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Biomedical subjects

R H Davis

Publications and source records attributed to R H Davis.

At least 91 records · Page 5Linked to original sources
PubMed1988

Polyamine transport in Neurospora crassa.

Polyamine transport in Neurospora crassa is concentrative and energy dependent in a dilute buffer. The saturable systems governing the uptake of putrescine (Km = 0.6 mM), spermidine (Km = ca. 0.24 mM), and spermine (Km = 0.07 mM) share components, as indicated by mutual inhibition among the polyamines. In addition, nonsaturable components prevail for putrescine and spermidine, particularly the former. Radiolabeled substrates, once in the cell, are released only slowly, even if unlabeled polyamines are included in the incubation medium. Permeabilization of cells with n-butanol leads to partial release of internalized 14C-polyamines, and the remainder is almost wholly exchangeable with added, unlabeled polyamine. Polyamine uptake was inhibited by the polyamines themselves and by a polyamine analog, methylglyoxal bisguanylhydrazone, but only weakly and incompletely by the basic amino acids arginine and ornithine. Uptake of putrescine and spermidine was inhibited by monovalent cations, Ca2+, and certain other components of the growth medium. As a result, uptake from the growth medium was very slow and largely by way of the nonsaturable uptake mechanism.

Biological Transport↗
PubMed1988

Abnormal gastroduodenal motility in children and adolescents with recurrent functional abdominal pain.

To determine whether motor activity of the stomach and proximal small intestine is a factor in recurrent abdominal pain in adolescents, we prospectively investigated eight patients with recurrent abdominal pain and compared them with seven normal adolescents. All patients underwent a detailed examination to exclude other known organic causes of the pain. The gastroduodenal motor activity during fasting was studied with a semiconductor recording probe. The recordings were analyzed for periodicity, duration, and propagation velocity of the activity front of the migrating motor complex. The amplitude of the antral and duodenal contractions was also determined. The patients with recurrent abdominal pain had more frequent migrating motor complexes, but these were shorter in duration and moved more slowly down the intestine (slower propagation velocities). The patients also had high-pressure duodenal contractions that were associated with abdominal pain during the study period. These studies suggest that altered intestinal motility may be the underlying mechanism of recurrent abdominal pain in some children.

Abdomen↗
PubMed1988

Effect of dietary sodium nitroprusside as a source of cyanide on the selenium status of chicks given diets of varying selenium concentration.

1. The interaction between dietary cyanide, given in the form of sodium nitroprusside (SNP), and selenium has been studied in two experiments with growing chicks from 14 to 38 d of age. 2. In experiment 1, dietary selenium at 10 mg Se/kg reduced growth, food intake and efficiency of utilisation, and increased relative liver size and selenium content. All of these effects were eliminated by the addition of 0.1 g SNP/kg except for liver selenium content, which progressively declined towards control values as SNP was increased to 0.4 g/kg in increments of 0.1 g/kg. At 0.3 g SNP/kg, cyanide toxicity, as judged by decreased growth, reached significance. 3. In experiment 2, similar effects were observed with selenium at 10 mg Se/kg and SNP at 0.3 g/kg, but selenium deficiency was not evident from growth indicators when selenium supplementation of the diet was omitted completely, nor did these indicators suggest that deficiency was induced by cyanide. 4. In both experiments, plasma and liver glutathione peroxidase activity reflected the dietary selenium content. There was an interaction with dietary SNP content. With selenium intake at a toxic level, SNP increased enzyme activity, further evidence of alleviation of selenium toxicity, but when selenium intake was low and normal, SNP decreased activity in liver, an indication that cyanide could induce deficiency. 5. A possible mechanism for alleviation of selenium toxicity is proposed.

Animals↗
PubMed1988

Sodium nitroprusside, a convenient source of dietary cyanide for the study of chronic cyanide toxicity.

1. The suitability of sodium nitroprusside (SNP) as a source of dietary cyanide has been tested with growing chicks and with hens by measuring plasma or urinary thiocyanate; the major detoxication product of cyanide. 2. Growth and food intake were depressed and plasma thiocyanate concentration was increased in chicks in a progressive manner as the dietary concentration of SNP was increased from 0 to 0.5 g/kg. 3. In laying hens, urinary excretion of thiocyanate was increased fivefold by including SNP in the diet at 0.3 g/kg. 4. It is concluded that SNP is a suitable and convenient source of dietary cyanide for the investigation of chronic cyanide toxicity.

Animals↗
PubMed1987

Ornithine decarboxylase from Neurospora crassa. Purification, characterization, and regulation by inactivation.

Ornithine decarboxylase, a highly regulated enzyme of the polyamine pathway, was purified 670-fold from mycelia of Neurospora crassa that were highly augmented for enzyme activity. The enzyme is significantly different from those reported from three other lower eucaryotic organisms: Saccharomyces cerevisiae, Physarum polycephalum, and Tetrahymena pyriformis. Instead, the enzyme closely resembles the enzymes from mammals. The Mr = 110,000 enzyme is a dimer of 53,000 Da subunits, with a specific activity of 2,610 mumol per h per mg of protein. Antisera were raised to the purified enzyme and were rendered highly specific by cross-absorption with extracts of a mutant strain lacking ornithine decarboxylase protein. With the antisera, we show that the inactivation of the enzyme in response to polyamines is proportional to the loss of ornithine decarboxylase protein over almost 2 orders of magnitude. This is similar to the inactivation process in certain mammalian tissues, and different from the process in S. cerevisiae and P. polycephalum, in which enzyme modification, without proportional loss of antigen, accompanies enzyme inactivation. The N. crassa enzyme is therefore suitable as a microbial model for studies of the molecular regulation of the mammalian enzyme.

Eflornithine↗
PubMed1987

Arginine-specific carbamoyl phosphate metabolism in mitochondria of Neurospora crassa. Channeling and control by arginine.

Citrulline is synthesized in mitochondria of Neurospora crassa from ornithine and carbamoyl phosphate. In mycelia grown in minimal medium, carbamoyl phosphate limits citrulline (and arginine) synthesis. Addition of arginine to such cultures reduces the availability of intramitochondrial ornithine, and ornithine then limits citrulline synthesis. We have found that for some time after addition of excess arginine, carbamoyl phosphate synthesis continued. Very little of this carbamoyl phosphate escaped the mitochondrion to be used in the pyrimidine pathway in the nucleus. Instead, mitochondrial carbamoyl phosphate accumulated over 40-fold and turned over rapidly. This was true in ornithine- or ornithine carbamoyltransferase-deficient mutants and in normal mycelia during feedback inhibition of ornithine synthesis. The data suggest that the rate of carbamoyl phosphate synthesis is dependent to a large extent upon the specific activity of the slowly and incompletely repressible synthetic enzyme, carbamoyl-phosphate synthetase A. In keeping with this conclusion, we found that when carbamoyl-phosphate synthetase A was repressed 2-10-fold by growth of mycelia in arginine, carbamoyl phosphate was still synthesized in excess of that used for residual citrulline synthesis. Again, only a small fraction of the excess carbamoyl phosphate could be accounted for by diversion to the pyrimidine pathway. The continued synthesis and turnover of carbamoyl phosphate in mitochondria of arginine-grown cells may allow rapid resumption of citrulline formation after external arginine disappears and no longer exerts negative control on ornithine biosynthesis.

Arginine↗
PubMed1987

Spectrophotometric assay for ornithine decarboxylase.

A rapid and sensitive spectrophotometric assay for ornithine decarboxylase is described. It is based on the observation that the product of ornithine decarboxylase, putrescine, reacts with 2,4,6-trinitrobenzenesulfonic acid to give a colored product soluble in 1-pentanol whereas ornithine does not. The amount of putrescine produced by the enzyme was determined by measuring the absorbance of the 1-pentanol extract of the reaction mixture at 420 nm, and by comparing the results to those obtained by the trapping of 14CO2 and by HPLC assays. The three assays were found to be equivalent in sensitivity, with the spectrophotometric assay having the advantages of being relatively rapid, requiring only common laboratory equipment, and not requiring the use of radioactive isotopes.

Ornithine Decarboxylase↗
PubMed1987

Detoxication of cyanide in the chicken by conversion to thiocyanate, as influenced by the availability of transferable sulphur.

The urinary excretion of thiocyanate by hens after dosage with cyanide (30 mumol) has been studied in a series of acute experiments involving 6 hr urine collection periods. More than half of the dose could be recovered as thiocyanate when cyanide was given by intravenous infusion and the rate of excretion closely paralleled plasma thiocyanate concentration. Little cyanide was excreted directly. The excretion of thiosulphate fell by an amount that suggested that availability of sulphane sulphur might limit the extent of conversion. However, neither thiosulphate nor sulphur amino acids enhanced thiocyanate excretion when they were infused together with cyanide; indeed, thiocyanate excretion decreased as the level of sulphur compound given was increased. Both nitrite and sulphite depressed thiocyanate excretion also but they differed in their effects on plasma thiocyanate levels and the pattern of urinary excretion. Comparison of excretion from both sides of the kidneys separately emphasised the importance of the first pass of cyanide in its conversion to thiocyanate. The results suggest that although sulphur availability may be limited the in vivo production of sulphite also restricts cyanide detoxication.

Animals↗
PubMed1987

Control of hypertension in elderly patients with felodipine and metoprolol: a double-blind, placebo-controlled clinical trial.

1. Forty-nine patients aged 65-80 years, whose Phase V diastolic blood pressure (dBP) was above 95 mmHg after 4 weeks open treatment with metoprolol 50 mg twice daily were randomized to receive, double-blind, the calcium antagonist felodipine (n = 32) 2.5 mg twice daily or placebo (n = 17) in addition to metoprolol for 2 weeks. If the dBP remained greater than 95 mmHg, the dose of felodipine or placebo was doubled for a further 2 weeks; if the dBP was still greater than 95 mmHg, the dose of felodipine was doubled again to 10 mg twice daily or the corresponding placebo dose given. The duration of the double-blind period was 6 weeks, all patients receiving metoprolol 50 mg twice daily throughout. 2. At the end of the double-blind period, the seated dBP was reduced from 103 +/- 5 (mean +/- s.d.) to 88 +/- 7 mmHg (P less than 0.001) by felodipine and from 105 +/- 100 +/- 11 mmHg (NS) by placebo. The differences between these reductions (P less than 0.01) and between the final dBPs (P less than 0.001) were significant. Eighty-nine per cent of patients receiving felodipine and 33% of those receiving placebo (P less than 0.001) had controlled (dBP less than or equal to 95 mmHg) BPs. Half (14/27 completing) of the patients receiving felodipine required 2.5 mg throughout; 9/27 needed 5 mg and 4/27 10 mg twice daily. Adverse events occurred with equal frequency in the two groups, but the profile was different.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1987

Nonsense mutations of the ornithine decarboxylase structural gene of Neurospora crassa.

Ornithine decarboxylase (ODC) (EC 4.1.1.17) is an early enzyme of polyamine synthesis, and its activity rises quickly at the onset of growth and differentiation in most eucaryotes. Some have speculated that the enzyme protein may have a role in the synthesis of rRNA in addition to its role in catalyzing the decarboxylation of ornithine (G. D. Kuehn and V. J. Atmar, Fed. Proc. 41:3078-3083, 1982; D. H. Russell, Proc. Natl. Acad. Sci. USA 80:1318-1321, 1983). To test this possibility, we sought mutational evidence for the indispensability of the ODC protein for normal growth of Neurospora crassa. We found three new, ODC-deficient mutants that lacked ODC protein. Among these and by reversion analysis of an earlier set of mutants, we found that two ODC-deficient mutants carried nonsense mutations in the ODC structural gene, spe-1. Allele LV10 imparted a complete deficiency for enzyme activity (less than 0.006% of normal) and had no detectable ODC antigen. Allele PE4 imparted a weak activity to cells (0.1% of derepressed spe+ cultures) and encoded a lower-molecular-weight ODC subunit (Mr = 43,000) in comparison to that of the wild-type strain (Mr = 53,000). Strains carrying either mutation, like other spe-1 mutants, grew at a normal rate in exponential culture if the medium was supplemented with spermidine, the main end product of the polyamine pathway in N. crassa. Unless an antigenically silent, N-terminal fragment with an indispensable role persists in the LV10-bearing mutant, we conclude that the ODC protein has no role in the vegetative growth of this organism other than the synthesis of polyamines. The data extend earlier evidence that spe-1 is the structural gene for ODC in N. crassa. The activity found in mutants bearing allele PE4 suggests that the amino acids nearest the carboxy terminus do not contribute to the active site of the enzyme.

Alleles↗