Association of cystatin C with prostasomes in human seminal plasma

It was recently elucidated that cystatin C, a protein targeted to the classical secretory pathway by its signal peptide sequence, can also be secreted in association with exosomes. Accordingly, we wanted to investigate whether there is a secretory link between cystatin C and prostasomes in human seminal plasma. Cystatin C concentrations in seminal plasma from 50 men including 6 vasectomized men were measured by turbidimetry on an Architect Ci8200. Some of the seminal plasma samples were also analysed utilizing an Epics Profile XL-MCL cytometer. We found high concentrations of cystatin C in seminal plasma. The 2.5-97.5 percentiles, performed by bootstrap estimation, were 25.8 [95% confidence interval (CI): 22.3-29.4] to 77.0 mg/L (95% CI: 71.9-82.1). Cystatin C is present in approximately 50 times higher concentration in seminal plasma compared with blood plasma. There was no clear difference as regards seminal plasma content of cystatin C between vasectomized men and the rest of the group. Immunoblot analysis with chicken anti-cystatin C antibody revealed a firm association of cystatin C with prostasomes. Flow cytometric analysis demonstrated that cystatin C was linked to prostasomes also meaning an at least partial prostasomal membrane surface localization.