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Biomedical subjects

A F Bennett

Publications and source records attributed to A F Bennett.

At least 55 records · Page 3Linked to original sources
PubMed1985

Kinetics of inhibition and hysteresis of sheep liver cytoplasmic aldehyde dehydrogenase with glyoxylic acid: further evidence relating to the two-site model for aldehyde oxidation.

Despite the fact that it is an aldehyde, glyoxylic acid is not a substrate for sheep liver cytoplasmic aldehyde dehydrogenase; instead it functions as an inhibitor of both the esterase and dehydrogenase activities. From a consideration of the inhibition patterns it is concluded that glyoxylic acid does not bind in the catalytic propionaldehyde-binding domain, thus confirming the two-site model as proposed previously. Since the corresponding neutral methyl ester is a substrate it is suggested that the catalytic binding domain must contain a negatively charged group which prevents the binding of glyoxylic acid. Steady-state and pre-steady-state kinetic studies indicate that glyoxylic acid inhibits the dehydrogenase activity by converting the enzyme into a dead-end form which cannot undergo the catalytically essential conformational change. Incubation of the enzyme with NAD+ and glyoxylic acid for 10 min before the addition of propionaldehyde gave rise to hysteresis effects which can be explained on the basis of a slow isomerization of the enzyme X NAD+ X glyoxylic acid complex.

Aldehyde Dehydrogenase↗
PubMed1985

Temperature and muscle.

Rates of force development, contraction and relaxation of vertebrate skeletal muscle are temperature dependent with Q10 values of approximately 2. Maximal forces developed have a low or negative thermal dependence. The functional basis of these patterns is poorly understood. Muscle performance generally does not acclimate. There appears to have been some evolutionary adaptation among species and classes to different thermal regimes, such that muscles from cold-adapted species maintain better mechanical performance at low temperatures than do those from warm-adapted animals. However, rate processes remain strongly thermally dependent even in animals with low or variable body temperatures. This thermal dependence of muscle in vitro is reflected in behavioural performance: maximal force generation in vivo is temperature independent and time-dependent activities are more rapid at higher muscle temperatures.

Animals↗
PubMed1984

The effect of body temperature on the locomotory energetics of lizards.

Oxygen consumption (VO2), carbon dioxide production (VCO2), and stamina were measured in the lizard Tupinambis nigropunctatus running at sustainable and non-sustainable velocities (v) on a motor-driven treadmill. Three experimental groups were measured: field-fresh animals at body temperature (Tb) = 35 degrees C and laboratory-maintained animals at Tb = 35 and 25 degrees C. Mean preferred Tb was determined to be 35.2 degrees C. At 35 degrees C, field-fresh animals had a greater maximal oxygen consumption (VO2max corr) (4.22 vs 3.60 ml O2 g-0.76h-1) and a greater endurance. The net cost of transport (slope of VO2 on v) did not differ between the groups (= 2.60 ml O2 g-0.76)km-1). Velocity at which VO2max is attained (MAS) is 0.84 km h-1. The respiratory exchange ratio (R) exceeded 1.0 at v above MAS, indicating supplementary anaerobic metabolism. At 25 degrees C, VO2max corr was lower (2.34 ml O2 g-0.76h-1) as was endurance, MAS occurring at 0.5 km h-1. Net cost of transport was not significantly different than at 35 degrees C. The effect of Tb on locomotory costs was analyzed for this lizard and other species. It was concluded that the net cost of transport is temperature independent in all species examined and the total cost of locomotion (VO2 v-1) is temperature dependent in Tupinambis (Q10 = 1.4-2.0) and all other species examined except one. The energetic cost of locomotion [(VO2active-VO2rest)v-1], previously reported to be temperature independent in lizards, is temperature dependent in Tupinambis (Q10 = 1.3-1.6) and in two other species.2r

Animals↗
PubMed1984

Thermal dependence of muscle function.

Maximal isometric forces during both twitch and tetanus are largely temperature independent in muscles from both endothermic and ectothermic vertebrates. Anuran muscle can develop maximal force at lower temperatures than mammalian muscle. Tetanic tension is maximal at normally experienced body temperatures in a variety of animals, but twitch tension seldom is. Thermal dependence of twitch tension varies with muscle fiber type: tension decreases with increasing temperature in fast-twitch muscles and remains constant in slow-twitch muscles. In contrast to the low temperature dependence of force generation, rates of development of tension (time to peak twitch tension and tetanic rise time) and maximal velocity of shortening and power output are markedly temperature dependent, with average temperature coefficient (Q10) values of 2.0-2.5 Q10 values for rate processes of anuran muscle are only slightly lower than those of mammalian muscle. High body temperatures permit rapid rates of muscle contraction; animals active at low body temperatures do not achieve the maximal rate performance their muscles are capable of delivering. Thermal acclimation or hibernation does not appear to result in compensatory adjustments in either force generation or rate processes. In vivo, dynamic processes dependent on contractile rates are positively temperature dependent, although with markedly lower Q10 values than those of isolated muscle. Static force application in vivo is nearly temperature independent.

Adaptation, Physiological↗
PubMed1983

A two-site model for the esterase and dehydrogenase activities of sheep liver aldehyde dehydrogenase.

Although aldehyde dehydrogenase (ALDH) from sheep liver cytosol has a broad specificity, it will not oxidize the aldehyde group of glyoxylic acid which is in fact an inhibitor of the enzyme. The inhibition pattern is non-linear but competitive at high propionaldehyde concentrations (2-20 mM); however, a simple non-competitive pattern is observed at low (less than 100 microM) propionaldehyde concentrations (Ki = 1.6 mM). The esterase activity was unaffected by glyoxylic acid in the absence of NAD+ but a simple competitive inhibition pattern (Ki = 2.5 mM) was observed with respect to 4-nitrophenyl acetate in the presence of NAD+. The data require a two-site model in which ester and aldehyde binding sites are distinct but with a second propionaldehyde molecule, and glyoxylic acid, binding at or near the ester binding site. Consistent with this model is the fact that chloral hydrate was a non-competitive inhibitor of the esterase activity in the presence of NAD+ but a competitive inhibitor in its absence. The enzyme exhibited hysteretic behavior governed by the protonated form of an ionizable group with an apparent pKa of 7.55.

Aldehyde Dehydrogenase↗
PubMed1982

Proton release during the pre-steady-state oxidation of aldehydes by aldehyde dehydrogenase. Evidence for a rate-limiting conformational change.

A transient release of protons with an amplitude corresponding to one proton per active site has been observed for the oxidation of propionaldehyde, acetaldehyde, and benzaldehyde by sheep liver cytoplasmic aldehyde dehydrogenase at pH 7.6 with phenol red as indicator. At saturating substrate levels, the rate constants for the proton burst are in each case the same, and for acetaldehyde and propionaldehyde show the same dependence on the concentrations of the substrates, as the rate constants for the transient production of NADH reported previously [MacGibbon, A.K.H., Blackwell, L.F., & Buckley, P.D. (1977) Biochem. J. 167, 469-477]. Although, with propionaldehyde as a substrate, a full proton burst is also observed at pH 6.0, no proton burst is observed at pH 9.0. For 4-nitrobenzaldehyde, there is no burst in NADH production, but a burst in proton release is observed, showing that proton release precedes hydride transfer. No protons were released during the binding of the substrate analogues acetone and chloral hydrate nor on reaction of the enzyme with the inhibitor tetraethylthiuram disulfide (disulfiram). A model is proposed in which the rate-limiting step in the pre-steady-state phase of the reaction is a conformational change which occurs after the binding of aldehydes to the enzyme. As a result of the conformational change, the environment of a functional group on the enzyme, which initially has a pKa of about 8.5, is perturbed to give a final pKa value for the group of less than 5. Computer simulations were used to show that the model accurately reproduces all of the experimental data. The lack of observation of a second transient proton release, as required by the overall stoichiometry, argues that its release occurs in a slow step prior to NADH dissociation.

Acetaldehyde↗
PubMed1982

Acid-base imbalance in lizards during activity and recovery.

1. The effects of treadmill exercise on oxygen consumption (V02), carbon dioxide production (VCO2), arterial blood lactate concentration ([L-]a), arterial blood pH and arterial gas tensions (PaO2 and PaCO2) were measured in 3 species of lizards (Varanus salvator, V. exanthematicus, Iguana iguana) 2. Varanus salvator was exercised 45 min at an intensity which required 85% of its VO2 max. V. salvator utilized supplementary anaerobic metabolism during the first 10 min of this sustainable exercise, as evidenced by a 16 mmol/l increase in [L-]a. Respiratory exchange ratios (R, where R = VCO2/VO2) exceeded 1.2 when [L-]a and [H+]a were maximal. One half of the accumulated lactate was removed from the blood during the remainder of the 45 min exercise period, while blood pH returned to resting levels. 3. In a second set of experiments, high intensity exercise led to exhaustion after 5 to 10 min in all three species, resulting in large lactate (+ delta[L-]a = 14-20 mmol/l) and hydrogen ion (+ delta[H+]a = 23-57 nmol/l) accumulations. R values ranged from 1.2-1.8 at exhaustion. 4. Recovery from both sustainable and non-sustainable exercise was characterized as a period of rapid lactate removal. Respiratory exchange ratios were low (0.3-0.5) as metabolic CO2 was retained, replacing depleted bicarbonate stores. 5. We conclude that all three lizard species make ventilatory adjustments during and after exercise that minimize disturbances to resting hydrogen ion concentrations and acid-base balance. Varanus salvator demonstrate the ability to re-establish resting acid-base status during sustained exercise requiring 85% of their VO2,max. Changes in R appear to be a useful noninvasive indicator of net blood lactate accumulation.

Acid-Base Equilibrium↗
PubMed1981

Pulmonary oxygen transport during activity in lizards.

Oxygen consumption (MO2), effective alveolar ventilation (Veff), arterial and alveolar PO2 (PaO2, PAO2) and the alveolar-arterial PO2 difference [(A--a)PO2] were determined in the lizards Varanus exanthematicus and Iguana iguana at rest and during treadmill exercise at 35 degrees C. In both species, Veff increased more rapidly than MO2 giving rise to an increased PAO2. In contrast, PaO2 remained unchanged through the highest levels of MO2 attained. As a result, the (A--a)PO2 increased with increasing MO2. We suggest that the observed increase in (A--a)PO2 may be due to a rather low pulmonary oxygen diffusing capacity (DLO2) and limited capacity to increase DLO2 during exercise. Arterial desaturation was prevented by a compensatory hyperventilation, thus enhancing the gradient for alveolar-capillary gas exchange. These results indicate that both lizard species increase pulmonary oxygen transport sufficiently so that it is not a limiting factor to aerobic scope under the conditions of this study.

Animals↗
PubMed1981

A reinvestigation of the purity, isoelectric points and some kinetic properties of the aldehyde dehydrogenases from sheep liver.

1. Cytoplasmic aldehyde dehydrogenase was shown to be free of contamination by the mitochondrial enzyme by isoelectric focusing. 2. Both enzymes showed multiple banding in activity stains. The cytoplasmic enzyme gave two very close bands pI = 5.22 +/- 0.03 whereas the mitochondrial enzyme showed seven bands, a pair at pI = 5.22 and five further bands of pI 5.48 +/- 0.09, 5.56 +/- 0.07, 5.65 +/- 0.06, 5.70 +/- 0.03 and 5.76 +/- 0.02. Possible origins of the isoenzymes are discussed. 3. Disulfiram in a fourfold excess reduced the activity of the cytoplasmic enzyme to 9% of the initial value. The residual activity represents the activity of the disulfiram-modified enzyme and is not due to mitochondrial contamination. This casts doubt on the role of an essential thiol group. 4. The mitochondrial enzyme shows a low amplitude (22%) burst in the production of 4-nitrophenoxide ion during the hydrolysis of 4-nitrophenyl acetate at pH 7.6. The burst rate constant was 7.3 +/- 1 s-1 and the steady-state rate constant was 0.2 s-1, values similar to those previously reported for the cytoplasmic enzyme. 5. The mitochondrial enzyme shows a burst in the release of protons during the oxidation of propionaldehyde at pH 7.6. The burst rate constant was 6 s-1 and the amplitude was equal to half the formal enzyme concentration. The significance of these results for the steady-state mechanism is discussed.

Aldehyde Dehydrogenase↗
PubMed1981

Ventilation and acid-base balance during graded activity in lizards.

Arterial PCO2, hydrogen ion ([H+]a), and lactate ([L]a) concentrations, rates of metabolic CO2 production (VCO2) and O2 consumption (VO2), and effective alveolar ventilation (Veff) were determined in the lizards Varanus exanthematicus and Iguana iguana at rest and during steady-state treadmill exercise at 35 degrees C. In Varanus, VCO2 increased ninefold and VO2 sixfold without detectable rise in [L]a at running speeds below 1.0 to 1.5 km x h-1. In this range, Veff increased 12-fold resulting in decreased levels of PaCO2 and [H+]a. At higher speeds [L]a rose. Increments of 5 mM [L]a were accompanied by hyperventilation, reducing PaCO2 and thus maintaining [H+]a near its resting level. When [L]a increased further, [H+]a increased. Sustainable running speeds (0.3-0.5 km x h-1 and below) were often associated with increased VO2, VCO2, and [L]a in Iguana. Sixfold increases in VCO2 and 9-mM increments in [L]a were accompanied by sufficient increase in Veff (9-fold) to maintain [H+]a at or below its control level. When [L]a increased further, [H+]a increased. These results indicate that both lizard species maintain blood acid-base homeostasis rather effectively via ventilatory adjustments at moderate exercise intensities.

Acid-Base Equilibrium↗
PubMed1981

Thermal dependence of endurance and locomotory energetics in a lizard.

The thermal dependencies of endurance and the rates of oxygen cnsumption (VO2) and carbon dioxide production (VCO2) were determined for the lizard Dipsosaurus dorsalis walking on a treadmill. The thermal dependencies of endurance and maximal VO2 (VO2 max) are nerly identical. The maximal sustainable speed and the speed at which VO2 max is attained at each experimental body temperature are the same. The net cost of locomotion (the slope of VO2 vs. speed) is termperature independent. Resting VO2 and the incremental change in VO2 required to support locomotion at a sustainable speed are higher at 40 degrees C than at 25 degrees C. Therefore, the total cost of locomotion (VO2 divided by speed) is greater at 40 degrees C. However, the minimum cost of transport, attained at the maximal sustainable speed, is temperature independent. The results clearly indicate that maintenance of a high body temperature is energetically expensive for Dipsosaurus, but that the associated expanded activity capacity enables this lizard to avoid an obligate increase in the energetic requirements of locomotion.

Animals↗
PubMed1980

Antiquity of the vertebrate pattern of activity metabolism and its possible relation to vertebrate origins.

Vertebrates generally possess well developed capacities for anaerobic metabolism, resulting in formation of lactic acid. Those capacities have traditionally been interpreted in terms of adaptation to hypoxic environments or to special situations such as diving. However, anaerobic metabolsim in striated muscle tissue is frequently a major source of ATP utilized during periods of intense activity. The evolutionary significance of anaerobically supported activity has not been discussed, although the interrelationships of capacities for aerobiosis and activity have received considerable attention. We present here evidence that the pattern of activity metabolism utilized by extant species probably dates back to the earliest vertebrates. It is also postulated that the evolution of extensive capacity for anaerobically supported burst activity may have been closely related to the evolution of vertebrates from invertebrate chordates.

Aerobiosis↗