PubMed Health⌕ Search

Biomedical subjects

L K Keefer

Publications and source records attributed to L K Keefer.

At least 109 records · Page 6Linked to original sources
PubMed1978

Chemical models for possible nitrosamine artifact formation in environmental analysis.

Preliminary data concerning two different phenomena of potential importance to those studying the analysis and formation of environmental N-nitroso compounds are presented. First of all, we report that inorganic nitrite in the solid phase can serve as an effective nitrosating agent for solutions of amines in certain nonaqueous media. Secondly, we describe evidence suggesting that the appearance of nitrosamines as contaminants in deionized water (Cohen, 1977; Gough et al., 1977; Fiddler et al., 1977) might result at least partly from simple, acid-catalyzed nitrosation of the amine/ammonium functional groups on the anion exchange resins used in the demineralization process. Possible implications of both phenomena are discussed and potentially useful measures for their control are suggested.

Catalysis↗
PubMed1977

Chronic oral administration of 1-nitrosopiperazine at high doses to MRC rats.

1-Nitrosopiperazine was fed to two groups of rats as drinking water solutions containing 400 mg/liter (3.5 millimolar) and 800 mg/liter (7.0 millimolar), respectively. The treatment was 20 ml per rat per day, 5 days per week for life. In both groups many animals died with olfactory tumors (mostly esthesioneuroblastomas), the first at 36 weeks in the higher dose group, the first at 64 weeks in the lower dose group. There was also a small number of liver tumors in both groups. None of these tumors was seen in the untreated controls. The similarity of this tumor distribution to that produced by 1,4-dinitrosopiperazine suggests that the observed carcinogenicity of 1-nitrosopiperazine may be entirely due to its disproportionation in the acidic medium of the rat stomach. Chemical data supporting this interpretation are presented.

Administration, Oral↗
PubMed1977

Selective induction of intestinal tumors in rats by methyl(acetoxymethyl)nitrosamine, an ester of the presumed reactive metabolite of dimethylnitrosamine.

Methyl(acetoxymethyl)nitrosamine (DMN-OAc) was synthesized and tested for toxicity and carcinogenicity in rats to test the hypothesis that alpha-hydroxylation is required for metabolic activation of dimethylnitrosamine (DMN) to a reactive, proximate carcinogen. The acute median lethal doses (LD50) of DMN-OAc and DMN injected ip into 5-week-old male Sprague-Dawley (Charles River (CD) rats were determined to be 0.19 and 0.59 mmole/kg body weight or 25 mg DMN-OAc/kg and 44 mg DMN/kg body weight, respectively. Single ip injections of one-half the LD50 DMN-OAc (13 mg/kg body weight) in 5-week-old rats of both sexes resulted in a high incidence of epithelial tumors of the intestinal tract. Mean survival times for rats with intestinal tumors were 353 days for males and 433 days for females. Tumors were rarely found at other sites. DMN at equivalent toxic (one-half the LD50, 22 mg/kg) and molar (= one-sixth LD50, 7.0 mg/kg) dose levels, yielded (as expected) tumors of kidneys, lungs, and occasionally other organs, but at a much lower incidence. The finding of the potent carcinogenicity of DMN-OAc supported the postulate that alpha-hydroxylation of DMN in vivo generates a proximate carcinogen.

Adenocarcinoma↗
PubMed1976

Reduction of rat liver carcinogenicity of 4-nitrosomorpholine by alpha-deuterium substitution.

Groups of 30 males Sprague-Dawley rats were given 4-nitrosomorpholine-3,3,5,5-d in their drinking water at concentrations of 0.35 and 0.07 X 10(-3) M for 30 weeks. Two similar groups of rats were simultaneously given unlabeled 4-nitrosomorpholine (NM) at the same malar concentrations; all animals were observed throughout their lives. Those receiving the alpha-deuterium-labeled compound had significantly fewer liver tumors than did the corresponding animals receiving the unlabeled compound. The difference in potency appeared to be at least fivefold, a magnitude consistent with a primary kinetic isotope effect on the carcinogenic action of NM. Thus breakage of a bond linking a hydrogen (deuterium) atom with carbon adjacent to the nitrosamino function may be involved in a rate-limiting step of carcinogenesis by NM.

Animals↗
PubMed1975

Carcinogen chemistry. I. Reactions of protonated dialkylnitrosamines leading to alkylating and aminoalkylating agents of potential metabolic significance.

Three distinct modes of protolytic dialkylnitrosamine fragmentation were observed when we followed the time dependence of the nuclear magnetic resonance (NMR) spectra of seven nitrosamines in superacid solution: 1) In equimolar HSO3F: SbF5 ("magic acid"), dimethylnitrosamine was cleaved to the protonated Schiff base of formaldehyde and methylamine, and diethylnitrosamine was similarly converted to the protonated acetaldehydeethylamine Schiff base.2) By contrast, of the five dipropyl-and dibutylnitrosamines were studied, all cleaved nonoxidatively under these conditions (with loss of nitrogen gas) to the corresponding propyl or butyl cations. The carbocations thus produced underwent condensation and fragmentation to form the tert-butyl cation as the principal product ultimately observable by NMR. 3) Thethird fragmentation mechanism, which involved denitrosation to the dialkylammonium ion, was observed only as a minor pathway in the sulfuric or fluorosulfuric acid protolysis of dimethylnitrosamine. The mechanisms that are postulated for these cleavage reactions, if functioning in vivo, could account for several metabolic observations that have proved difficult to reconcile with previous conceptions of nitrosamine metabolism.

Alkylating Agents↗
PubMed1973

N-nitrosation by nitrite ion in neutral and basic medium.

Formaldehyde catalyzed the conversion of various secondary amines to nitrosamines in the pH range 6.4 to 11.0. Chloral was also an effective catalyst. The reaction proceeds easily enough to have potential synthetic applications; the proposed mechanism could explain some reported anomalies regarding the synthesis of carcinogenic N-nitroso compounds in vivo and in vitro.

Amines↗
PubMedDate not supplied

Pathophysiology of a sickle cell trait mouse model: human alpha(beta)(S) transgenes with one mouse beta-globin allele.

As a potential model for sickle cell trait (AS), we examined mice containing one normal mouse beta-globin allele in combination with a human hemoglobin S (h(alpha)beta(S)) transgene (m(beta)/hS). The mice segregated into two subpopulations containing low and high proportions of hemoglobin S (m(beta)/hS1 and m(beta)/hS2, respectively) that was associated with one or two human h(alpha)beta(S) transgenes. We noted striking kidney pathology (cortical cysts, hyperplastic tubules, and glomerulonephritis), increasing with age and with greater severity in m(beta)/hS1. mBeta/hS2 animals were largely tolerant to 5% O(2) for 1 h, whereas 80% of m(beta)/hS1 mice died, exhibiting acute sequestration of erythrocytes in spleen, liver, and heart. These pathologies appear to result from a decreased oxygen affinity of the hybrid (human alpha/mouse beta) hemoglobins with a mild beta-thalassemia phenotype. Thus, these mouse models of sickle trait seem to manifest their renal pathology and sensitivity to hypoxia by mechanisms related to low tissue oxygen delivery and are different from the human syndrome. Analyses of parameters such as P(50), red cell indices, and genetic background are necessary in establishing potential relevance of any mouse model of the sickle cell syndromes.

Alleles↗
PubMedDate not supplied

Inactivity of fecapentaene-12 as a rodent carcinogen or tumor initiator.

The possible carcinogenic activity of synthetic fecapentaene-12 (FP-12) was studied in several mammalian test systems: (a) for carcinogenicity by intrarectal instillation in male F344/NCr rats as well as by intrarectal and subcutaneous application in male B6C3F1 mice; (b) for initiation by skin painting in female SENCAR mice followed by repeated applications of 12-O-tetradecanoylphorbol-13-acetate (TPA), with 7,12-dimethylbenz[a]anthracene (DMBA) followed by TPA as positive control; (c) in a rat subcutaneous granuloma pouch assay in which mutagenicity was measured by induction of 6-thioguanine (6-TG) resistance and carcinogenicity was determined by induction of subcutaneous tumors in the pouch. There was no significant increase in tumor incidence after 72-78 weeks in test (a), although 2 rats receiving FP-12 intrarectally developed colon polyps. FP-12 did not initiate any skin tumors in test (b), nor did it significantly convert DMBA-initiated papillomas into carcinomas when 8 of the positive control mice were given FP-12 weekly for 10 weeks after 10 weeks on the DMBA-TPA regimen. Although FP-12 and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were comparably mutagenic in test (c), FP-12 induced no tumors after more than a year in 133 rats at risk while MNNG induced 7 tumors in 107 rats. These rodent assays provide no evidence that FP-12 is a strong carcinogen, although the possibility remains that it may possess weak carcinogenic activity not revealed by these experiments.

9,10-Dimethyl-1,2-benzanthracene↗
PubMedDate not supplied

Conversion of proteins to diazeniumdiolate-based nitric oxide donors.

Michael reaction of the methoxymethyl-protected monodiazeniumdiolate of piperazine (MOM-PIPERAZI/NO) with 4-maleimidobutyric acid followed by its conversion to the N-hydroxy-succinimido ester produces a reagent capable of transferring the nitric oxide (NO)-donating diazeniumdiolate group to the terminal amines of the lysine residues contained in proteins. The reagent has been used to produce diazeniumdiolated bovine serum albumin (D-BSA) and diazeniumdiolated human serum albumin (D-HSA) containing 22 and 19 modified lysyl groups, respectively. Upon dissolution in pH 7.4 phosphate buffer at 37 degrees C, these albumin derivatives gradually released all of their contained NO (approximately 40 mol/mol of protein) with initial rates of about 30-40 pmol/min/mg and half-lives on the order of 3 weeks. This methodology is now available for use in exploiting the unique specific metabolic interactions of proteins to target NO therapy to specific physiological processes in vivo.

Cross-Linking Reagents↗
PubMedDate not supplied

Decomposition and quality control considerations in biological work with fecapentaene preparations.

Solutions of synthetic fecapentaene 12 (FP-12) intended for carcinogenicity studies were found to decompose extremely rapidly during customary dosage procedures. Apparent half-lives as short as 15 min were observed. While rates and even the qualitative course of decomposition were surprisingly variable in replicate experiments, high concentration and exposure to air were confirmed to be especially important destabilizing influences. The results suggested a primary role for a radical decomposition mechanism in the presence of atmospheric oxygen. Consistent with this hypothesis, FP-12 solutions were significantly stabilized by the radical chain-breaking antioxidant vitamin E. On the other hand, dithiothreitol greatly destabilized FP-12, presumably because of its nucleophilicity. The diacetyl diester of FP-12 was more soluble than the parent diol, but its decomposition rates in the presence and absence of vitamin E were similar to those of unesterified FP-12. Ultraviolet irradiation of an all-trans-FP-12 solution decreased its concentration by 70% in 0.5 min. The mutagenicities of the decomposition/isomerization products of FP-12, as studied in Salmonella typhimurium tester strain TA 100, ranged from negligible to comparable with all-trans-FP-12 itself. It is concluded that unchecked decomposition of fecapentaene preparations can profoundly affect biological tests therewith. While this can be largely controlled through the use of rigorous precautions, including protection from air, light, nucleophiles, and acids as well as selection of the lowest concentration compatible with the application at hand, the data argue strongly for inclusion of appropriate quality control measures in all future dosing operations to prove that the biological activity reported is that of the fecapentaene itself rather than that of a decomposed dosing solution.

Chromatography, High Pressure Liquid↗
PubMedDate not supplied

The Fenton degradation as a nonenzymatic model for microsomal denitrosation of N-nitrosodimethylamine.

The microsomal metabolism of the carcinogen N-nitrosodimethylamine (NDMA) was suggested to be initiated by hydrogen atom abstraction to form an alpha-nitrosamino radical, which either oxidizes further to an alpha-hydroxy nitrosamine as the initial product of the activating dealkylation pathway or fragments to the nitric oxide radical and N-methylformaldimine as the first step of the presumably inactivating denitrosation route. To examine the chemistry of the alpha-nitrosamino radical in a nonenzymatic setting, we exposed NDMA to the Fenton reagent, which is known to be capable of abstracting hydrogen atoms from organic species. The products observed were those expected of a denitrosation model. Solutions containing 13 mM [14C]NDMA, 15 mM FeSO4, 15 mM H2O2, and 7.5 mM H2SO4 were kept at 4-10 degrees C for 1 h and then basified to yield methylamine (3.2 +/- 0.5 mM, mean +/- SD, n = 8), formaldehyde (3.1 +/- 0.9 mM), and unreacted nitrosamine (10.2 +/- 0.7 mM) as the only radioactive species detected, with total nitrate/nitrite also being found at a level of 2.8 +/- 0.5 mM. N-Methylformaldiminium ion was identified as an intermediate. The parallels between these results and those seen in the microsomal reaction support the hypothesis that the alpha-nitrosamino radical is a common intermediate in enzymatic denitrosation versus dealkylation of NDMA.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalysis↗
PubMedDate not supplied

Stereoselectivity in the microsomal conversion of N-nitrosodimethylamine to formaldehyde.

The possibility that N-nitrosodimethylamine (NDMA) might be metabolized preferentially at either the syn (relative to the nitroso oxygen) or the anti methyl group has been examined by comparing the rates of formaldehyde production when unlabeled NDMA, its fully deuteriated analogue (NDMA-d6), and (Z)- or (E)-N-nitrosomethyl(methyl-d3)amine (NDMA-d3) were incubated in turn at concentrations of 0-2.4 mM with acetone-induced rat liver microsomes. The Km values for the conversion of (Z)- and (E)-NDMA-d3 to formaldehyde were identical to each other within experimental error (32 +/- 2 and 35 +/- 1 microM, respectively) but different from those for NDMA (24 +/- 6 microM) and NDMA-d6 (116 +/- 3 microM); similar Vmax values were observed for the four isotopic variants [7.5-8.1 nmol/(mg of protein.min)]. The observed similarity of kinetic parameters for (Z)- and (E)-NDMA-d3 suggested that the isotopic composition of the methyl group is an energetically more important determinant of its rate of oxidation at the NDMA demethylase active site than is its orientation relative to the nitroso oxygen atom. The absence of syn vs anti stereospecificity was confirmed via product isolation studies, in which the formaldehyde generated from each of the four isotopomers was trapped as the dimedone adduct and assayed for deuterium content by mass spectrometry; again, a strong preference for metabolism at CH3 vs CD3 regardless of stereochemistry was observed, though the data on CH2O generation suggested that there may be a slight net excess of anti attack. The results indicate that the microsomal enzymes employed display little regioselectivity in metabolizing the syn vs anti methyl groups of NDMA.

Animals↗