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Biomedical subjects

R D Allison

Publications and source records attributed to R D Allison.

At least 19 recordsLinked to original sources
PubMed2002

The effect of pasture nitrate concentration and concentrate intake after turnout on embryo growth and viability in the lactating dairy cow.

This study investigated the effects on embryo growth and survival rate of feeding heavily-fertilised spring grass, containing high levels of quickly-degradable nitrogen, to pregnant cows. Forty-eight lactating Holstein cows between 2 and 8 weeks pregnant were turned-out, after a one-week transition period onto high- or low-nitrate pasture and fed a high- or low-concentrate supplement. Cows grazing the High nitrate pasture had significantly higher milk and plasma urea concentrations than cows grazing the Control pasture, while cows which were fed less concentrate had a notably higher plasma ammonia. However, there was no evidence that an increased quickly-degradable nitrogen (QDN) intake from pasture affected embryo survival or growth from 20 days onwards. This suggests that the impact of turnout on fertility mainly affects ovulation, fertilisation and/or the early embryo.

Ammonia↗
PubMed2000

Effect of vitamin E supplementation on the health and fertility of dairy cows: a review.

The currently recommended intake of vitamin E for dairy cows is based on the prevention of nutritional myopathy, a calf disease. However, it is likely that the vitamin E requirement of the modern dairy cow is very different from that of a calf. This review of the literature investigates the effect of vitamin E supplementation on the health and fertility of the dairy cow. Supplementation of high levels of vitamin E (at least 1000 iu per day) during the dry period and early lactation can reduce the incidence of mastitis, possibly because of an increase in immune system activity and function, but there appears to be little benefit of supplementation on infectious diseases other than mastitis. The evidence for a response in the reproductive system is more equivocal. In herds with a history of selenium deficiency and a high incidence of retained fetal membranes, supplementation, in conjunction with selenium, can reduce retention, but the evidence for an effect of supplementation on other reproductive diseases, such as cystic ovarian disease and metritis, is based on a very limited number of cases. The literature suggests that the current recommendations for vitamin E are inadequate. In particular, it suggests that the current linking of requirement to dry matter intake is incorrect, because vitamin E requirement is probably at its highest when intake is at its lowest However, the majority of the data on which this conclusion is based, come from North America where cows will encounter significantly different levels of oxidative stress from cows in the EU.

Animal Feed↗
PubMed1997

Inhibition of endothelial cell amino acid transport System y+ by arginine analogs that inhibit nitric oxide synthase.

A variety of N omega-monosubstituted L-arginine analogs are established inhibitors of nitric oxide synthase; in all cases, initial binding is competitive with the substrate L-arginine. The efficacy of such compounds in vivo will depend on their transport into the relevant nitric oxide synthase-containing cells; in fact, inhibition may actually be augmented if cellular uptake of L-arginine is also blocked by the analogs. Because vascular endothelial cells synthesize vasoactive nitric oxide under both physiological and pathophysiological conditions, we have performed inhibition analyses with novel arginine analogs to determine the substrate specificity of the primary L-arginine transport system. Na(+)-independent System y+, present in porcine pulmonary artery endothelial cells. As reported by others, no Na(+)-independent System bo,+ activity was detectable. For System y+. Dixon plots suggest competitive inhibition and apparent Ki values, which ranged between 0.1 and 0.8 mM, estimated for each inhibitor. Some influence of amino acid side chain structure could be detected, but in general, the data establish that this transport system accepts a broad range of arginine derivatives. Loading the cells with individual arginine analogs resulted in trans-stimulation of arginine uptake suggesting that they serve as substrates of System y+ as well as inhibitors. These results indicate that plasma membrane transport is unlikely to be a limiting factor in drug development for nitric oxide synthase inhibitors.

Animals↗
PubMed1993

Glutamine inhibits the ammonia-dependent activities of two Cys-1 mutants of human asparagine synthetase through the formation of an abortive complex.

Cys-1 mutants of recombinant human asparagine synthetase were constructed and their ability to catalyze the glutamine-dependent nitrogen transfer reaction required for asparagine biosynthesis was determined. In agreement with previous work, altering Cys-1 to either Ala or Ser eliminated the glutamine-dependent activity while only minimally affecting the kinetic properties of the ammonia-dependent reaction. A lack of glutaminase activity in these mutants also allowed examination of glutamine binding in studies of the ability of glutamine to inhibit the ammonia-dependent production of asparagine. In both mutants, analysis of the observed kinetics indicated that glutamine inhibited ammonia-dependent asparagine synthesis through the formation of an abortive complex. This unanticipated observation suggests that the commonly accepted mechanism for nitrogen transfer from the primary amide of glutamine to aspartic acid in asparagine synthetase may have to be re-examined. A novel mechanistic proposal which is consistent with the formation of an abortive complex in the two Cys-1 mutants is presented.

Ammonia↗
PubMed1992

Steady-state kinetic mechanism of bovine brain tubulin: tyrosine ligase.

The ATP-dependent resynthesis of tubulin from tyrosine and untyrosinated tubulin was examined to establish the most probable steady-state kinetic mechanism of the tubulin: tyrosine ligase (ADP-forming). Three pair-wise sets of initial rate experiments, involving variation of two substrates pair-wise with the third substrate held at a high (but non-saturating) level, yielded convergent-line data, a behaviour that is diagnostic for sequential mechanisms. Michaelis constants were 14 microM, 1.9 microM and 17 microM for ATP, untyrosinated tubulin and L-tyrosine respectively, and the maximal velocity was 0.2 microM/min. AMP was a competitive inhibitor with respect to ATP, and a non-competitive inhibitor versus either tubulin or tyrosine. Likewise, L-dihydroxyphenylalanine acted competitively relative to tyrosine and non-competitively with respect to either ATP or tubulin. These findings directly support a random sequential mechanism. Product inhibition patterns with ADP were also consistent with this assignment; however, inhibition studies were not practical with either orthophosphate or tyrosinated tubulin because both were very weak inhibitors. Substrate protection of the enzyme against alkylation by N-ethylmaleimide and thermal inactivation, along with evidence of enzyme binding to ATP-Sepharose and tubulin-Sepharose, also supports the idea that this three-substrate enzyme reaction exhibits a random substrate addition pathway.

Adenosine Diphosphate↗
PubMed1983

Concerning the equilibrium exchange kinetics and mechanism of the sheep brain glutamine synthetase reaction.

Isotope exchange measurements of sheep brain glutamine synthetase have yielded conflicting experimental findings and interpretation, leaving in doubt the question of whether the enzyme operates via ordered or random pathways of enzyme-substrate interactions. We now report new experimental evidence that demonstrates the earlier discrepant results may be attributed to the choice of reaction conditions used to achieve equilibration of the chemical reaction prior to addition of isotopic tracer. A random kinetic mechanism, perhaps not of the rapid-equilibrium type, is most compatible with the exchange data. We also discuss other potential time-dependent processes that may compromise the equilibration of reaction systems and affect the outcome of exchange experiments, and criteria for equilibration are suggested for the aid of other workers.

Adenosine Diphosphate↗
PubMed1982

Interconversion of leukotrienes catalyzed by purified gamma-glutamyl transpeptidase: concomitant formation of leukotriene D4 and gamma-glutamyl amino acids.

The reversible conversion of leukotriene C4 to leukotriene D4 and of the latter to leukotriene E4 were studied with highly purified homogeneous preparations of gamma-glutamyl transpeptidase, dipeptidase, and aminopeptidase M. The conversion of leukotriene C4 to leukotriene D4, catalyzed by gamma-glutamyl transpeptidase, is significantly more rapid when carried out in the presence of an amino acid mixture a closely approximating that found in blood plasma and is accompanied by gamma-glutamyl amino acid formation. Because gamma-glutamyl transpeptidase is bound to the external surface of cell membranes and thus is readily accessible to plasma amino acids, it appears that conversion of leukotriene C4 to leukotriene D4 under physiological conditions is coupled with the formation of gamma-glutamyl amino acids. The apparent Km value for leukotriene C4 in this reaction is about 6 X 10(-6) M, a value close to that found for glutathione. Conversion of leukotriene D4 to leukotriene C4 is effectively catalyzed by gamma-glutamyl transpeptidase in the presence of relatively low concentrations of glutathione. The conversion of leukotriene D4 to leukotriene E4 is catalyzed much more rapidly by renal dipeptidase than by renal aminopeptidase M. Incubation of leukotriene E4 with gamma-glutamyl transpeptidase and glutathione leads to formation of a compound with the properties of gamma-glutamyl leukotriene E4; this reaction is analogous to that shown previously in which gamma-glutamyl cystine is formed by transpeptidation between glutathione and cystine.

Aminopeptidases↗
PubMed1981

Evidence that transpeptidation is a significant function of gamma-glutamyl transpeptidase.

gamma-Glutamyl transpeptidase (purified from rat kidney) was incubated with glutathione and a mixture of amino acids that closely approximates the amino acid composition of blood plasma, and the relative extents of transpeptidation and hydrolysis were determined by quantitative measurement of the products formed (glutamate, cysteinylglycine, gamma-glutamyl amino acids). At pH 7.4, in the presence of 50 microM glutathione and the amino acid mixture, about 50% of the glutathione that was utilized participated in transpeptidation. Studies in which the formation of individual gamma-glutamyl amino acids was determined in the presence of glutathione and the amino acid mixture showed that L-cystine and L-glutamine are the most active amino acid acceptors, and that other neutral amino acids also participate in transpeptidation to a significant extent. These in vitro experiments are consistent with a number of other findings which indicate that transpeptidation is a significant physiological function of gamma-glutamyl transpeptidase.

Animals↗
PubMed1977

Steady state and equilibrium exchange kinetic studies of the sheep brain glutamine synthetase reaction.

The kinetic mechanism of the sheep brain glutamine synthetase has been examined by both initial rate kinetics using the glutamate analog beta-glutamate and by isotope exchange measurements at equilibrium. Results of the initial rate studies were compatible with a number of sequential mechanisms but not with a partially or fully ordered rapid equilibrium or a ping-pong mechanism. Kinetic parameters at 37 degrees and pH 7.2 were K beta-Glu = 16 mM, KATP = 0.28 mM, and KNH2OH = 1.4 mM. For all equilibrium exchanges studied (ATP in equilibrium ADP, ATP in equilibrium Pi, and Glu in equilibrium Gln), the rate of exchange rose smoothly to a maximum as all substrates and products were simultaneously raised in a constant ratio. This result is in accord with a random order of substrate addition. A brief treatment of equilibrium exchange rates in cases where all substrate/product pairs are varied together is also presented.

Adenosine Diphosphate↗