From wiki:
In human prostatic secretions, the protein content is less than 1% and includes proteolytic enzymes, prostatic acid phosphatase, and prostate-specific antigen. The secretions also contain zinc with a concentration 500-1,000 times the concentration in blood.
Other than that I believe the prostate's secretions are alkaline which helps neutralize the acidity in the vagina... - how essential it is not entirely sure...