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T Nohmi

Publications and source records attributed to T Nohmi.

At least 109 records · Page 6Linked to original sources
PubMed1988

RecA-mediated cleavage activates UmuD for mutagenesis: mechanistic relationship between transcriptional derepression and posttranslational activation.

The products of the SOS-regulated umuDC operon are required for most UV and chemical mutagenesis in Escherichia coli. It has been shown that the UmuD protein shares homology with LexA, the repressor of the SOS genes. In this paper we describe a series of genetic experiments that indicate that the purpose of RecA-mediated cleavage of UmuD at its bond between Cys-24 and Gly-25 is to activate UmuD for its role in mutagenesis and that the COOH-terminal fragment of UmuD is necessary and sufficient for the role of UmuD in UV mutagenesis. Other genetic experiments are presented that (i) support the hypothesis that the primary role of Ser-60 in UmuD function is to act as a nucleophile in the RecA-mediated cleavage reaction and (ii) raise the possibility that RecA has a third role in UV mutagenesis besides mediating the cleavage of LexA and UmuD.

Bacterial Proteins↗
PubMed1987

Metabolic activation of phenacetin and phenetidine by several forms of cytochrome P-450 purified from liver microsomes of rats and hamsters.

Metabolic activation of phenacetin by liver microsomes proceeds via both phenetidine and N-hydroxyphenacetin to direct-acting mutagens, i.e., N-hydroxyphenetidine and p-nitrosophenetole. Five different molecular species of cytochrome P-450 have been purified from liver microsomes of drug-pretreated Wistar rats or Syrian hamsters and their abilities to activate phenetidine and phenacetin were compared using reconstituted microsome systems. High-spin forms of cytochrome P-450 purified from 3-methylcholanthrene-pretreated rats (MC-P-448-H) or hamsters (P-488 ham-II) showed higher catalytic activity for N-hydroxylation of phenetidine than three other low-spin forms of cytochrome P-450 purified from the same animals or from phenobarbital-pretreated rats. MC-P-448-H and P-488 ham-II required the presence of cytochrome b5 for their maximum activities in the reconstituted system. The five forms of cytochrome P-450, however, exhibited no measurable activity for N-hydroxylation of phenacetin either with or without cytochrome b5. The mutagenicity of phenacetin and phenetidine toward Salmonella typhimurium TA100 was generated when the reconstituted microsomes containing MC-P-488-H or P-488 ham-II were used as activating enzymes. From these results, it was suggested that high-spin forms of cytochrome P-450 (MC-P-448-H and P-448 ham-II) played an important role in the metabolic activation of phenacetin to the direct-acting mutagens.

Aminophenols↗
PubMed1986

Purification and characterization of a form of cytochrome P-450 with high specificity for aflatoxin B1 from 3-methylcholanthrene-treated hamster liver.

A form of cytochrome P-450 highly active in inducing mutagenicity of aflatoxin B1 was purified to a specific content of 15.1 nmol/mg of protein from 3-methylcholanthrene-treated hamster liver. This species of cytochrome P-450, having its absorption maximum at 448.5 nm in carbon monoxide-complex of reduced form and low spin ferric ion, is of molecular weight of 56,000 and distinctly different in physicochemical and catalytic properties from major forms of cytochrome P-450 purified from phenobarbital- or 3-methylcholanthrene-treated rat liver. In the induction of aflatoxin B1 mutagenicity, this hamster cytochrome P-450 is 50 times more potent than those from rat liver.

Aflatoxin B1↗
PubMed1984

Mutations in Salmonella typhimurium and inactivation of Bacillus subtilis transforming DNA induced by phenylhydroxylamine derivatives.

Phenylhydroxylamine (PHA) and its derivatives such as monomethyl (2-Me, 3-Me, 4-Me) and dimethyl (2,3-diMe, 2,4-diMe, 2,5-diMe, 2,6-diMe, 3,4-diMe, 3,5-diMe) were tested for their mutagenicity and for their inducing ability to inactivate transforming DNA. All these compounds except PHA and 3,5-diMePHA were found to be mutagenic in Salmonella typhimurium TA100 even in the absence of S9 mix, and their mutagenic potency was in the order: 2,6-diMe- greater than 2,4-diMe- = 3,4-diMe- greater than 4-Me- greater than 2,3-diMe- = 2,5-diMe- greater than 2-Me- = 3-MePHA. Besides mutagenicities, all the PHA derivatives except 2,6-diMePHA caused severe reductions in the activity of Bacillus subtilis transforming DNA. To establish the structure-activity relationship, we examined the correlation between these activities and the stabilities of the PHA derivatives, and the results indicated that the more chemically unstable the PHA derivatives were, the more active they were with respect to the mutations and to the inactivation of the transforming DNA. The mutagenic activity of 2,6-diMePHA was the sole exception, because it was most stable, but its induced mutation frequency was highest. From these results, we suggest that all the PHA derivatives, except 2,6-diMePHA, cause DNA damage through the generation of active molecular species, such as nitrenium ions, without any enzymatic activation, while 2,6-diMePHA requires further metabolic activation by bacterial enzymes to stimulate mutagenesis.

Animals↗
PubMed1984

Primary mutagenicity screening of food additives currently used in Japan.

Salmonella/microsome tests (Ames tests) and chromosomal aberration tests in vitro using a Chinese hamster fibroblast cell line were carried out on 190 synthetic food additives and 52 food additives derived from natural sources, all of which are currently used in Japan. Fourteen out of 200 tested in the Ames assay showed positive effects and 54 out of 242 were positive in the chromosome test. Three additives (erythorbic acid, chlorine dioxide and beet red) were positive only in the Ames test, although their mutagenic potentials were relatively weak, while 43 additives were positive only in the chromosome test. Eleven additives (calcium hypochlorite, cinnamic aldehyde, L-cysteine monohydrochloride, Food Green No. 3 (Fast Green FCF), hydrogen peroxide, potassium bromate, sodium chlorite, sodium hypochlorite, sodium nitrite, cacao pigment and caramel) were positive in both the Ames test and the chromosome test. The usefulness of such primary screening tests combining two different genetic end-points, gene mutation and chromosomal aberration, and some correlation between mutagenicity and carcinogenicity of food additives are discussed.

Animals↗
PubMed1983

Species difference in the metabolic activation of phenacetin by rat and hamster liver microsomes.

Phenacetin is mutagenic in Salmonella typhimurium TA 100 when liver 9,000 X g supernatant fractions from PCB-treated hamsters instead of rats are used. A mechanism of the species difference in phenacetin mutagenicity was investigated. By high-performance liquid chromatography analysis, it was found that phenacetin is activated to direct-acting mutagens through N-hydroxylation and deacetylation by hamster liver microsomes. Although no significant species difference was observed in N-hydroxylation, rates of deacetylation were 9 to 150 times higher in hamsters than in rats. The results indicate that the marked species difference in phenacetin mutagenicity is due to the difference in deacetylation activity between rat and hamster liver microsomes.

Animals↗
PubMed1982

Difference in liver homogenates from Donryu, Fischer, Sprague-Dawley and Wistar strains of rat in the drug-metabolizing enzyme assay and the Salmonella/hepatic S9 activation test.

Comparison studies for detecting differences between liver microsome and S9 preparations from 4 strains (Donryu, Fischer, Sprague-Dawley, Wistar) of young male rats were carried out with pretreatment of the animals by inducers such as PCBs and PB plus 5,6-BF. Each microsome fraction was assayed for the enzymic activity of metabolism of model substrates such as aniline, benzophetamine, BP, DMN and 7-ethoxycoumarin. The hepatic S9 sample was also compared, as regards its metabolizing ability to activate 9 pre-mutagens (2AA, AAF, o-AAT, BP, DAB, DMBA, DMN, m-PDA, quinoline) to directly acting mutagens in the Salmonella/hepatic S9 activation test by using TA98, TA100 and TA1537 strains with or without cytochrome P450 inhibitors (SKF-525A, metyrapone, 7,8-benzo-flavone). In the enzymic assay with PCBs-induced microsomes, BP hydroxylation a strain-specific difference: the microsomes from Fischer and Wistar rats were more effective for metabolizing BP than those from the other strains of rat. The effect of induction by BP plus 5,6-BF for Fischer rats showed relatively higher enzymic activity in the same induction group. Other microsomes prepared from rats with and without induction by PB plus, 5,6-BF did not show a clear-cut strain dependency in the enzymic activities assayed. In the mutation experiments with hepatic S9 samples, the examination of DAB and quinoline revealed a marked strain difference when S9 samples prepared from PCBs-pretreated and PB-plus-5,6-BF-induced rats were used: the S9 sample from Fischer rats was available for activating the two pre-mutagens to directly acting mutagens. No marked difference in the metabolic activation of the remaining 7-pre-mutagens was observed on other S9 preparations. In examinations of mutagenicity activities with the use of three inhibitors, the two S9 preparations made with the two induction methods showed inhibition profiles closely similar to each other. However, there were minor differences in the profiles by these inhibitors. From these findings it was concluded that Fischer rat-liver S9 is useful for detecting mutagens in the metabolic activation test, when induction by PB plus 5,6-BF was used in the Ames Salmonella test.

Animals↗
PubMed1982

Effects of 7, 8-benzoflavone and SKF 525-A on the enzyme-mediated mutagenicity of phenylenediamines.

The effects of microsomal enzyme inhibitors (7, 8-BF and SKF 525-A) on the S-9-mediated mutagenicity of o-, m- and p-phenylenediamine were investigated using Salmonella typhimurium TA98. SKF 525-A did not affect the enzyme-mediated mutagenicity of m- and p-phenylenediamine, while 7, 8-BF reduced significantly the mutagenicity of all three isomers of phenylenediamine. When the enzyme reactions in the agar overlayer were stopped successively by adding 7, 8-BF directly onto the plate, the number of revertants increased linearly with time at least for 6 hours. These data suggest that cytochrome P-448 takes a main role in the activation of phenylenediamines and that in the agar layer this microsomal enzyme remain active for a period as long as 6 hours at 37 degrees C.

Animals↗