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Biomedical subjects

A T James

Publications and source records attributed to A T James.

At least 19 recordsLinked to original sources
PubMed1989

Statistical modelling of mitochondrial power supply.

By experiment and theory, formulae are derived to calculate the response of mitochondrial power supply, in flux and potential, to an ATP consuming enzyme load, incorporating effects of varying amounts of (i) enzyme, (ii) total circulating adenylate, and (iii) inhibition of the ATP/ADP translocase. The formulae, which apply between about 20% and 80% of maximum respiration, are the same as for the current and voltage of an electrical circuit in which a battery with potential, linear in the logarithm of the total adenylate, charges another battery whose opposing potential is also linear in the same logarithm, through three resistances. These resistances produce loss of potential due to dis-equilibrium of (i) intramitochondrial oxidative phosphorylation, (ii) the ATP/ADP translocase, and (iii) the ATP-consuming enzyme load. The model is represented geometrically by the following configuration: when potential is plotted against flux, the points lie on two pencils of lines each concurrent at zero respiration, the two pencils describing the respective characteristics of the mitochondrion and enzyme. Control coefficients and elasticities are calculated from the formulae.

Adenosine Triphosphate↗
PubMed1985

Estimation of a derivative by a difference quotient: its application to hepatocyte lactate metabolism.

Values of the derivative of a convex function are estimated from data for the function by assigning the difference quotients of the observations to appropriate values of the determining variable. The procedure is illustrated for the time course of hepatocytes incubated with lactate, of amount l, for which pyruvate, of amount p, is also observed. An empirical linear relation between dp/dt and ln(l/p) is established.

Animals↗
PubMed1983

The interaction of mammalian medium-chain hydrolase with yeast fatty acid synthetase.

The interaction of rat mammary gland medium-chain thioesterase with yeast fatty acid synthetase has been investigated. Medium-chain thioesterase interacts with yeast fatty acid synthetase causing premature chain termination of the fatty acids synthesized from acetyl-CoA and malonyl-CoA. This effect is most marked under conditions of rate-limiting malonyl-CoA availability. Immobilized yeast fatty acid synthetase specifically binds rat mammary gland medium-chain thioesterase. This interaction has been used to purify medium-chain thioesterase to near homogeneity from samples of rat mammary gland cytosol. The stoichiometry of binding of medium-chain thioesterase to yeast fatty acid synthetase has been investigated. Yeast fatty acid synthetase binds 5.7 +/- 1 mol medium-chain thioesterase/mol yeast fatty acid synthetase. It is concluded that yeast fatty acid synthetase has a medium-chain thioesterase binding site.

Animals↗
PubMed1980

Desaturation of positional and geometric isomers of monoenoic fatty acids by microsomal preparations from rat liver.

A range of cis- and trans-monoenoic fatty acids was tested as substrates for desaturation in microsomal preparations from rat liver. Trans-monoenoic acids were generally desaturated in the delta 9 position to the same extent as stearic acid. Acids with delta 7-trans- and delta 11-trans-olefinic unsaturation produced delta 7-trans, 9-cis- and delta 9-cis, 11-trans-conjugated dienoic acids, respectively, but the delta 8-trans-and delta 10-trans-monoenoic acids did not give delta 8,9- or delta 9,10-allenes. Of the cis-monoenoic acids examined, only those with double bonds at or beyond the delta 14 position gave any measurable delta 9 desaturation. When delta 9 desaturation of long chain saturated acids was inhibited by adding sterculic acid, these saturated acids were desaturated at the delta 5 and delta 6 positions. Many of the monoenoic acids tested were also desaturated at the delta 5 and/or delta 6 positions, although the percentage conversions were always low. delta 9-cis, 11-trans-, delta 9-cis, 12-trans- and delta 9-cis, 13-trans-dienoic acids, produced in situ by delta 9 desaturation of the corresponding monoenoic acids, were extensively desaturated in the delta 6 position. These results are discussed in terms of: (a) the various models proposed to explain the substrate specificities of the desaturases, and (b) the metabolism of unnatural fatty acids ingested from dietary sources.

Animals↗
PubMed1980

The delta 5 and delta 6 desaturation of fatty acids of varying chain length by rat liver: a preliminary report.

The delta 6 desaturase of rat liver can accommodate substrates with a wide range of chain length. delta 9-cis, 12-cis-Dienoic acids of chain lengths 14-22 carbon atoms were all desaturated at the delta 6 position by microsmal preparations from rat liver. By contrast, the delta 5 desaturase appeared much more chain-length sensitive. The percentage delta 5 desaturation of a series of delta 8-cis- and delta 9-trans-monoenoic acids increased with increasing chain length (from C16 to C20).

Animals↗
PubMed1979

Stearolyl-CoA desaturase: a control enzyme in hepatic lipogenesis.

1. Hepatocytes were isolated by perfusion of the liver with collagenase/salt solutions and incubated in culture after attachment to plastic culture dishes for periods up to 48 h. 2. The cells, when incubated in serum-free culture medium in the presence of insulin, showed enhanced stearolyl-CoA desaturase activity which was not observed when 50 muM cycloheximide was included. When insulin was omitted from the medium, the cells lost 80% of their original desaturase activity. 3. Cells isolated from animals fed 20% (w/w) sucrose for two weeks prior to sacrifice, showed high levels of fatty acid synthesis, stearolyl-CoA desaturase activity and triacylglycerol synthesis when compared with cells isolated from animals fed a corn oil supplemental diet. 4. The observations are discussed in terms of the influence of stearoyl-CoA desaturase activity on hepatic lipogenesis.

Acetates↗
PubMed1979

The control of lipogenesis by dietary linoleic acid and its influence on the deposition of fat.

The replacement of dietary starch by sucrose results in an increase in hepatic lipogenesis in the rat. When corn oil (4% by weight or 9% of the energy content of the diet) was included with the sucrose (20% by weight, 20% of the energy content) the lipogenic effect of the sucrose was completely suppressed. In contrast, when beef tallow replaced the corn oil, the induced activity caused by the sucrose was reduced by only approximately 20%. No significant differences were observed between males and females. These diets containing sucrose supplemented with either 4% (w/w) corn oil or 4% (w/w) beef tallow, were then used to ascertain whether or not the effects on hepatic lipogenesis were reflected in changes in the amount of fat deposited during growth from 4--24 weeks of age. It was shown that the percentage body fat was only statistically different (P less than 0.05) when animals fed sucrose-supplemented diets were compared with animals fed diets supplemented with sucrose and beef tallow. However, there were no significant differences in total carcass weight of these rats. The results are discussed in terms of the relative contribution of liver and adipose tissue to total lipogenesis and the factors which control the lipogenic activity in the two tissues.

Animals↗
PubMed1978

Evidence for the different responses of delta9-, delta6- and delta5-fatty acyl-CoA desaturases to cytoplasmic proteins.

Microsomes prepared from the livers of 4-week-old rats were, after extraction with 0.1 M potassium phosphate buffer, pH 7.4, unable to catalyse either the delta6 desaturation of alpha-linolenic acid (9c.12c.15c., 18 : 3) into 6c.9c.12c.15c., 18 : 4 or the delta5 desaturation of eicosatrienoic acid (8c.11c.14c., 20 : 3) into arachidonic acid (5c.8c.11c.14c., 20 : 4). Both these enzymes only showed full activity after incubation of the microsomes with either the 100 000 X g supernatant fraction or with purified bovine catalase. Bovine serum albumin, while capable of restoring 50% of the delta5 desaturase activity has no effect on the delta6 desaturase. In contrast the delta9 desaturase activity of microsomes was never completely lost after extraction with buffer but could be stimulated by optimum concentrations of both bovine serum albumin and catalase. The significance of the different responses of the three desaturases to the cytoplasmic components is discussed.

8,11,14-Eicosatrienoic Acid↗
PubMed1977

Properties of rat liver microsomal stearoyl-coenzyme A desaturase.

1. Rat liver microsomal stearoyl-CoA desaturase activity was shown to be stimulated by both bovine serum albumin and a basic cytoplasmic protein from rat liver. 2. Partially purified desaturase is unaffected by either of these two proteins. 3. Bovine serum albumin appears to exert its effect on the crude system by protecting the desaturase substrate, stearoly-CoA, from the action of endogenous thiolesterases. 4. By using partially purified enzyme preparations, it was possible to establish the substate specificity of the delta9-fatty acyl-CoA desaturase with the C14, C15, C16, C17, C18 and C19 fatty acyl-CoA substrates. Maximum enzyme activity was shown with stearoyl-CoA decreasing with both palmitoyl-CoA and nonadecanoyl-CoA, as reported previously for free fatty acids. 5. Both cytochrome b5 and NADH-cytochrome b5 reductase (EC 1.6.2.2) are required for these studies and a method is described for the purification of homogeneous preparations of detergent-isolated cytochrome b5 from rat liver. 6. From amino acid analyses, a comparison was made of the hydrophobicity of the membrane portion of cytochrome b5 with the hydrophobicity reported for stearoyl-CoA desaturase. The close resemblance of the two values suggested that unlike cytochrome b5 and its reductase, the stearoyl-CoA desaturase may be largely buried in the endoplasmic reticulum.

Amino Acids↗
PubMed1977

Interrelationship between the dietary regulation of fatty acid synthesis and the fatty acyl-CoA desaturases.

In this paper we present further evidence for the close control of fatty acid synthetase and stearoyl-CoA desaturase. Furthermore, we have established that whereas dietary palmitic acid may influence the activity of this desaturase but not of fatty acid synthetase, dietary linoleic acid appears to control both these enzymes. Finally, we have studied the influence of dietary fat and carbohydrate on the activities of the delta6 and delta5 desaturases. The former is only slightly affected by these dietary components. The delta5 desaturase activity is stimulated as the dietary fat content rises but is unaffected by dietary carbohydrate. The control of these enzymes is therefore independent of the control of fatty acid synthetase and stearoyl-CoA desaturase. From the data presented, the magnitude of the controlling effect of polyunsaturated fatty acids on fatty acid synthetase and stearoyl-CoA desaturase activity is determined and its relevance to lipogenesis in man based on daily intake of carbohydrate and linoleic acid is discussed.

Animals↗
PubMed1976

The effect of soluble rat liver proteins on the activity of microsomal stearoyl-CoA and linoleoyl-CoA desaturase.

1. The influence of bovine serum albumin and soluble rat liver proteins on the activity of rat liver microsomal delta9 and delta6 desaturases has been studied. 2. In the absence of bovine serum albumin, the delta9 desaturase which converts stearoyl-CoA into oleoyl-CoA, shows a non-linear correlation between enzyme activity and protein concentration. 3. Optimum concentrations of bovine serum albumin have three main effects on the enzyme activity: (i) establishes a linear relationship between enzyme activity and protein concentration, (ii) stimulates the enzyme activity 2--3-fold and (iii) raises the optimum substrate concentration from 10 to 100 muM. 4. A highly purified soluble liver protein of molecular weight 24 000 also stimulated the enzyme activity and brought about a linear relationship between enzyme activity and protein concentration. 5. It was concluded that the non-linear kinetics were due to limiting amounts of substrate binding protein in the microsomal preparations. 6. The delta6 desaturase which converts linoleoyl-CoA into gamma-linolenoyl-CoA was also stimulated by bovine serum albumin and soluble liver proteins. 7. The significance of the fatty acid-binding proteins is discussed.

Animals↗
PubMed1975

Factors effecting the solubilisation of stearoyl-coA desaturase of hen liver microsomes.

1. The lipid requirement for maximum desaturase activity was investigated using acetone/water mixtures. It was shown that for maximum stearoyl-CoA desaturase activity of hen liver microsomes neither the total neutral lipid fraction nor 44% of the phospholipid fraction were required. 2. The effect of sodium deoxycholate, Triton X-100, Nonidet P-40 and Bio-solv on the enzyme activity indicated that the neutral detergents had a milder effect than the ionic detergent but both classes could cause considerable irreversible loss of activity. 3. The treatment of the microsomes with 2.5% (v/v) water in acetone greatly improved the effective solubilising power of Triton X-100. The yield of desaturase in the 100 000 X g supernatant obtained by treating the microsomal fraction in this way was strongly dependent upon protein concentration. Maximum solubilisation was achieved with25 mg protein per ml 1% (w/v) Triton X-100 in 0.1 M potassium phosphate buffer pH 7.4. 4. A comparison of the properties of the solubilised and membrane-bound enzyme was made by an investigation of: (i) the temperature and pH optimum, (ii) activation energy and (iii) the effect of inhibitors on the enzyme activity.

4-Chloromercuribenzenesulfonate↗
PubMed1975

Metabolism of trans-3-hexadecenoic acid in broad bean.

1. Broad bean (Vicia faba) leaves contain rather high concentrations (about 4% of total fatty acids) of the trans-3-hexadecenoic acid. 2. Amounts of the acid increase with the age of the leaves and are absent from etiolated tissue. 3. Changes in the levels of trans-delta-4-hexadecenoic acid can be produced by subjecting the intact plants to various light/dark periods. 4. Chloroplasts isolated from broad-bean leaves show high rates of fatty acid synthesis from [1-14C]acetate. Synthesis is dependent on coenzyme A and ATP but is insensitive to the addition of exogenous acyl carrier protein. 5. The pattern of acids made includes about 20% palmitic, 5% hexadeconoic, 10% stearic and 60% oleic. trans-3-Hexadecenoic acid synthesis was most active in chloroplasts from plants exposed to the dark for 5 days and light for 3 days. 6. Arsenite addition inhibited stearate formation by isolated chloroplasts but resulted in a two-fold stimulation of overall synthesis. 7. The rate of fatty acid synthesis by isolated chloroplasts paralleled the changes in endogenous trans-3-hexadecenoic acid levels in the leaves from which they were isolated.

Acetates↗