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T C Jenkins

Publications and source records attributed to T C Jenkins.

At least 37 records · Page 2Linked to original sources
PubMed2001

Direct methylation procedure for converting fatty amides to fatty acid methyl esters in feed and digesta samples.

Two direct methylation procedures often used for the analysis of total fatty acids in biological samples were evaluated for their application to samples containing fatty amides. Methylation of 5 mg of oleamide (cis-9-octadecenamide) in a one-step (methanolic HCl for 2 h at 70 degrees C) or a two-step (sodium methoxide for 10 min at 50 degrees C followed by methanolic HCl for 10 min at 80 degrees C) procedure gave 59 and 16% conversions of oleamide to oleic acid, respectively. Oleic acid recovery from oleamide was increased to 100% when the incubation in methanolic HCl was lengthened to 16 h and increased to 103% when the incubation in methoxide was modified to 24 h at 100 degrees C. However, conversion of oleamide to oleic acid in an animal feed sample was incomplete for the modified (24 h) two-step procedure but complete for the modified (16 h) one-step procedure. Unsaturated fatty amides in feed and digesta samples can be converted to fatty acid methyl esters by incubation in methanolic HCl if the time of exposure to the acid catalyst is extended from 2 to 16 h.

Animal Feed↗
PubMed2001

Telomerase inhibitors for the treatment of cancer: the current perspective.

Telomerase is a holoenzyme responsible for the maintenance of telomeres, the protein-nucleic acid complexes at the ends of eukaryotic chromosomes that serve to maintain chromosomal stability and integrity. Telomerase activity is essential for the sustained proliferation of most immortal cells, including cancer cells. Since the discovery that telomerase activity is detected in 85-90% of all human tumours and tumour-derived cell lines but not in most normal somatic cells, telomerase has become the focus of much attention as a novel and potentially highly-specific target for the development of new anticancer chemotherapeutics. Herein we review the current perspective for the development of telomerase inhibitors as cancer chemotherapeutics. These include antisense strategies, reverse transcriptase inhibitors and compounds capable of interacting with high-order telomeric DNA tetraplex ("G-quadruplex") structures, so as to prevent enzyme access to the necessary linear telomere substrate. Critical appraisal of each individual approach is provided together with highlighted areas of likely future development.

Animals↗
PubMed2001

Aerobic nitroreduction by flavoproteins: enzyme structure, mechanisms and role in cancer chemotherapy.

NQO1 (DT-diaphorase) and its truncated isoenzyme, the metalloenzyme NQO2, can reduce quinone substrates by two-electron transfer. While NQO1 is a known detoxification enzyme, the function of NQO2 is less well understood. Both rat NQO1 and human NQO2 reductively bioactivate the dinitroarene CB 1954 to a cytotoxic product that behaves as a difunctional DNA-crosslinking species with potent anti-tumour activity, although human NQO1 is much less effective. A FMN-dependent nitroreductase from E. coli B also reduces quinones and reductively bioactivates CB 1954. However, this enzyme reduces CB 1954 to the 2- and 4-hydroxylamines in equivalent yield, whereas NQO1 and NQO2 generate only the 4-isomer. The reduction profile is a key factor in the development of anti-tumour prodrugs, where distinct delivery strategies are being evaluated: prodrug therapy, antibody-, macromolecule and gene-directed enzyme prodrug therapy (ADEPT, MDEPT or GDEPT). The flavoprotein enzymes are explored in terms of structure and bioreduction mechanism, particularly for use in the design of novel prodrugs with potential application as chemotherapeutic agents.

Aerobiosis↗
PubMed2001

DNA tetraplex-binding drugs: structure-selective targeting is critical for antitumour telomerase inhibition.

Four-stranded tetraplex ("G-quadruplex") DNA represents a new paradigm for the design of DNA-interactive antitumour drugs, as the formed DNA-drug complexes have been suggested to interfere with critical telomerase function. The unique structural features presented by tetraplex over duplex DNA have stimulated the design of small ligand molecules able to selectively promote the formation and/or stabilisation of such higher-order DNA structures. Current developments in tetraplex-targeted telomerase inhibitors, and importantly their DNA structural selectivity, are explored.

Antineoplastic Agents↗
PubMed2001

Comparison of fatty acid content of milk from Jersey and Holstein cows consuming pasture or a total mixed ration.

Holstein (n = 19) and Jersey (n = 18) cows were used to study effects of two feeding systems on fatty acid composition of milk. Confinement cows were fed a total mixed ration with corn silage and alfalfa silage and pastured cows grazed a crabgrass (90%) and clover (10%) pasture and were allowed 5.5 kg of grain per head daily. Two milk samples were collected from each cow at morning and afternoon milkings 1 d each week for four consecutive weeks in June and July 1998. One set of milk samples was analyzed to determine fatty acid composition, and the second set was used for crude protein and total fat analyses. Data were analyzed by the general linear models procedure of SAS, using a split-plot model with breed, treatment, and breed x treatment as main effects and time of sampling and week as subplot effects along with appropriate interactions. Milk from pastured cows was higher than milk from confinement cows for the cis-9, trans-11 octadecadienoic acid isomer of conjugated linoleic acid (CLA). Also, milk from Holsteins was higher than milk from Jerseys for C16:1, C18:1, and CLA and lower than Jerseys for C6:0, C8:0, C10:0, C12:0, and C14:0. Several treatment x week interactions existed, but main effects were still important; for example, proportions of CLA in milk of grazed cows were relatively constant across weeks (0.66, 0.64, 0.64, and 0.69% +/- 0.02%, respectively), but the CLA in milk of confinement cows increased in wk 4 (0.35, 0.31, 0.31, and 0.48% +/- 0.02% for wk 1 to 4, respectively). There are potentially important differences in fatty acid composition of milk from cows consuming a warm season pasture species compared with milk from cows consuming a total mixed ration, as well as differences between Holstein and Jersey breeds.

Animal Feed↗
PubMed2000

Bioactivation of 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) by human NAD(P)H quinone oxidoreductase 2: a novel co-substrate-mediated antitumor prodrug therapy.

A novel prodrug activation system, endogenous in human tumor cells, is described. A latent enzyme-prodrug system is switched on by a simple synthetic, small molecule co-substrate. This ternary system is inactive if any one of the components is absent. CB 1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide] is an antitumor prodrug that is activated in certain rat tumors via its 4-hydroxylamine derivative to a potent bifunctional alkylating agent. However, human tumor cells are resistant to CB 1954 because they are unable to catalyze this bioactivation efficiently. A human enzyme has been discovered that can activate CB 1954, and it has been shown to be commonly present in human tumor cells. The enzyme is NQO2 [NAD(P)H quinone oxidoreductase 2], but its activity is normally latent, and a nonbiogenic co-substrate such as NRH [nicotinamide riboside (reduced)] is required for enzymatic activity. There is a very large (100-3000-fold) increase in CB 1954 cytotoxicity toward either NQO2-transfected rodent or nontransfected human tumor cell lines in the presence of NRH. Other reduced pyridinium compounds can also act as co-substrates for NQO2. Thus, the simplest quaternary salt of nicotinamide, 1-methyl-3-carboxamidopyridinium iodide, was a co-substrate for NQO2 when reduced to the corresponding 1,4-dihydropyridine derivative. Increased chain length and/or alkyl load at the 1-position of the dihydropyridine ring improved specific activity, and compounds more active than NRH were found. However, little activity was seen with either the 1-benzyl or 1-(2-phenylethyl) derivatives. A negatively charged substituent at the 3-position of the reduced pyridine ring also negated the ability of these compounds to act as cosubstrates for NQO2. In particular, 1-carbamoylmethyl-3-carbamoyl-1,4dihydropyridine was shown to be a co-substrate for NQO2 with greater stability than NRH, with the ability to enter cells and potentiate the cytotoxicity of CB 1954. Furthermore, this agent is synthetically accessible and suitable for further pharmaceutical development. NQO2 activity appears to be related to expression of NQO1 (DT-diaphorase), an enzyme that is known to have a favorable distribution toward certain human cancers. NQO2 is a novel target for prodrug therapy and has a unique activation mechanism that relies on a synthetic co-substrate to activate an apparently latent enzyme. Our findings may reopen the use of CB 1954 for the direct therapy of human malignant disease.

Animals↗
PubMed2000

Energetics of DNA intercalation reactions.

Isothermal titration calorimetry has been used to determine the binding enthalpy and heat capacity change (DeltaC(p)()) for a series of DNA intercalators, including ethidium, propidium, daunorubicin, and adriamycin. Temperature-dependent binding enthalpies were measured directly for the ligands, from which DeltaC(p)() values of -140 to -160 cal mol(-)(1) K(-)(1) were calculated. Published van't Hoff plots were reanalyzed to obtain DeltaC(p)() values of -337 to -423 cal mol(-)(1) K(-)(1) for the binding of actinomycin D to several DNA oligonucleotide duplexes with defined sequences. Heat capacity changes for DNA intercalation were found to correlate with the alterations in solvent-accessible surface area calculated from available high-resolution structural data. Multiple linear regression was used to derive the relationship DeltaC(p)() = 0. 382(+/-0.026)DeltaA(np) - 0.121(+/-0.077)DeltaA(p) cal mol(-)(1) K(-)(1), where DeltaA(np) and DeltaA(p) are the binding-induced changes in nonpolar and polar solvent-accessible surface areas (in square angstroms), respectively. The DeltaC(p)() terms were used to estimate the hydrophobic contribution to intercalative binding free energies, yielding values that ranged from -11.2 (ethidium) to -30 kcal mol(-)(1) (actinomycin D). An attempt was made to parse the observed binding free energies of ethidium and propidium into five underlying contributions. Such analysis showed that the DNA binding behavior of these simple intercalators is driven almost equally by hydrophobic effects and van der Waals contacts within the intercalation site.

Animals↗
PubMed2000

Targeting multi-stranded DNA structures.

The design of agents targeted toward a structure-specific molecular recognition of DNA triplexes or tetraplexes ( quadruplexes ) is discussed, where such structures are relevant to antigene-based chemotherapies and the in situ cellular inhibition of telomerase function, respectively. Using principles that stem from the development of earlier synthetic duplex-binding ligands, together with recent findings that probe structure thermodynamic linkages and kinetic features of stability, a rational approach is developed to exploit the distinct molecular templates offered by these high-order nucleic acid biotarget systems. Such analytical techniques can usefully augment conventional drug design methods, particularly where detailed structural information is unavailable or the mode of binding to form a persistent DNA biotarget ligand complex is not established. Examples from the author s laboratory are used to illustrate structure-specific (or structure-preferential) recognition and subsequent stabilization of DNA triplexes using intercalative or groove-mediated binding mechanisms, and the successful targeting of DNA tetraplexes using planar extended-aromatic ligands. In each case, chemical manipulation of the molecule by exploiting either (i) geometric isomers, (ii) redistribution of charged groups and/or H-bond donors/acceptors, or (iii) optimization of intermolecular pi-overlap can be used to improve the affinity or specificity of the underlying DNA drug binding events.

Animals↗
PubMed2000

Site of administration and duration of feeding oleamide to cattle on feed intake and ruminal fatty acid concentrations.

Two experiments were conducted to determine effects of oleamide on feed intake and ruminal fatty acids when the oleamide was introduced in the feed vs through a ruminal fistula (Exp. 1) or the oleamide was fed for an extended (9-wk) length of time (Exp. 2). In Exp. 1, four nonlactating Holstein cows, each fitted with a ruminal cannula, were fed four diets in a 4 x 4 Latin square design. Each period lasted 2 wk. Diets consisted of 48% corn silage and 52% concentrate on a DM basis. One diet contained no added fat (control) and a second diet contained 4.2% oleic acid. The remaining two diets were designed to expose cows to 4.2% amide (as oleamide) either through the feed (AF) or by administering oleamide into the rumen (AR) each day through the ruminal cannula. The AF diet reduced DMI similarly to results reported previously for lactating dairy cows and sheep. Intake of the oleic acid diet was intermediate between the control and AF diets. Dry matter intake was reduced by AR similarly to the AF diet. The acetate:propionate ratio in samples of ruminal contents was reduced by oleic acid but not by AF or AR. In Exp. 2, 12 steers were divided into three equal groups of two Angus and two Simmental x Angus crosses, and each group was assigned a diet containing either no added fat (control), 4% oleamide, or 4% high-oleic canola oil. All steers had ad libitum access to feed and water. Dry matter intake by steers fed the canola oil diet was not different from that by steers fed the control diet when averaged over the first 3 wk, the last 3 wk, or over the entire 9-wk study. Oleamide reduced DMI 4 kg/d over the first 3 wk of the study. However, DMI of the oleamide diet consistently increased over the 9-wk study, resulting in wk 7 to 9 DMI that was not different from that of steers fed the control diet. These results show that the reduction in feed intake when oleamide is added to cattle rations can be attributed more to physiological responses than to an undesirable unique taste or odor of the oleamide. In finishing beef steers, the decreased intake induced by oleamide was most severe during the first 1 or 2 wk of feeding but gradually lessened over time until it nearly returned to normal by wk 9.

Administration, Oral↗
PubMed2000

Feeding oleamide to lactating Jersey cows 1. Effects on lactation performance and milk fatty acid composition.

Oleamide was previously reported to resist ruminal biohydrogenation and elevate milk oleic acid concentration when fed to lactating Holstein cows. To determine if Jersey cows responded similarly to oleamide, four lactating Jersey cows (mean 417 kg of body weight and 64 days in milk) were fed four diets in a 4x4 Latin square with 2-wk periods. Diets were total mixed ration containing 47% corn silage and 53% concentrate (dry matter basis) and were supplemented with no added fat (control), or with 3.5% added fat from either higholeic canola oil, a commercial source of oleamide, or oleamide synthesized from oleic acid and urea. The canola oil supplement had no effect on milk yield or composition. Compared to canola oil, the oleamide supplements reduced milk yield, dry matter intake, and milk fat and protein contents. Milk oleic acid concentration increased from 17.4% of total fatty acids for the control diet to 22.1% for the canola oil diet. Both oleamides further increased milk oleic acid to 30.0 and 27.1% of total fatty acids for the commercial and synthesized oleamides, respectively. Milk palmitic acid was reduced and stearic acid was increased by all fat supplements but more so by the oleamides than by the canola oil. Consistent with previous reports that fatty acyl amides resist ruminal biohydrogenation, feeding oleamide to Jersey cows in this study increased milk oleic acid concentration but had negative effects on feed intake and milk yield.

Animal Feed↗
PubMed2000

Feeding oleamide to lactating Jersey cows. 2. Effects on nutrient digestibility, plasma fatty acids, and hormones.

Six lactating Jersey cows were used in a 6 x 6 Latin square with 14-d periods to evaluate different ratios of canola oil and oleamide on nutrient digestibility, plasma fatty acids, and plasma hormones. The control diet contained no added fat. All other diets contained 3.5% added fat consisting of 0, 25, 50, 75, and 100% as oleamide and the remainder as canola oil. Data were collected during the final 4 d of each period. Dry matter intake was reduced by the addition of canola oil to the diet, and further reduced by replacing canola oil with oleamide. Milk yield was not affected by diet but increasing oleamide proportion in the fat supplement caused linear increases in cis-C18:1 and linear decreases in C4 to C16 fatty acids in milk. Adding canola oil reduced total tract digestibilities of fiber and fatty acids, but had no effect on the digestibilities of dry matter or protein. Replacing canola oil with oleamide increased protein digestibility linearly, and increased digestibility of fiber (quartic relationship) and fatty acids (quadratic relationship). Oleic acid concentration in plasma increased by adding canola oil to the diet, and was further increased by replacing canola oil with oleamide. Diet had no effect on plasma concentrations of insulin or IGF-I. Oleamide fed to Jersey cows in this study was highly digestible and had no deleterious effects on total tract digestility of fiber or protein. Increasing oleic acid concentration in plasma lipids while maintaining a constant level of added fat in the ration had no effect on circulating concentrations of insulin or IGF-I in Jerseys.

Animals↗
PubMed2000

Effects of amount and source of fat on the rates of lipolysis and biohydrogenation of fatty acids in ruminal contents.

Because the percentage loss of unsaturated fatty acids across the rumen has varied considerably in previous in vivo studies, we conducted five experiments to identify potential factors that might affect the in vitro rates of lipid lipolysis and biohydrogenation in ruminal contents. The factors examined included the amount of fat added to the substrate, the source of added fat, the diet fed to the donor fistulated cow, and the time of collection of inoculum from the donor cow. Lipolysis and biohydrogenation were expressed as the rates of disappearance of neutral lipid and unsaturated fatty acids, respectively, from the culture contents over time using a first-order model. The rate of lipolysis of soybean oil declined from 44%/h to less than 30%/h as the percentage of soybean oil in the culture substrate increased from 2 to 10%. The overall rate of biohydrogenation of C18:2 was 14.3%/h, but declined 1.2%/h for each percentage unit increase in C18:2 added to the substrate. Compared with C18:2, the rates of biohydrogenation of C18:1 were generally lower (averaged 3.6 %/h) for all fat sources. The rate of biohydrogenation of C18:2 in soybean oil was not affected by the amount of grain or fat fed to the donor cow, or the time after feeding that ruminal inoculum was collected. Based on these findings, high linoleic acid concentrations in the diet would possibly reduce biohydrogenation and increase the postruminal flow of this unsaturated fatty acid. Also, lipolysis may vary considerably due to amount and source of lipid added to the diet, but this has little influence on the initial disappearance rates of linoleic or oleic acids from ruminal contents.

Animal Feed↗
PubMed1999

Design, synthesis, and evaluation of a novel sequence-selective epoxide-containing DNA cross-linking agent based on the pyrrolo[2, 1-c][1,4]benzodiazepine system.

Synthetic routes have been investigated to prepare a novel C8-epoxide-functionalized pyrrolo[2,1-c][1,4]benzodiazepine 6 as a potential sequence-selective DNA cross-linking agent (Wilson et al. Tetrahedron Lett. 1995, 36, 6333-6336). A successful synthesis was accomplished via a 10-step route involving a pro-N10-Fmoc cleavage method that should have general applicability to other pyrrolobenzodiazepine (PBD) molecules containing acid- or nucleophile-sensitive groups. During the course of this work, a one-pot reductive cyclization procedure for the synthesis of PBD N10-C11 imines from nitro dimethyl acetals was also discovered, although this method results in C11a racemization which can reduce DNA binding affinity and cytotoxicity. The target epoxide 6 was shown by thermal denaturation studies to have a significantly higher DNA-binding affinity than the parent DC-81 (3) or the C8-propenoxy-PBD (15), which is structurally similar but lacks the epoxide moiety. The time course of effects upon thermal denaturation indicated a rapid initial binding phase followed by a slower phase consistent with the stepwise cross-linking of DNA observed for a difunctional agent. This was confirmed by an electrophoretic assay which demonstrated efficient induction of interstrand cross-links in plasmid DNA at concentrations >1 microM. Higher levels of interstrand cross-linking were observed at 24 h compared to 6 h incubation. A Taq polymerase stop assay indicated a preference for binding to guanine-rich sequences as predicted for bis-alkylation in the minor groove of DNA by epoxide and imine moieties. The pattern of stop sites could be partly rationalized by molecular modeling studies which suggested low-energy models to account for the observed binding behavior. The epoxide PBD 6 was shown to have significant cytotoxicity (45-60 nM) in the A2780, CH1, and CH1cis(R) human ovarian carcinoma cell lines and an IC(50) of 0.2 microM in A2780cis(R). The significant activity of 6 in the cisplatin-resistant CH1cis(R) cell line (IC(50) = 47 nM) gave a resistance factor of 0.8 compared to the parent cell line, demonstrating no cross-resistance with the major groove cross-linking agent cisplatin.

Antineoplastic Agents↗
PubMed1999

Effect of A-ring modifications on the DNA-binding behavior and cytotoxicity of pyrrolo[2,1-c][1,4]benzodiazepines.

Several A-ring-modified analogues of the DNA-binding antitumor agent DC-81 (5) have been synthesized in order to study structure-reactivity/cytotoxicity relationships. For two molecules (23 and 30) the modifications required the addition of a fourth ring to give the novel dioxolo[4,5-h]- and dioxano[5,6-h]pyrrolo[2,1-c][1, 4]benzodiazepin-11-one (PBD) ring systems, respectively. Another three analogues (34, 38, and 48) have the native benzenoid A-ring replaced with pyridine, diazine, or pyrimidine rings to give the novel pyrrolo[2,1-c][1,4]pyridodiazepine, pyrrolo[2,1-c][1, 4]diazinodiazepine, and pyrrolo[2,1-c][1,4]pyrimidinodiazepine systems, respectively. The other new analogues (16a,b) have extended chains at the C8-position of the DC-81 structure. During the synthesis of these compounds, a novel tin-mediated regiospecific cleavage reaction of the dioxole intermediate 18 was discovered, leading to the previously unknown iso-DC-81 (20). In addition, an unusual simultaneous nitration-oxidation reaction of 4-(3-hydroxypropoxy)-3-methoxybenzoic acid (8) was found to produce 3-(4-carboxy-2-methoxy-5-nitrophenoxy)propanoic acid (9), a key intermediate, in high yield. In general, the results of cytotoxicity and DNA-binding studies indicated that none of the changes made to the A-ring of the PBD system significantly improved either binding affinity or cytotoxicity in comparison to DC-81. This result suggests that the superior potency of natural products such as anthramycin (1), tomaymycin (2), and sibiromycin (3) is due entirely to differences in C-ring structure, and in particular exo or endo unsaturation at the C2-position and C2-substituents containing unsaturation. This study also provided information regarding the influence of A-ring substitution pattern on the relative stability of the interconvertible N10-C11 carbinolamine, carbinolamine methyl ether, and imine forms of PBDs.

Animals↗