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X X Ding

Publications and source records attributed to X X Ding.

16 recordsLinked to original sources
PubMed2006

High-aspect-ratio aluminum borate nanowire bundles supported by sucrose.

High-aspect-ratio and single-crystal aluminum borate (Al(18)B(4)O(33)) nanowire bundles with an ordered orientation were synthesized by using an innovative sucrose-assisted growth process. The process involves the dehydration and polycondensation of aluminum borate-sucrose solution to form a highly viscous precursor. The sucrose plays a crucial role in the growth of the nanowire bundles by supporting as a polymeric substrate and a type of adhesive template. Electron microscopy was used to characterize the high-aspect-ratio nanowire bundles. A possible growth mechanism for the nanowire bundles is proposed.

Journal Article↗
PubMed2005

From Al4B2O9 nanowires to BN-coated Al18B4O33 nanowires.

A novel route was proposed to completely coat aluminum borate nanowires by in situ providing the precursor for BN coating. Uniformly BN-coated Al18B4O33 nanowires could be obtained by the reaction of Al4B2O9 nanowires with ammonia at high temperature. The high-temperature unstable Al4B2O9 nanowires were converted into Al18B4O33 nanowires, simultaneously evaporated boron oxide. The reaction between the in situ generated vapors and ammonia ensures that the BN layers are attached tightly on the surface of the as-formed Al18B4O33 nanowires.

Letter↗
PubMed2005

Metal oxide coating on carbon nanotubes by a methanol-thermal method.

A methanol-thermal method has been developed to fabricate one-dimensional composite nanowires by coating multiwalled carbon nanotubes with lanthanum oxide and iron oxide, respectively. The coating structure and composition have been investigated by transmission electron microscopy and the X-ray energy dispersive spectrum. Magnetic measurement for the iron oxide coatings indicated that the coercivity has been enhanced after coating. The method reported here provides a novel procedure for the fabrication of one-dimensional composite nanostructures.

Crystallization↗
PubMed2004

MgO nanowire growth from Mg metal and SiO2.

Comparative study on the diameter distribution of MgO nanowires has been carried out. MgO nanowires could be synthesized by the direct reaction between metallic magnesium and silica, and the obtained nanowires have diameters ranging from 50 to 200 nm and lengths of several hundreds nanometers, exhibiting a straight wire. The diameter can be downscaled to smaller than 50 nm, and the nanowire exhibits a curved and twisted one-dimensional structure with lengths up to several micrometers, when a fine support catalyst was used as the reactant. The diameter-controlled growth mechanism was also explained in this work.

Crystallization↗
PubMed1992

Cytochrome P450: progress and predictions.

The cytochrome P450 gene superfamily encodes many isoforms that are unusual in the variety of chemical reactions catalyzed and the number of substrates attacked. The latter include physiologically important substances such as steroids, eicosanoids, fatty acids, lipid hydroperoxides, retinoids, and other lipid metabolites, and xenobiotics such as drugs, alcohols, procarcinogens, antioxidants, organic solvents, anesthetics, dyes, pesticides, odorants, and flavorants. Accordingly, it is not surprising that these catalysts have come under intensive study in recent years in fields as diverse as biochemistry and molecular biology, endocrinology, pharmacology, toxicology, anesthesiology, nutrition, pathology, and oncology. In this review, recent advances in our knowledge of the catalytic properties, reaction mechanisms, and regulation of expression and activity of the P450 enzymes are briefly summarized. In addition, the prospects for research in this field are considered, and advances are predicted in four broad areas: improved basic knowledge of enzyme catalysis and regulation; synthesis of fine chemicals, including drug design and screening; removal of undesirable environmental chemicals; and biomedical applications related to steroid, drug, carcinogen, and alcohol metabolism.

Animals↗
PubMed1991

cDNA and derived amino acid sequence of rabbit nasal cytochrome P450NMb (P450IIG1), a unique isozyme possibly involved in olfaction.

Olfactory-specific cytochrome P450NMb was previously purified to electrophoretic homogeneity from microsomes of rabbit nasal mucosa in this laboratory. In the present study, a cDNA library made from poly(A)+ RNA from rabbit nasal mucosa was screened with antibodies to this P450, and eight immunopositive clones were isolated and characterized. The sequence determined from two overlapping clones contained an open reading frame of 1446 nucleotides, with the predicted first 39 amino acids corresponding to residues 12 to 50 of purified NMb, except for position 46, where Leu was encoded instead of the Glu residue that was found earlier by Edman degradation analysis. The complete polypeptide, including residues 1 to 11, contains 494 amino acid residues and has a molecular weight of 56,640. Sequence comparisons indicated that NMb is more than 50% identical to members of the rabbit P450 gene II family, including IIB4, IIC3, IIC5, IIE1, and IIE2, and 83% identical to rat P450olf1 (IIG1). Hybridization of NMb to electrophoretically fractionated rabbit nasal poly(A)+ RNA revealed 3.6- and 2.1-kb species, but with a probe derived from the 3'-nontranslated portion of the cDNA only the 3.6-kb band was observed, suggesting the use of alternate polyadenylation sites or splicing. In agreement with the known tissue-specific distribution of NMb protein, NMb transcripts were found in olfactory mucosa, but not in liver, lung, intestine, or kidney. Genomic hybridization analysis indicated that there may be only one copy of the NMb gene present in the rabbit genome.

Amino Acid Sequence↗
PubMed1991

Purification and characterization of cytochrome P450 2E2 from hepatic microsomes of neonatal rabbits.

The alcohol-inducible P450 2E subfamily in the rabbit has two known members that differ in only 16 amino acid residues scattered throughout the polypeptide chain. P450 2E1 has been thoroughly characterized, and is known to have diverse inducers and substrates. Little is known, however, about the properties of P450 2E2, since efforts to isolate this isozyme from adult rabbits have been unsuccessful. In the present study, 2E2 was purified to electrophoretic homogeneity from liver microsomes of neonatal rabbits with the use of 4-methylpyrazole as a stabilizing agent. The purified cytochrome was identified as 2E2 by NH2-terminal amino acid sequence analysis as well as by immunoblot analysis with three different antibodies to 2E1. Purified 2E2, in contrast to 2E1, is predominantly low-spin in the presence of 20% glycerol, but is in a mixed high- and low-spin state as the concentration of glycerol is decreased. The catalytic properties of purified 2E1 and 2E2 were compared in the reconstituted system with a variety of substrates, including alcohols, ethers, nitrosamines, and aromatic compounds. Differences between the two enzymes in catalytic activity and in the interaction with cytochrome b5 were observed with some but not all of the substrates tested. Purified 2E1 and 2E2 both consume molecular oxygen relatively rapidly during NADPH oxidation in the absence of an added substrate, and stoichiometric determinations indicated that only about 20% of the O2 was reduced to H2O2, with the remainder apparently undergoing four-electron reduction to water.

Amino Acid Sequence↗
PubMed1991

Differences in the developmental expression of rabbit cytochromes P-450 2E1 and 2E2.

The alcohol-inducible CYP2E subfamily in rabbits contains two genes; CYP2E1 encodes the cytochrome earlier termed P-450 3a, and CYP2E2 encodes a cytochrome that is 97% identical in amino acid sequence to cytochrome P-450 (P-450) 2E1. In the present studies, the ontogenic expression of these two cytochromes was examined. In liver, P-450 2E2 mRNA is detectable immediately after birth and reaches slightly greater than the adult level at 2 weeks of age; in contrast, P-450 2E1 mRNA is not detectable until day 14 and increases rapidly to approximately twice the adult level at 5 weeks of age. P-450 2E protein is present in liver immediately after birth, coincident with the appearance of P-450 2E2 mRNA, peaks at 2 weeks, and then, despite the continued elevation in P-450 2E mRNA, decreases to the adult level at 5 weeks. In kidney, P-450 2E2 mRNA is not detectable at any age; P-450 2E1 mRNA, however, is present at 1 week, and the level increases to about half the adult level at 5 weeks of age. P-450 2E protein in this tissue is elevated at 2 weeks, relative to mRNA levels, and reaches approximately half the adult level at 5 weeks. The lack of close correlation between mRNA and protein levels in the liver and kidney of newborn rabbits indicates that the posttranscriptional control of P-450 2E enzyme levels that predominates in adult animals is also operative during the neonatal period. Monooxygenase activities with ethanol and p-nitrophenol as substrates reflect the developmental increase in P-450 2E protein, as well as the appearance and levels of spectrally detectable P-450, cytochrome b5, and NADPH-P-450 reductase in hepatic microsomes. The expression of P-450 2E2, but not P-450 2E1, in early neonates suggests that these two closely related cytochromes may have functional differences that are important during the first few weeks of life.

Aging↗
PubMed1990

Rabbit nasal cytochrome P-450 NMa has high activity as a nicotine oxidase.

Rabbit nasal olfactory and respiratory microsomes demonstrate high activity toward [3H]-(S)-nicotine, with specific activities of 22.2 and 6.5 nmol/min/mg protein, respectively. The major metabolite produced is (S)-nicotine delta 1'; 5'-iminium ion, with lesser amounts of nornicotine and the N'-oxide. Reconstitution of the rabbit nasal microsomal system with cytochromes P-450 NMa and NMb indicated that only P-450 NMa has significant activity toward nicotine, and the metabolite profile and turnover are similar to that observed with nasal microsomes. The low Km (35 microMs) and high Vmax (28 min-1) suggest that a significant portion of inhaled nicotine is metabolized by nasal tissues in the rabbit.

Animals↗
PubMed1990

Immunochemical characterization of multiple forms of cytochrome P-450 in rabbit nasal microsomes and evidence for tissue-specific expression of P-450s NMa and NMb.

Two unique forms of cytochrome P-450 (P-450), designated NMa and NMb, were recently isolated in this laboratory from nasal microsomes of rabbits. In the present study, polyclonal antibodies to the purified nasal cytochromes were prepared. Immunochemical analysis with specific rabbit anti-NMa and sheep anti-NMb antibodies indicated that P-450 isozymes identical to or having a high structural homology with NMa are present in both olfactory and respiratory mucosa, as well as in liver, but NMb was detected only in the olfactory mucosa. Neither form was detected in other tissues examined, including brain, esophageal mucosa, heart, intestinal mucosa, kidney, and lung. The specific occurrence of NMb in the olfactory mucosa was further substantiated by the detection and specific inhibition by anti-NMb of the formation of unique NMb-dependent metabolites of testosterone in olfactory microsomes but not in microsomes from liver or respiratory mucosa. Similar experiments with antibodies to previously purified rabbit hepatic P-450 isozymes indicated that not all of the hepatic cytochromes are expressed in the nasal tissues. Thus, P-450 isozymes structurally homologous to hepatic forms 2, 3a, and 4, but not 3b and 6, were found in the olfactory mucosa. On the other hand, only form 2 was detected in the respiratory mucosa. Immunoquantitation experiments revealed that NMa and NMb are the major P-450 forms in olfactory microsomes, whereas NMa and P-450 form 2 (or its homolog) constitute the major portion of the respiratory nasal microsomal P-450. The level of NMa in the liver is relatively low, accounting for less than 3% of total microsomal P-450 in this tissue. In addition, evidence is provided that NMa is the major catalyst in the dealkylation of two nasal carcinogens, hexamethylphosphoramide and phenacetin, in both olfactory and respiratory nasal microsomes.

Animals↗
PubMed1988

Purification and characterization of two unique forms of cytochrome P-450 from rabbit nasal microsomes.

Two forms of cytochrome P-450, designated P-450NMa and P-450NMb, were purified to electrophoretic homogeneity from rabbit nasal microsomes. The purified cytochromes, which contained 14-16 nmol of P-450/mg of protein, exhibited apparent monomeric molecular weights of 49,500 and 51,000, respectively. As indicated by several criteria, including the amino acid composition, absorption spectra, and peptide maps, the two nasal forms of P-450 are distinct from each other. Furthermore, as judged by the NH2-terminal amino acid sequences, they are distinct from all other P-450 cytochromes described to date. In the ferric form, P-450NMa is in the low-spin state, whereas P-450NMb is predominantly in the high-spin state. When reconstituted with NADPH-cytochrome P-450 reductase and phospholipid, P-450NMa is very active in the oxidation of ethanol as well as several nasal procarcinogens, including the N-deethylation of N-nitrosodiethylamine, the O-deethylation of phenacetin, and the N-demethylation of hexamethyl-phosphoramide. P-450NMb also metabolizes these substrates, but at lower rates. Both nasal forms are also active with testosterone, with P-450NMa oxidizing the substrate in the 17-position to give androstenedione and P-450NMb catalyzing hydroxylation in the 15 alpha-, 16 alpha-, and 19-positions. The two cytochromes represent the major portion of the total P-450 in nasal microsomes, but the corresponding forms could not be detected in hepatic microsomes.

Amino Acid Sequence↗
PubMed1986

Immunochemical identification of cytochrome P-450 isozyme 3a (P-450ALC) in rabbit nasal and kidney microsomes and evidence for differential induction by alcohol.

Polyclonal and monoclonal antibodies to rabbit liver microsomal alcohol-inducible cytochrome P-450 isozyme 3a (P-450ALC) were used to examine the tissue distribution of the cytochrome. Isozyme 3a or an immunochemically indistinguishable variant of this protein was detected on immunoblots of kidney and nasal mucosa microsomes, but not of microsomes prepared from brain, lung, adrenal, heart, intestine, ovary, spleen, testis, or uterus from untreated or ethanol-treated rabbits. The presence of isozyme 3a was also indicated by inhibition by anti-3a IgG of microsomal aniline hydroxylation and butanol oxidation. The identity of isozyme 3a was further substantiated by peptide-mapping analysis of the immunoaffinity-purified proteins. The amount of isozyme 3a was increased in kidney, but not in nasal microsomes, by chronic ethanol treatment. The induction of isozyme 3a in the kidney was reflected in a more than 2-fold increase in the total rates and a 7-fold increase in the isozyme 3a-dependent rates of aniline and butanol metabolism. Based on immunoblot quantitation, the specific content of isozyme 3a is about 10 pmol/mg of protein in kidney and 80 pmol/mg of protein in nasal microsomes of untreated rabbits. After ethanol treatment of the animals, the content increases to 50 pmol/mg of protein in kidney but is unchanged in nasal microsomes. The presence of isozyme 3a may play a significant role in the toxicity of foreign compounds.

Animals↗
PubMedDate not supplied

Induction of cytochrome P-450 isozyme 3a (P-450IIE1) in rabbit olfactory mucosa by ethanol and acetone.

Cytochrome P-450 isozyme 3a, the alcohol-inducible form of cytochrome P-450 (P-450IIE1), was previously identified in rabbit nasal microsomes with the use of immunochemical techniques; the occurrence of this cytochrome in the nasal mucosa was subsequently confirmed through RNA hybridization experiments. However, in contrast to the well established inducibility of isozyme 3a in liver and kidney by alcohol treatment of the animals, no induction was observed in the nasal tissue with the use of a polyclonal anti-3a antibody for immunochemical quantitation. Recently, two new P-450 isozymes, designated NMa and NMb, were identified in rabbit nasal microsomes, and were found to have overlapping substrate specificity with isozyme 3a. Moreover, the two new cytochromes cross-react with the polyclonal anti-3a antibody that was used in the earlier study for quantitation of nasal isozyme 3a. These recent findings invalidate our previous conclusion that isozyme 3a is not induced by ethanol treatment of rabbits. In the present study, immunoblot quantitation of isozyme 3a was performed with a monoclonal anti-3a antibody that does not recognize either NMa or NMb, and the nasal microsomal metabolism of butanol was examined at various substrate concentrations. We have found that the level of isozyme 3a protein in nasal mucosa is elevated about 2-fold after treatment of the animals with ethanol and about 6-fold after treatment with acetone. Furthermore, corresponding increases in the rate of microsomal butanol oxidation were observed at low substrate concentrations. Thus, we conclude that P-450 isozyme 3a is, in fact, inducible in the nasal tissues by ethanol or acetone treatment of rabbits.

Acetone↗
PubMedDate not supplied

Cytochrome P-450-dependent formation of ethylene from N-nitrosoethylamines.

Ethylene was identified as a metabolite of N-nitrosodiethylamine (NDEA) as well as of N-nitrosomethylethylamine and N-nitrosoethylbutylamine. The formation of ethylene from these carcinogenic compounds is cytochrome P-450- and NADPH-dependent. With NDEA, the rate of the reaction in rabbit liver microsomes is increased by treatment of the animals with phenobarbital or ethanol, which are known to induce the synthesis of P-450 form 2 and P-450 form 3a (P-450ALC), respectively, and the activity is inhibited by anti-P-450 form 2 or form 3a IgG. The rate of ethylene formation is about 10-fold higher in nasal microsomes than in hepatic microsomes. In a reconstituted system, P-450 form 3a is the most active in ethylene formation from NDEA, followed by forms 2 and 4. It has been proposed by others that the alpha-hydroxylation of N-nitrosodialkylamines leads to the formation of an aldehyde and an alkyl diazonium ion, which, with the loss of N2, yields an alkyl carbonium ion capable of producing the corresponding alcohol or binding macromolecules through electrophilic reactions. In the case of the N-nitrosoalkylethylamines we have studied, the ethyl carbonium ion produced could, in addition, generate ethylene by proton elimination.

Animals↗