mutant mouse dihydrofolate reductase. After transfection and selection in increasing concentrations of methotrexate, several hundred copies per cell of the sequence encoding OTC were detected by blot analysis. Immunoprecipitation of extracts of radiolabeled cells with anti-OTC antiserum revealed newly synthesized mature OTC subunits. Furthermore, OTC enzymatic activity in cell extracts was comparable to that of control human liver, and mitochondrial localization of OTC was demonstrated by immunofluorescence. When we incubated transfected HeLa cells with dinitrophenol, a known inhibitor of mitochondrial import, the only form of newly synthesized OTC detected was the precursor. We estimated the rate of import of precursor by performing an inhibitor-free chase; precursor was converted to mature subunit with a half-life of less than two minutes. When a HeLa transformant was incubated with the arginine analogue canavanine, the major form of newly synthesized OTC detected was a species migrating slightly more slowly than the normal precursor; little mature-sized subunit was recovered. This indicates that substitution of the analogue for arginine in the OTC precursor interferes with mitochondrial import and processing. Thus, arginine residues in the OTC precursor--most likely the four residues contained in its NH2-terminal leader sequence--probably play an important role in mitochondrial import and/or processing.

Amino Acid Sequence↗
PubMed1984

Structure and expression of a complementary DNA for the nuclear coded precursor of human mitochondrial ornithine transcarbamylase.

Most mitochondrial proteins are encoded in the nucleus and are translated on free cytoplasmic ribosomes as larger precursors containing amino-terminal "leader" sequences, which are removed after the precursors are taken up by mitochondria. We have deduced the complete primary structure of the precursor of a human mitochondrial matrix enzyme, ornithine transcarbamylase (OTC), from the nucleotide sequence of cloned complementary DNA. The amino-terminal leader peptide of OTC is 32 amino acids in length and contains four arginines but no acidic residues. Cleavage of the leader peptide from the "mature" protein occurs between glutamine and asparagine residues. The sequence of mature human OTC resembles that of the subunits of both OTC and aspartate transcarbamylase from Escherichia coli. The biological activity of the cloned OTC complementary DNA was tested by joining it with SV40 (an animal virus) regulatory elements and transfecting cultured HeLa cells, which do not normally express OTC. Both the precursor and mature forms of the OTC subunit were identified; in stable transformants, enzymatic activity was also detected.

Amino Acid Sequence↗
PubMed1984

Biogenesis of the mitochondrial enzyme methylmalonyl-CoA mutase. Synthesis and processing of a precursor in a cell-free system and in cultured cells.

Methylmalonyl-CoA mutase (EC 5.4.99.2; mutase), a cytoplasmically synthesized mitochondrial matrix enzyme, is translated in a cell-free system programmed with rat liver RNA as a larger precursor polypeptide, designated pre-mutase, which appears to be 3-4 kDa larger than the subunit of purified mutase (77.5 kDa). When pre-mutase is incubated with intact rat liver mitochondria, it is taken up by them and proteolytically processed to the size of the mature subunit. The overall reaction is inhibited by compounds such as dinitrophenol which disrupt mitochondrial energy metabolism. The final, proteolytic step can be carried out by the mitochondrial matrix in the presence of added Zn2+ and is inhibited by metal ion chelators and by certain protease inhibitors (e.g. leupeptin and p-aminobenzamidine). Newly synthesized mutase was also detected in intact, cultured Buffalo rat liver cells labeled with [3H] leucine in the presence of dinitrophenol. When dinitrophenol is removed in a pulse-chase protocol, the accumulated pre-mutase is rapidly (t1/2 = 6-9 min) converted to mutase. On the other hand, when the chase is performed in the presence of the inhibitor, the labeled pre-mutase persists for greater than 5 h. This long term stability of pre-mutase contrasts sharply with the instability previously reported for unprocessed precursors of two other mitochondrial enzymes.

Animals↗
PubMed1981

Immunochemical studies on cultured fibroblasts from patients with inherited methylmalonic acidemia.

We developed a radioimmunoassay to quantitate material crossreacting immunochemically with human methylmalonyl-CoA mutase (methylmalonyl-CoA CoA-carbonylmutase, EC 5.4.99.2), and have applied this assay to extracts of fibroblasts from controls and from 32 patients with methylmalonic acidemia due to inherited deficiencies in mutase activity. Four control lines had an average of 237 ng of crossreacting material (CRM) per mg of cell protein (range, 193-297 ng/mg). Mutant lines from each of the four cbl complementation groups of inherited methylmalonic acidemia, which have normal amounts of mutase activity in vitro, contained quantities of CRM comparable to those of control lines. On the other hand, 28 cell lines from the mut complementation group, which express mutations at the structural gene locus for the mutase apoenzyme, contained widely diverse amounts of CRM. Each of seven lines from the mut- subgroup, whose residual mutase activity reflects the presence of a structurally altered mutase protein with reduced affinity for cofactor, had detectable CRM ranging from 20% to 100% of control. The 21 lines from the mut0 group, which have no detectable mutase activity in vitro, fell into two populations with regard to CRM: 9 lines had detectable CRM ranging from 3% to 40% of control; 12 others had no detectable CRM (limit of detectability, less than 1% of control). These results emphasize the wide range of mutations at a single structural gene locus that can result in deficient enzyme activity.

Cell Line↗