Microfluorometric assessment of the DNA-DNP complex in human spermatozoa

During spermiogenesis, the DNA-nucleoprotein complex undergoes alterations that are reflected in a decreasing capacity for binding DNA-specific dyes, such as ethidium bromide (EB). Human spermatozoa with a low or high capacity for EB binding were depleted of RNA and most nuclear proteins by exposure to RNAse, EDTA and trypsin, with and without additional high salt buffer (HSB) treatment. When treated with RNAse, EDTA and trypsin only, the haploid DNA fluorescence value (calculated from the diploid value of the standard cell population) was found at EB concentrations of 6.5 to 12.5 micrograms/mL. At these EB concentrations, a significantly lower fluorescence was found in the material also treated with HSB, probably reflecting an unwinding of the highly negatively supercoiled DNA loops that are induced by HSB treatment. Maximal fluorescence was not found until a concentration of 50 micrograms EB/mL. This may be due to an overwinding of the DNA by the positive supercoiling caused by EB. The significant difference in EB uptake initially found between the two groups whose spermatozoa showed low and high capacities for EB binding disappeared after removal of the nucleoproteins, suggesting that this compartment of the nucleoprotein-DNA complex is responsible for the different uptakes of EB.