Restricted repair of aflatoxin B1 induced damage in alpha DNA of monkey cells

Acute oral toxicity test and assessment of combined toxicity of cadmium and aflatoxin B1 in kunming mice

Cadmium and aflatoxin B₁ (AFB₁) are both common and widespread pollutants in food and feed. There are several reports on toxicity induced by Cadmium or AFB₁ alone, but few address the toxicity caused by co-exposure to the two substances. In this study, 42 female and 42 male Kunming (KM) mice were divided into seven groups to test the acute oral toxicity of CdCl₂ and AFB₁, using Karber's method. The combined toxicity was assessed using the Keplinger evaluation system. Acute toxicity symptoms, deaths, and body and organ weights were evaluated, and hematological, blood biochemical, and histopathological analyses were conducted. The results revealed the following median lethal doses (LD₅₀): LD_{50(Female KM mice)} = 62.56 mg/kg; LD_{50(Male KM mice)} = 48.79 mg/kg; LD_{50(KM mice)}=55.27 mg/kg. The combined toxicity of AFB₁ and CdCl₂ showed an additive effect in mice, and an increase in the mixed dose of AFB₁ and CdCl₂ resulted in greater toxicity. These results demonstrated that the combined toxicity of AFB₁ and CdCl₂ was greater than the toxicities of the individual components in mice; thus, this may cause particular challenges when addressing these hazards in food and feed and the associated risk to human and animal health.

DNA Sequence Modulates Geometrical Isomerism of the trans-8,9- Dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)- 9-hydroxy Aflatoxin B1 Adduct

Aflatoxin B₁ (AFB₁), a mycotoxin produced by Aspergillus flavus, is oxidized by cytochrome P450 enzymes to aflatoxin B₁-8,9-epoxide, which alkylates DNA at N7-dG. Under basic conditions, this N7-dG adduct rearranges to yield the trans-8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxy aflatoxin B₁ (AFB₁–FAPY) adduct. The AFB₁–FAPY adduct exhibits geometrical isomerism involving the formamide moiety. NMR analyses of duplex oligodeoxynucleotides containing the 5′-XA-3′, 5′-XC-3′, 5′-XT-3′, and 5′-XY-3′ sequences (X = AFB₁–FAPY; Y = 7-deaza-dG) demonstrate that the equilibrium between E and Z isomers is controlled by major groove hydrogen bonding interactions. Structural analysis of the adduct in the 5′-XA-3′ sequence indicates the preference of the E isomer of the formamide group, attributed to formation of a hydrogen bond between the formyl oxygen and the N⁶ exocyclic amino group of the 3′-neighbor adenine. While the 5′-XA-3′ sequence exhibits the E isomer, the 5′-XC-3′ sequence exhibits a 7:3 E:Z ratio at equilibrium at 283 K. The E isomer is favored by a hydrogen bond between the formyl oxygen and the N⁴-dC exocyclic amino group of the 3′-neighbor cytosine. The 5′-XT-3′ and 5′-XY-3′ sequences cannot form such a hydrogen bond between the formyl oxygen and the 3′-neighbor T or Y, respectively, and in these sequence contexts the Z isomer is favored. Additional equilibria between α and β anomers and the potential to exhibit atropisomers about the C5–N⁵ bond do not depend upon sequence. In each of the four DNA sequences, the AFB₁–FAPY adduct maintains the β deoxyribose configuration. Each of these four sequences feature the atropisomer of the AFB₁ moiety that is intercalated above the 5′-face of the damaged guanine. This enforces the R_a axial conformation for the C5–N⁵ bond.

AFB(1) -induced mutagenesis of the gpt gene in AS52 cells

Aflatoxin B(1) (AFB(1) ) is a potent mutagen and an important risk factor for hepatocellular carcinoma (HCC) in humans. Transgenic mouse strains and cells in culture have been used to detect different types of mutations caused by AFB(1) and investigate the molecular determinants of their location and frequency. The AFB(1) mutational spectrum in the gpt gene was markedly different in AS52 cells compared with the liver in gpt delta B6C3F1 transgenic mice. The results demonstrate the importance of metabolism, chromosomal location, transcription and selection conditions on mutational spectra.

Transcription-coupled DNA repair in prokaryotes

Transcription-coupled repair (TCR) is a subpathway of nucleotide excision repair (NER) that acts specifically on lesions in the transcribed strand of expressed genes. First reported in mammalian cells, TCR was then documented in Escherichia coli. In this organism, an RNA polymerase arrested at a lesion is displaced by the transcription repair coupling factor, Mfd. This protein recruits the NER lesion-recognition factor UvrA, and then dissociates from the DNA. UvrA binds UvrB, and the assembled UvrAB* complex initiates repair. In mutants lacking active Mfd, TCR is absent. A gene transcribed by the bacteriophage T7 RNA polymerase in E. coli also requires Mfd for TCR. The CSB protein (missing or defective in cells of patients with Cockayne syndrome, complementation group B) is essential for TCR in humans. CSB and its homologs in higher eukaryotes are likely functional equivalents of Mfd.

Excision-repair patch lengths are similar for transcription-coupled repair and global genome repair in UV-irradiated human cells

We have used the buoyant density shift method to measure excision-repair patch lengths in UV-irradiated repair-proficient human cells and in primary fibroblasts belonging to xeroderma pigmentosum complementation group C (XP-C), in which excision repair of UV-induced photoproducts is dependent upon transcription. The patch size was found to be about 30 nucleotides for both cell types. This agrees with the size of the DNA fragments excised in vitro by the dual incisions of the structure-specific nucleases XPG and ERCC1-XPF. We conclude that the XPC protein is not required to target the excision nucleases to sites of DNA cleavage in transcribed strands of expressed genes or to protect the newly incised DNA from further processing by exonucleases.

A review of the dose-response induction of DNA adducts by aflatoxin B1 and its implications to quantitative cancer-risk assessment

The induction of DNA adducts by aflatoxin B1 in the liver has been extensively reviewed in a quantitative cancer-risk assessment of aflatoxins (CDHS, 1990). Rat is the most sensitive species for aflatoxin tumorigenesis and liver is the most sensitive site. In vitro DNA-adduct studies were mostly on adduct identification and specificity of binding. In vivo studies provided dose-response relationship of aflatoxin B1, binding to DNA and DNA-adduct formation. Most in vivo studies were conducted in rats. The dose-response curves of DNA-adduct induction after ingestion or injection treatments in this species were reviewed. A linear dose-response relationship was observed in both injection and ingestion studies at low doses. For cancer-risk assessment, this observation is consistent with the assumption of the linear dose-response risk-assessment model for genotoxic agents, and justifies the use of this model for quantitative cancer-risk assessment for aflatoxins.

Intragenomic repair heterogeneity of DNA damage

The mutagenic and carcinogenic consequences of unrepaired DNA damage depend upon its precise location with respect to the relevant genomic sites. Therefore, it is important to learn the fine structure of DNA damage, in particular, proto-oncogenes, tumor-suppressor genes, and other DNA sequences implicated in tumorigenesis. Both the introduction and the repair of many types of DNA lesions are heterogeneous with respect to chromatin structure and/or gene activity. For example, cyclobutane pyrimidine dimers are removed more efficiently from the transcribed than the nontranscribed strand of the dhfr gene in Chinese hamster ovary cells. In contrast, preferential strand repair of alkali-labile sites is not found at this locus. In mouse 3T3 cells, dimers are more efficiently removed from an expressed proto-oncogene than from a silent one. Persistent damage in nontranscribed domains may account for genomic instability in those regions, particularly during cell proliferation as lesions are encountered by replication forks. The preferential repair of certain lesions in the transcribed strands of active genes results in a bias toward mutagenesis owing to persistent lesions in the nontranscribed strands. Risk assessment in environmental genetic toxicology requires assays that determine effective levels of DNA damage of producing malignancy. The existence of nonrandom repair in the mammalian genome casts doubt on the reliability of overall indicators of carcinogen-DNA binding and lesion repair for such determinations. Tissue-specific and cell-specific differences in the coordinate regulation of gene expression and DNA repair may account for corresponding differences in the carcinogenic response to particular environmental agents.

DNA sequence selectivity of guanine-N7 alkylation by nitrogen mustards is preserved in intact cells

Nitrogen mustard alkylating agents react with isolated DNA in a sequence selective manner, and the substituent attached to the drug reactive group can impose a distinct sequence preference. It is not clear however to what extent the observed DNA sequence preferences are preserved in intact cells. The highly reiterated sequence of human alpha DNA has been used to determine the sites of guanine-N7 alkylation following treatment of cells with three nitrogen mustards, mechlorethamine, uracil mustard and quinacrine mustard, known to react in isolated DNA with distinctly different sequence preferences. Alpha DNA from drug treated cells was extracted, purified, end-labeled, and a 296 base pair, singly end-labelled, fragment isolated. Following the quantitative conversion of alkylation sites to strand breaks the fragments were separated on DNA sequencing gels. Clear differences were observed between the alkylation patterns of the three compounds, and the selectivities were qualitatively similar to those predicted and observed in the same sequence alkylated in vitro. In particular the unique preferences of uracil and quinacrine mustards for 5'-PyGC-3' and 5'-GT/GPu-3' sequences, respectively, were preserved in intact cells suggesting that the pattern of sequence dependent reactivity is not grossly affected by the nuclear milieu.

Preferential repair of DNA damage on the transcribed strand of the human metallothionein genes requires RNA polymerase II

To examine the possible role of transcription in directing repair of DNA damage in active genes, we compared repair of UV- and aflatoxin B1-induced damage on each strand of the human metallothionein (hMT) genes. Repair on the transcribed strand of an active hMT gene occurs at least 3 times faster than that on its nontranscribed strand. Both strands of inactive genes and both strands of a regulatory region 5' to an active gene are not repaired at this faster rate. Inducing higher levels of transcription with dexamethasone selectively increased the rate of repair on only the transcribed strand of the induced gene, while treatment of cells with alpha-amanitin eliminated the strand-selective repair. These results demonstrate that repair on the transcribed strand of a gene is independent of repair on the nontranscribed strand and that the transcriptional complex plays a role in directing repair to the transcribed strand of active genes.

Lack of sequence-specific removal of N-methylpurines from cellular DNA

The removal of N-methylpurines from the DHFR gene and an unexpressed adjacent locus located downstream occurs at similar rates and to a similar extent in dimethyl sulfate treated Chinese hamster ovary B11 cells. Furthermore, no significant differences in repair rates are observed between the strands of the active gene. These data primarily reflect the removal of the most abundant lesion produced by dimethyl sulfate, 7-methylguanine, and are in contrast to the results obtained for the removal of ultraviolet-induced cyclobutane pyrimidine dimers from the same region of the genome. Pyrimidine dimers are cleared preferentially from the transcribed strand of the DHFR gene and are removed poorly from the non-transcribed complementary strand and unexpressed adjacent regions. The results suggest that DNA lesions such as dimers that block transcription are removed preferentially from active genes, whereas lesions that do not interfere with nucleic acid synthesis (i.e. 7-methylguanine) are removed at similar rates from expressed and silent loci.

Evolution of chromosome bands: molecular ecology of noncoding DNA

Giemsa dark bands, G-bands, are a derived chromatin character that evolved along the chromosomes of early chordates. They are facultative heterochromatin reflecting acquisition of a late replication mechanism to repress tissue-specific genes. Subsequently, R-bands, the primitive chromatin state, became directionally GC rich as evidenced by Q-banding of mammalian and avian chromosomes. Contrary to predictions from the neutral mutation theory, noncoding DNA is positionally constrained along the banding pattern with short interspersed repeats in R-bands and long interspersed repeats in G-bands. Chromosomes seem dynamically stable: the banding pattern and gene arrangement along several human and murine autosomes has remained constant for 100 million years, whereas much of the noncoding DNA, especially retroposons, has changed. Several coding sequence attributes and probably mutation rates are determined more by where a gene lives than by what it does. R-band exons in homeotherms but not G-band exons have directionally acquired GC-rich wobble bases and the corresponding codon usage: CpG islands in mammals are specific to R-band exons, exons not facultatively heterochromatinized, and are independent of the tissue expression pattern of the gene. The dynamic organization of noncoding DNA suggests a feedback loop that could influence codon usage and stabilize the chromosome's chromatin pattern: DNA sequences determine affinities of----proteins that together form----a chromatin that modulates----rate constants for DNA modification that determine----DNA sequences. Theories of hierarchical selection and molecular ecology show how selection can act on Darwinian units of noncoding DNA at the genome level thus creating positionally constrained DNA and contributing minimal genetic load at the individual level.

Differential repair of DNA damage in the human metallothionein gene family

We studied the repair of UV- and aflatoxin B1 (AFB1)-induced damage in the human metallothionein (hMT) gene family. After exposure to either UV or AFB1, DNA damage was initially repaired faster in the DNA fragments containing the transcribed hMT-IA, hMT-IE, and hMT-IIA genes than in the genome overall. By 6 h posttreatment, there was at least twice as much repair in these genes as in the rest of the genome. Repair of UV damage in the hMT-IB gene, which shows cell-type specific expression, and in the hMT-IIB gene, which is a nontranscribed processed pseudogene, was about the same as in the rest of the genome, whereas repair of AFB1-induced damage was deficient in these two genes. Inducing transcription of the three expressed hMT genes with CdCl2 or of only the hMT-IIA gene with dexamethasone increased the initial rate of repair in the induced genes another twofold over the rate observed when they were transcribed at a basal level. The rates of repair in the hMT-IB and hMT-IIB genes were not altered by these inducing treatments. Transcription of the hMT genes was transiently inhibited after UV irradiation. Inducing transcription of the genes did not shorten this UV-induced delay. Thus, the efficiency of repair of damage in a DNA sequence is dependent on the level of transcriptional activity associated with that sequence. However, an increased efficiency in repair of a gene itself is not necessarily coupled to recovery of its transcription after DNA damage.

Repair of N-methylpurines in specific DNA sequences in Chinese hamster ovary cells: absence of strand specificity in the dihydrofolate reductase gene

We have developed a quantitative method for examining the removal of N-methylpurines from specific genes to investigate their possible differential repair throughout the genome. Chinese hamster ovary cells were exposed to dimethyl sulfate, and the isolated DNA was treated with an appropriate restriction endonuclease. The DNA was heated to convert remaining N-methylpurines to apurinic sites to render them alkaline-labile. Duplicate samples heated in the presence of methoxyamine to protect the apurinic sites from alkaline hydrolysis provided controls to assess total DNA. After alkaline hydrolysis, agarose gel electrophoresis, Southern transfer, and probing for the fragment of interest, the ratios of band intensities of the test DNA sample to its methoxyamine-treated control counterpart were calculated to yield the percentage of fragments containing no alkaline-labile sites. The frequency of N-methylpurines was measured at different times after dimethyl sulfate treatment to study repair. We found no differences between the rates of repair of N-methylpurines in the active dihydrofolate reductase gene and a nontranscribed region located downstream from it in treated cells. Also, similar rates of repair were observed in the transcribed and nontranscribed strands of the gene, in contrast to previous results for the removal of cyclobutane pyrimidine dimers. Thus, there does not appear to be a coupling of N-methylpurine repair to transcription in Chinese hamster ovary cells. However, the repair in the dihydrofolate reductase domain appears to be somewhat more efficient than that in the genome overall. Our method permits the quantifying at the defined gene level of abasic sites or of any DNA adduct that can be converted to them.

Differential repair of DNA damage in the human metallothionein gene family

We studied the repair of UV- and aflatoxin B1 (AFB1)-induced damage in the human metallothionein (hMT) gene family. After exposure to either UV or AFB1, DNA damage was initially repaired faster in the DNA fragments containing the transcribed hMT-IA, hMT-IE, and hMT-IIA genes than in the genome overall. By 6 h posttreatment, there was at least twice as much repair in these genes as in the rest of the genome. Repair of UV damage in the hMT-IB gene, which shows cell-type specific expression, and in the hMT-IIB gene, which is a nontranscribed processed pseudogene, was about the same as in the rest of the genome, whereas repair of AFB1-induced damage was deficient in these two genes. Inducing transcription of the three expressed hMT genes with CdCl2 or of only the hMT-IIA gene with dexamethasone increased the initial rate of repair in the induced genes another twofold over the rate observed when they were transcribed at a basal level. The rates of repair in the hMT-IB and hMT-IIB genes were not altered by these inducing treatments. Transcription of the hMT genes was transiently inhibited after UV irradiation. Inducing transcription of the genes did not shorten this UV-induced delay. Thus, the efficiency of repair of damage in a DNA sequence is dependent on the level of transcriptional activity associated with that sequence. However, an increased efficiency in repair of a gene itself is not necessarily coupled to recovery of its transcription after DNA damage.

Effects of 3-aminobenzamide on the rejoining of DNA-strand breaks in mammalian cells exposed to methyl methanesulphonate; role of poly(ADP-ribose) polymerase

The effect of 3-aminobenzamide (3AB), an inhibitor of poly(ADP-ribose) polymerase, on DNA-repair processes has been investigated after treating V79 hamster cells with methyl methanesulphonate (MMS). Repair activity was observed as changes in DNA-strand break levels. MMS induces transient strand breaks, the level of which slowly decreases with time. Addition of 3AB leads to a rapid increase in the number of breaks. The level of breaks increases linearly with time until it suddenly levels off. Increasing the concentration of 3AB does not change the slope of this curve, but the steady-state level of breaks increases. The incision-rejoining kinetics indicates that 3AB induces a delay in the strand-break rejoining process. In the absence of 3AB the breaks have a lifetime of 1-2 min and this is increased by a factor of 5 in the presence of 5 mM 3AB.

DNA repair by articular chondrocytes. IV. Measurement of Micrococcus luteus endonuclease-sensitive sites by alkaline elution in rabbit articular chondrocytes

The ability of resting and dividing rabbit articular chondrocytes to repair low doses of ultraviolet (UV) damage was measured through removal of UV endonuclease-sensitive sites (ESS, pyrimidines dimers) as measured by alkaline elution. The repair of damage was significantly (P less than 0.001) greater in dividing than non-dividing cells. An age-related decrease in repair capability was found in resting chondrocytes, but not in their dividing counterpart. These results support earlier findings of unscheduled DNA synthesis by the same cells. (Mech. Ageing Dev., 32: 39-55, 1985).

Processing of psoralen adducts in an active human gene: repair and replication of DNA containing monoadducts and interstrand cross-links

We have examined DNA repair in the dihydrofolate reductase (DHFR) gene in cultured human cells treated with 4'-hydroxymethyl-4,5',8-trimethylpsoralen (HMT) using a newly developed assay for interstrand DNA cross-linking in defined genomic sequences. Within 24 hr, 80% of the cross-links, but only 45% of the monoadducts, were removed from a 32 kb transcribed sequence, demonstrating that repair efficiency in an active human gene varies with the nature of the damage. HMT monoadducts were also detected in the replicated DHFR sequence at frequencies indicating little interference with replication. The existence of cross-linkable monoadduct sites in the replicated DNA implies strand continuity opposite those sites and a relatively error-free mechanism of bypass. Translesion replication could circumvent transcription blockage in a damaged gene. These findings have important implications for mechanisms of mutagenesis and DNA lesion tolerance in human cells.

Production of thymine glycols in DNA by radiation and chemical carcinogens as detected by a monoclonal antibody

In order to understand the role in carcinogenesis of damage indirectly induced by chemical carcinogens, it is important to identify the primary DNA lesions. We have measured the formation and repair of one type of DNA modification, 5,6-dihydroxydihydrothymine (thymine glycol), following exposure of cultured human cells to the carcinogens N-hydroxy-2-naphthylamine or benzo(a)pyrene. The efficiency of production of thymine glycols in DNA by these carcinogens was compared to that by ionizing radiation and ultraviolet light. Thymine glycols were detected using a monoclonal antibody against this product in a sensitive immunoassay. We found that thymine glycols were produced in DNA in a dose dependent manner after exposure to the carcinogens and that their production was reduced if either catalase or superoxide dismutase or both were present at the time of treatment. The efficiency of thymine glycol production following exposure to the chemical carcinogens was greater than that following equi-toxic doses of radiation. Thymine glycols were efficiently removed from the DNA of human cells following treatment with either the chemical carcinogens, ionizing radiation or ultraviolet light.

Cell-cycle-dependent repair of damage in alpha and bulk DNA of monkey cells

Excision repair of bulky chemical adducts in alpha DNA of confluent cultures of African green monkey cells has previously been shown to be deficient relative to that in the overall genome. We have compared the removal of adducts produced by treatment with aflatoxin B1 (AFB1) and N-acetoxy-2-acetylamino-fluorene (NA-AAF) from alpha DNA sequences in synchronized and exponentially growing cultures of monkey cells. Proficient removal of AFB1 adducts in alpha DNA was observed in exponentially growing cultures. However, as the cultures approached confluence, adduct removal from alpha DNA became deficient. Cells synchronized by subculturing confluent cultures exhibited proficient removal of adducts from both alpha and bulk DNA when treated in early G1 or late S/G2 while those cells treated in early S phase did not remove adducts from either alpha or bulk DNA. We conclude that the accessibility of chemical adducts to repair in alpha chromatin is influenced by the growth state and the cell cycle stage.

Rates of heat-induced DNA purine alterations in synthetic polydeoxyribonucleotides

Damage to DNA by heat can occur at physiological conditions. The effects of the varying conformational states adopted by double-stranded DNA on the incidences and distributions of thermally induced hydrolytic purine alterations are unknown. The possible role of conformational changes on damage by heat to purines in DNA polymers was therefore investigated. Model compounds used were the synthetic alternating copolymer poly(dG-dC):poly(dG-dC) and the homopolymer poly(dG):poly(dC). Base damages were assayed by high performance liquid chromatography using polymers radioactively labeled in guanine. Conformational states were assayed by circular dichroic spectral changes. Incubation and heating of the polymers in 1 mM Mn2+ caused the spectral shift reported for the left-handed Z-DNA conformation in the alternating copolymer and the change reported for the triple helix in the homopolymer. After incubation at 85 degrees C., incidences of base damages were compared between the polymers. No deamination of guanine to xanthine was observed under any conditions. The presence of manganese reduced depurination in both polymers. Rates of guanine imidazole ring openings to yield 2,6-diamino-4-hydroxy-5-formamidopyrimidine were increased in the presence of the cation and constituted the chief form of purine damage in the homopolymer. Therefore, the distribution of heat-induced DNA alterations within the genome may be determined by DNA conformational states. This observed opening of purine imidazole rings in the presence of manganese ions may have mutagenic consequences and may be involved in carcinogenesis by metals.

Nonrandom binding of the carcinogen N-hydroxy-2-acetylaminofluorene to repetitive sequences of rat liver DNA in vivo

We have examined the distribution of individual adducts in repetitive DNA sequences of rat liver in vivo after a single dose of the carcinogen N-hydroxy-2-acetyl-aminofluorene. Repetitive fragments [82, 125, 179, 225, and 370 base pairs (bp)] were isolated by digestion of hepatic DNA with HindIII restriction endonuclease (EC 3.1.23.21) and gel electrophoresis. As assayed by 32P postlabeling, no qualitative differences were observed between the DNA-bound metabolites in repetitive sequences and total DNA, but preferential binding to these sequences occurred. After 1 day of treatment, the amounts of N-hydroxy-2-acetylaminofluorene-induced adducts were found to be 13.8, 2.0, and 3.0 times higher in 179-, 225-, and 370-bp repeats, respectively, than in total DNA, while 82- and 125-bp repeats showed no differences. The relative distribution of individual adducts varied among the various sequences. After 9 days, all five sequences showed 1.3-1.7 times higher binding as compared to total DNA. In contrast, a random binding was observed when DNA reacted in vitro with an active metabolite, N-acetoxy-2-acetylaminofluorene. Taken together, these results suggest that the enrichment and differential excision of adducts in the repetitive DNA sequences may be a function of the nuclear organization of DNA. This application of the 32P assay constitutes a means to study the DNA damage and excision repair in vivo in chromatin structural components, including transcribed and nontranscribed multiple-copy genes, in a much more sensitive and precise way than has been hitherto possible.

Formation and repair of furocoumarin adducts in alpha deoxyribonucleic acid and bulk deoxyribonucleic acid of monkey cells

We have extended our previous finding that excision repair of furocoumarin photoadducts is deficient in the highly repetitive alpha DNA sequences in cultured African green monkey cells. The formation and removal from DNA of the individual photoadducts of 4'-(hydroxymethyl)-4,5',8-trimethylpsoralen (HMT) were monitored by analysis of DNA hydrolysates using a high-pressure liquid chromatography procedure that separated the major adducts from each other and also resolved the two diastereomers of the most frequent monoadduct. The overall deficiency in removal of HMT adducts from alpha DNA was similar to that previously observed by us with 4'-(aminomethyl)-4,5',8-trimethylpsoralen and angelicin. The two diastereomers of the furan-T monoadducts were formed in the same relative amounts in alpha DNA and bulk DNA whether photoaddition was in vivo or in vitro, and they were removed from cellular DNA at the same relative rates. Therefore, the deficient removal of furocoumarin adducts from alpha cannot be due to preferential formation of some adduct inherently refractory to repair. However, in vivo, the photochemical conversion of the furan-T monoadducts to diadducts was markedly reduced in alpha DNA, relative to that in bulk DNA. This indicates a possible conformational constraint in the internucleosomal DNA in alpha-chromatin which may account for the deficiency in repair.