On sperm nuclear zinc and chromatin decondensation : an in vitro study on the physiology of the ejaculated human spermatozoon

Abstract

Ejaculated human spermatozoa were studied in vitro with respect to their capacity to decondense the chromatin in sodium dodecyl sulfate (SDS). The content of zinc in sperm heads was studied in epididymal, vasal, and ejaculated human spermatozoa. These were the main results: (1) Soon after ejaculation most spermatozoa decondensed the chromatin in SDS with zinc-chelating EDTA. Only few spermatozoa decondensed in SDS alone. (2) During storage, many spermatozoa lost the capacity to decondense in SDS-EDTA. Half of all spermatozoa lost this capacity within the first hour after ejaculation whether they were washed and stored in a buffered salt solution (BSS) within 20 minutes after ejaculation, or simply stored in the original seminal plasma. During prolonged storage (24 hours) the capacity was better retained among spermatozoa stored in BSS, than in those stored in seminal plasma. Furthermore, among spermatozoa treated with EDTA before storage, the loss of capacity for decondensation during storage was enhanced. In contrast, the initial capacity for decondensation was completely prevented in spermatozoa supplemented with zinc during 24 h storage in BSS. (3) The ejaculated human sperm head contained significant amounts of zinc bound within the nuclear matrix. With EDTA-treatment, 90% of sperm head zinc could be removed soon after ejaculation. After 24 h storage in seminal plasma significantly less zinc could be released by exposure to EDTA. (4) Epididymal and vasal sperm heads had significantly lower contents of zinc than ejaculated sperm heads. (5) The zinc content of ejaculated sperm heads from various portions of split­ ejaculates was neither correlated to the total seminal plasma zinc concentration, nor to the concentrations of measured subfractions of zinc bound to various groups of zinc-ligands. However, most of the variations in sperm head zinc could be explained by variations in total sperm number and concentration of fructose secreted by the seminal vesicles. The results seem to justify the conclusion that the human spermatozoon takes up zinc at ejaculation from the concomitantly expelled prostatic fluid, and that zinc subsequently acts as a reversible stabilizer of the sperm chromatin. The results also imply that inappropriate stabilization by zinc of the sperm chromatin is likely to occur in spermatozoa from men with prostatic dysfunction, men expelling the spermatozoa mainly in vesicular fluid, and men expelling high total numbers of spermatozoa. The possibility is discussed that zinc stabilizes the quarternary structure of the sperm chromatin by chelating between e.g. amino- and thiol-groups of adjacent nucleoprotein fibers. Concomitantly, zinc would protect these thiol-groups from being comitted into superstabilizing disulfide-bridge crosslinks. Thereby would zinc preserve a potential of the chromatin for rapid decondensation in the ooplasm