Endogenous protein carboxyl methylation in hamster spermatozoa: changes associated with capacitation in vitro

Protein carboxyl methylase (PCM) and its substrate(s), methyl acceptor protein(s) (MAP), are present in the spermatozoa of rat, rabbit and man. In the present study, PCM activity and MAP capacity were measured in homogenates of hamster testes and isolated spermatozoa, and found to be similar to those of other species. Using solubilized preparations of spermatozoa from the cauda epididymis of hamster, PCM activity was twice as high in sperm tails as in heads while the reverse was true for MAP capacity. Since in this and in previous studies this enzyme reaction has been measured in broken cells, we thought it pertinent to measure methylation under physiological conditions (i.e., in motile spermatozoa). To accomplish this, an assay was developed which depends upon the conversion of [3H-methyl]methionine to S-adenoxyl-L [methyl-3H]methionine which, in turn, serves as a methyl donor for PCM. In sperm that had been labelled with [3H]methionine the MAP for the endogenous methylation reaction was not solubilized with Triton X-100, and was found primarily in sperm tails. When hamster spermatozoa were incubated in medium containing taurine, epinephrine and bovine serum albumin to induce capacitation, endogenous methylation was stimulated 8- to 9-fold; when taurine was omitted from this medium, methylation was stimulated 14-fold. Taurine, however, was essential for in vitro fertilization as the number of eggs fertilized declined from 92% in complete medium to 0% in medium minus taurine. The decrease in fertilization rate was also associated with a decrease in sperm motility. In previous studies we have demonstrated an association between PCM and sperm motility, and have suggested that this enzyme system could be involved in other functions such as the acrosome reaction. From the results in the present study we conclude that the conditions that lead to capacitation in vitro are associated with a marked change in endogenous protein carboxyl methylation.