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C R Pullinger

Publications and source records attributed to C R Pullinger.

47 records · Page 3Linked to original sources
PubMed1983

The relationship between the rate of hepatic sterol synthesis and the incorporation of [3H]water.

The true rate of sterol synthesis in liver cells was determined by measurement of the weight of desmosterol produced over a given time period during incubations in the presence of triparanol. The simultaneous presence of tritiated water (3H2O) during the incubations permitted a direct observation of the weight of tritium incorporated into a given mass of newly synthesized sterol. The incorporation of tritium per atom of sterol carbon (H/C ratio) was lower than some previously reported values and suggests that a sizeable proportion of the reducing equivalents (NADPH) required for sterol synthesis arises via the pentose phosphate pathway. The H/C ratio changed significantly with length of the incubation period. The value of the ratio was also dependent upon whether the acetyl-CoA units utilized for sterol synthesis were derived predominantly from a carbohydrate or a fatty acid source.

Animals↗
PubMed1982

Effects of oleate and compactin on the metabolism and secretion of cholesterol and cholesteryl ester by rat hepatocytes.

Incubation of rat hepatocytes with oleate for a period of 1 h gave rise to a decrease in the total (esterified plus unesterified) cholesterol associated with very-low-density lipoprotein (VLDL). This effect was no longer apparent after longer incubation periods. The rate of cholesterol biosynthesis decreased during the first hour of incubation in the presence of oleate. After longer incubation periods, however, more cholesterol was synthesised in the presence of oleate than in its absence. The extracellular presence of oleate gave rise to a 2-fold increase in the concentration of cellular cholesteryl ester. Under these conditions cholesteryl ester contributed a larger proportion of the total cholesterol secreted with the VLDL. The cholesteryl ester associated with VLDL was derived predominantly from cholesteryl ester synthesised intracellularly. Inhibition of cholesterol synthesis with compactin did not significantly alter the rate of secretion of VLDL-cholesterol. Newly synthesised non-esterified cholesterol equilibrated with the bulk of pre-existing cellular cholesterol before secretion with the VLDL. This was true irrespective of the rate of endogenous cholesterol synthesis.

Animals↗
PubMed1980

Metabolism of hydroxy sterols by rat liver.

5 alpha-[16-3H]lanost-8-ene-3 beta,15 alpha-diol and 5 alpha-[16-3H]lanost-8-ene-3 beta,15 beta-diol were both extensively metabolised by rat liver enzymes in vitro. Quantitatively, the most important product in both cases was a more polar compound, tentatively identified as a 5 alpha-lanost-8-enetriol. In addition, 5 alpha-[16-3H]lanost-8-ene-3 beta,15 beta-diol gave rise to the corresponding 3 beta,15beta-diol diester, whilst with 5 alpha-[16-3H]lanost-8-ene-3 beta,15 alpha-diol only the 3 beta-hydroxyl group was esterified. The enzymes involved may normally be responsible for metabolising spontaneously produced non-enzymic oxidation products of dietary or cellular cholesterol. High concentrations of 5 alpha-[16-3H]lanost-8-ene-3 beta,15 beta-diol stimulated ester formation. With both substrates, carbon monoxide inhibited formation of the polar sterol metabolite but stimulated ester formation. Under all conditions, cholesterol was a relatively minor metabolic product of either of the 5 alpha-lanost-8-ene-3 beta,15-diols.

Animals↗
PubMed1979

Studies on the mechanism of lanosterol 14 alpha-demethylation. A requirement for two distinct types of mixed-function-oxidase systems.

Carbon monoxide inhibited the removal of C-32 of dihydrolanosterol (I), but not of its metabolites 5 alpha-lanost-8-ene-3 beta,32-diol (II) and 3 beta-hydroxy-5 alpha-lanost-8-en-32-al (III). It appears therefore that cytochrome P-450 is a component of the enzyme system required to initiate oxidation of the 14 alpha-methyl group, but not of that responsible for the subsequent oxidation steps required for elimination of C-32 as formic acid. Non-radioactive compounds (II) and (III), when added to cell-free systems actively converting dihydrolanosterol into cholesterol, inhibited 14 alpha-demethylation measured by the rate of formation of labelled cholesterol from dihydro[1,7,15,22,26,30-14C]lanosterol or of labelled formic acid from dihydro[32-14C]lanosterol. However, neither compound (II) nor compound (III) accumulated radioactive label under these conditions. These observations could be attributed partly to inhibition of the initial oxidation of the 14 alpha-methyl group by compounds (II) and (III).

Animals↗
PubMed1979

Utilization of endogenous and exogenous sources of substrate for cholesterol biosynthesis by isolated hepatocytes.

The rates of cholesterol biosynthesis in isolated rat hepatocytes were determined by using a method based on measurement of the rate of formation of desmosterol (cholesta-5,24-dien-3beta-ol), which accumulates during inhibition of cholesterogenesis by the drug triparanol. Incubation of cells from normal or 24h-starved animals in a medium containing albumin, glucose, amino acids and acetate as the only organic constituents led to an accelerating rate of sterol formation during the earlier stages of a 6h incubation period. The contribution of exogenously added acetate (initial concentration 3.34mm) to sterol synthesis in both types of cells reached an early maximum and then continually declined. Exogenously added pyruvate and lactate were more efficient sources of sterol carbon than was acetate. Exogenous glucose even at relatively high concentrations (11.1mm) was incapable of providing more than 6% of the total sterol carbon. Although the proportion of total sterol carbon supplied from exogenous acetate increased with increasing concentrations of the extracellular substrate, the rates of total sterol synthesis in both types of cell remained unchanged. Similar observations were made when lactate or pyruvate was the cholesterogenic precursor in normal cells. These studies suggest that, although exogenous substrates were capable of expanding an intracellular pool of cholesterol precursor, the normal supply of intermediary metabolites was not rate-limiting for cholesterogenesis.

Acetates↗
PubMed1977

Measurement of the absolute rates of cholesterol biosynthesis in isolated rat liver cells.

Triparanol [2-(4-chlorophenyl)-1-(4-diethylaminoethoxyphenyl)-1-p-tolylethanol] at a concentration of 2 micronm has no effect on the overall conversion of [2=14C]acetate into C27 sterols by isolated liver cells. In the presence of triparanol, however, the formation of radioactive cholesterol is inhibited by 85-90% and the balance of radioactivity appears in the C27 sterol desmosterol (cholesta-5,24-dien-3beta-ol). The very small weights of desmosterol which accumulate under these conditions were, as a routine, quantitatively converted into the heptafluorobutyrate 3-enol ester of cholesta-4,24-dien-3-one. This derivative has a high electron-capturing capability, a property that enables extremely small quantities (less than 0.25pmol) of the material to be accurately measured by gas chromatography with electron-capture detection. Measurements of the mass and specific radioactivity of the newly biosynthesized desmosterol formed in the presence of triparanol provides an accurate assessment of the amount of cholesterol that would be synthesized by the liver cells in the absence of the drug.

Acetates↗