This study was initiated to determine the relationship between the fertilizing potential of spermatozoa and abnormalities in the compact packing of their chromatin which occurs in the final stage of male germ cell differentiation. Chromatin packing involves disulphide bridge covalent cross-linking. The degree of packing was determined from the accessibility of DNA to a fluorescent dye, ethidium bromide, following detergent treatment of the spermatozoa. The amount of dye bound was determined by flow cytometry in the presence or absence of heparin, a polyanion which removes only non-disulphide bridge-linked proteins. The results of a number of different sperm samples were compared with their results following in-vitro fertilization, and a relationship between disordered sperm chromatin packing and rates of embryo cleavage was observed. This study suggests that abnormal chromatin packing in spermatozoa may contribute to male fertility.
The phenomenon of active dissociation of the noncovalently binding vital dye Hoechst 33342 from DNA in living cells (DNA clearing) is described. Step-by-step selection with increasing concentrations of the dye resulted in a series of rodent cell lines that were resistant to the toxic action of Hoechst 33342. Some of the lines exhibited enhanced dissociation of the bisbenzimidazole dye–DNA complex. Two cell lines from this group (AA8HoeR-7 and LHoeR-3) were analysed in detail and compared with a Syrian hamster tumour cell line, a typical example of mdr-1-mediated multidrug-resistant cell lines. The markedly enhanced level of DNA clearing in AA8HoeR-7 and LHoeR-3 cells leads to high cellular resistance to the toxic effect of Hoechst 33342 and cross-resistance to mitomycin C, a minor-groove alkylating agent in clinical use. Our results suggest that DNA clearing is one of the mechanisms of multidrug resistance in tumour cells.
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