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Biomedical subjects

A Minn

Publications and source records attributed to A Minn.

At least 55 records · Page 3Linked to original sources
PubMed1989

NADPH:cytochrome P-450(c) reductase: biochemical characterization in rat brain and cultured neurons and evolution of activity during development.

NADPH:cytochrome P-450 (c) reductase is a microsomal enzyme which is involved in the cytochrome P-450-dependent biotransformation of many exogenous agents as well as of some endogenous molecules. Using cytochrome c as a substrate, the kinetic parameters of this enzyme were determined in brain microsomes. The comparison of the NADPH:cytochrome P-450 reductase's Vmax values and cytochrome P-450 contents in both fractions, suggests a role of cerebral NADPH:cytochrome P-450 reductase in cytochrome P-450 independent pathways. This is also supported by the different developmental pattern of brain enzyme as compared to the liver enzyme, and by the presence of a relatively high NADPH:cytochrome P-450 reductase activity in immature rat brain and neuronal cultures, while cytochrome P-450 was hardly detectable in these preparations. The enzyme activity was not induced by a phenobarbital chronic treatment neither in the adult brain nor in cultured neurons, suggesting a different regulation of the brain enzyme expression.

Animals↗
PubMed1988

A new aspect of the protective functions of the blood-brain barrier: activities of four drug-metabolizing enzymes in isolated rat brain microvessels.

The multiple functions of the blood-brain barrier (BBB) consist mostly in membrane properties controlling the bidirectional exchange of molecules between the general circulation and the central nervous system. As lipophilic molecules are able to easily penetrate the membrane of brain microvessels endothelial cells, an Achilles' heel exists in the brain's protective system. We measured the activities of some enzymes involved in the metabolism of lipophilic xenobiotics, i.e. cytochrome P-450-linked monooxygenases, epoxide hydrolase, NADPH:cytochrome P-450 reductase and 1-naphthol UDP-glucuronosyl transferase in isolated rat brain microvessels. The relatively high activities observed indicate the capacity of endothelial cells to metabolize xenobiotics, and, thus, to give an additional protection to the brain.

Animals↗
PubMed1988

Induction of plasma and tissue enzymes by drugs: significance in toxicological studies.

1. Observations of drug metabolism enzyme induction in animals during early toxicological trials, and phase I or II of clinical trials, are of importance for the development of a new chemical of pharmacological interest. 2. Induction can be detected by determination of enzyme activities or enzyme proteins in tissues (subcellular fractions or cells in culture). 3. Indirect methods for checking induction can involve determinations of endogenous (i.e. glucaric acid, 6-beta-hydroxycortisol) or xenobiotic (antipyrine) metabolites which are produced by the inducible enzyme systems. 4. Recent progress in the knowledge of biochemical mechanisms of induction and the large number of identified isoenzymes in each enzyme 'family' have complicated the interpretation in pharmaco-toxicological studies. 5. This paper describes the present state of the art concerning the relationships between the isoenzymes and the major classes of inducers concerning cytochromes P-450, UDP-glucuronosyltransferases, gamma-glutamyltransferase and epoxide hydrolases.

Animals↗
PubMed1987

Brain mitochondrial cytochrome P-450scc: spectral and catalytic properties.

The cytochrome P-450-dependent cholesterol side chain cleavage system of the brain has been studied using nonsynaptic mitochondria as the source of enzymatic activity. The system has been found to bind cholesterol and 11-deoxycorticosterone, producing type I difference spectra, whereas the binding of pregnenolone induced a reverse type I difference spectrum. Inhibitors of cytochrome P-450-linked monooxygenase activities produced type II spectra. The formation of labeled pregnenolone after incubation of brain mitochondria with [4-14C]cholesterol has been obtained, and this formation was inhibited by glutethimide, a specific inhibitor of cytochrome P-450scc. The functional significance of this enzymatic activity is discussed.

Aminoglutethimide↗
PubMed1987

Ethoxyresorufin O-deethylase activity in rat brain subcellular fractions.

Non-polar, lipid-soluble drugs or xenobiotics are able to cross the blood-brain barrier. The brain cytochrome P-450-dependent monooxygenases may oxidize these molecules to more polar and somewhat hazardous metabolites responsible for neurotoxicity. In order to characterize the cytochrome P-450 dependent aryl hydrocarbon hydroxylase activity in brain subcellular fractions, we used 7-ethoxyresorufin as a substrate, as its O-deethylation reflects specifically the activity of the cytochrome P-450 isoform which metabolizes and is induced by polycyclic aromatic hydrocarbons. The results reported here show that this enzymatic activity occurs in both microsomal and mitochondrial fractions, and that the induction after 3-methylcholanthrene treatment remains limited, thus preventing the formation of high levels of harmful metabolites.

Animals↗
PubMed1987

Changes of cerebral gamma glutamyltransferase activities after treatment with exogenous inducers.

The activity of gamma-glutamyltransferase localized in isolated brain synaptic membranes- and microvessels-enriched fractions was assayed after treatment of rats with either phenobarbital or ethanol. Phenobarbital increased the activity of gamma-glutamyltransferase in microvessels, without alteration of synaptic membranes activity. An increase of enzyme activity was also obtained after a chronic intoxication with ethanol. These results suggest that the isoform of gamma-glutamyltransferase localized in brain microvessels may respond to exogenous inducers.

Animals↗
PubMed1987

The activity of 1-naphthol-UDP-glucuronosyltransferase in the brain.

Cerebral microsomes catalysed efficiently the glucuronidation of 1-naphthol, this formation of glucuronide being activated by treatment with Triton X-100 or digitonin. Activated microsomes from the brain of the rat conjugated 1-naphthol with an apparent Km of 95 microM and a Vmax of 5.47 nmol/hr mg protein at 30 degrees C. Microsomal uridine diphosphate (UDP)-glucuronosyltransferase activity in brain towards 1-naphthol was not significantly induced by pretreatment of animals with 3-methylcholanthrene or phenobarbital. These data suggest that UDP-glucuronosyltransferases in brain are different from the hepatic enzymes with regard to biochemical parameters and in response to inducers of drug metabolism. The hepatic UDP-glucuronosyltransferase deficiency in Gunn rats was also observed in the brain.

Animals↗
PubMed1987

Quantitative measurement of cerebral cytochrome P-450 by second derivative spectrophotometry.

Second order derivative spectrophotometry was used to improve the accuracy and reproducibility of cytochrome P-450 measurements in subcellular fractions obtained from the brain. This method allowed better resolution of the overlapping bands of cytochrome P-450 and other iron proteins, as well as a reduction of the effects of turbidity and non-specific components. Using this method, the cytochrome P-450 content of rat brain mitochondrial and microsomal fractions was measured to be 74.2 +/- 2.3 and 5.9 +/- 0.3 pmol/mg protein, respectively.

Animals↗
PubMed1986

Subcellular distribution of cytochrome P-450 in the brain.

The subcellular distribution of the monooxygenase complexes in the brain was studied by using subcellular fractionation and characterization of these fractions by marker enzymes. Cytochrome P-450 was found to be mainly localized in both synaptic and non-synaptic mitochondria; only a small quantity of enzyme was also found in the microsomal fraction. Peeling off the outer membrane of mitochondria showed that the protein was retained in the inner membrane fraction. A comparative study among some other species confirmed the mitochondrial prevalence of cerebral cytochrome P-450. A partial purification of the rat brain mitochondrial P-450 was obtained.

Animals↗
PubMed1986

Cimetidine inhibits cerebral and hepatic mitochondrial respiration in rat.

The presence of cimetidine in the incubation medium of rat brain mitochondria caused decreased oxygen uptake, especially during oxidative phosphorylation (state 3). This inhibition of the respiratory control and of ATP synthesis was dose-dependent. The same observations were made for hepatic mitochondria. The significance of these results is discussed in terms of both the neurological side-effects of cimetidine and its effect on regulatory mechanisms of cerebral or hepatic blood flow.

Animals↗
PubMed1983

Uptake of L-glutamine into synaptosomes. Is the gamma-glutamyl cycle involved?

Transport of L-glutamine into rat cortical synaptosomes has been investigated by (14C)L-glutamine uptake experiments. This amino acid enters synaptosomes both by an active carrier mediated system, which may be the result of gamma-glutamyl cycle activity and by a Na+-dependent transport system. This view is supported by the following observations: a) as demonstrated previously (10), glutamine inside synaptosomes reaches concentrations higher than those of the incubation medium, and initial rates of uptake approach saturation kinetics; b) the uptake of glutamine is inhibited by uncouplers; c) the uptake is inhibited by methionine sulfoximine, a suicide-inhibitor of an enzyme of the gamma-glutamyl cycle; d) the initial rate of uptake is lowered by decreasing the Na+-level of the incubation medium or by adding ouabain. The validity of this hypothesis is discussed.

Amino Acids↗
PubMed1981

[L-Glutamine transport into synaptosomes isolated from rat brain].

Using a rapid centrifugation-stop procedure, intrasynaptosomal transport of L-glutamine has been measured in conditions allowing the determination of initial rates of this transport. The transport followed saturation kinetics (Km = 0.52 mM; Vmax = 78.24 nmol/min. mg protein), was inhibited at low incubation temperature values and needed a metabolic energy supply. These results demonstrate that L-glutamine is probably transported into synaptic endings via a carrier-mediated mechanism.

Animals↗
PubMed1981

Adenosine transport into guinea-pig synaptosomes.

Kinetics for transport of adenosine into guinea-pig neocortex synaptosomes were studied by incubating them with [14C]adenosine for up to 30 s. The apparent Km value of the high-affinity transport system for adenosine was 21.1 microM and the Vmax value was 257.3 pmol/min/mg protein. The transport system was inhibited by both compounds structurally related (compounds 554 and 555) and unrelated (dipyridamole) to adenosine. Because electrically stimulated synaptosomes release up to 1.5% of the adenosine derivative content per min, the physiological significance of adenosine uptake is discussed as a possible mechanism to compensate for the loss of adenine nucleotides from synaptosomal preparations.

Adenine Nucleotides↗