Molecular dynamics simulation of a tumorigenic benzo[a]pyrene metabolite bound to DNA at a single strand-double strand junction

Molecular modeling of the major benzo[a]pyrene N2-dG adduct in cases where mutagenesis results are known in double stranded DNA

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations (e.g. GC-->TA, GC-->AT, etc.). One hypothesis for this complexity is that different mutations are induced by different conformations of its major adduct [+ta]-B[a]P-N2-dG when bypassed during DNA replication (probably by different DNA polymerases). Previous molecular modeling studies suggested that B[a]P-N2-dG adducts can in principle adopt at least 16 potential conformational classes in ds-DNA. Herein we report on molecular modeling studies with the eight conformations most likely to be relevant to base substitution mutagenesis in 10 cases where mutagenesis has been studied in ds-DNA plasmids in E. coli with B[a]P-N2-dG adducts of differing stereoisomers and DNA sequence contexts, as well as in five cases where the conformation is known by NMR. Of the approximately 11,000 structures generated in this study, the computed lowest energy structures are reported for 120 cases (i.e. eight conformations and 15 examples), and their conformations compared. Of the eight conformations, four are virtually always computed to be high in energy. The remaining four lower energy conformations include two with the BP moiety in the minor groove (designated: BPmi5 and BPmi3), and two base-displaced conformations, one with the dG moiety in the major groove (designated: Gma5) and one with the dG in the minor groove (designated: Gmi3). Interestingly, these four are the only conformations that have been observed for B[a]P-N2-dG adducts in NMR studies. Independent of sequence contexts and adduct stereochemistry, BPmi5 structures tend to look reasonably similar, as do BPmi3 structures, while the base-displaced structures Gma5 and BPmi3 tend to show greater variability in structure. A correlation was sought between modeling and mutagenesis results in the case of the low energy conformations BPmi5, BPmi3, Gma5 and Gma3. Plots of log[(G-->T)/(G-->A)] versus energy[(conformation X)-(conformation Y)] were constructed for all six pairwise combinations of these four conformations, and the only plot giving a straight line involved Gma5 and Gmi3. While this finding is striking, its significance is unclear (as discussed).

Chromosome analysis of human spermatozoa following in vitro exposure to cyclophosphamide, benzo(a)pyrene and N-nitrosodimethylamine in the presence of rat liver S9

For the purpose of assessing mutagenic effects (clastogenicity) of metabolites derived from chemical mutagens/carcinogens on human sperm chromosomes, spermatozoa were exposed in vitro to cyclophosphamide (CP), benzo(a)pyrene (BP) or N-nitrosodimethylamine (NDMA) for 2h in the presence or absence of rat liver S9, a metabolic activator of these chemicals. After in vitro fertilization between human spermatozoa and zona-free hamster oocytes, chromosome complements of sperm origin were analyzed cytogenetically. In the absence of S9, none of three chemicals (20 microg/ml CP, 200 microg/ml BP and 20mg/ml NDMA) caused a significant increase in spermatozoa with structural chromosome aberrations (8.6, 10.0 and 7.5%), as compared with their matched controls (10.9, 11.0 and 8.5%). In the presence of S9, however, a significant increase in chromosomally abnormal spermatozoa was observed in CP (37.1%, P < 0.001) and BP (31.0%, P < 0.001), indicating that enzymatic activation of CP and BP induced chromosomal abnormalities in human sperm. In contrast, NDMA did not induce chromosome aberrations in human spermatozoa by S9 treatment, although positive results have been observed in somatic cells. The present results on in vitro clastogenicity of CP, BP and NDMA are consistent with the results in previous in vivo studies with murine spermatozoa. Our S9/human sperm chromosome assay seems to be useful for estimation of hereditary risk of chemicals in human. Because most chemicals need metabolic activation to bind to DNA.

Involvement of dopamine D1-like receptors in mediating increases in protein secretion from rabbit lacrimal gland

To identify and localize the dopamine receptor subtypes in rabbit lacrimal gland which mediate protein secretion, the effects were determined of different dopamine receptor subtype agonists, antagonists, and a beta adrenergic antagonist on this process. Protein secretion into the medium was quantified with the Bradford assay. Dopamine increased protein secretion between 10(-7) and 10(-4)M, and it could be maintained for a subsequent 80 min. The relatively selective D1-like receptor agonist, SKF-38393 (10(-4)M) had a similar effect which was suppressed by the D1-like receptor antagonist, SCH-23390. However, neither the D2-like receptor agonist, quinpirole (10(-4)M), nor the selective D2-like receptor antagonist, sulpiride (10(-4)M) altered either the basal level or the stimulated response to dopamine. The dopamine (10(-4)M)-elicited increases in protein secretion were completely suppressed in the presence of either 10(-4)M propranolol or 10(-4)M bretylium. Protein secretion in rabbit lacrimal gland is mediated by dopaminergic nerves through stimulation of the presynaptic D1-like receptor.