Isocitric and citric acid in human prostatic and seminal fluid: implications for prostatic metabolism and secretion

Human prostatic secretion is remarkably rich in citric acid but the mechanisms to account for this accumulation are not well understood. One factor may be the extent of citrate oxidation to isocitrate, catalyzed by aconitase. The citrate-to-isocitrate ratio will help characterize the relative significance of this reaction in prostatic production and secretion of citrate. Isocitric acid and citric acid were measured in samples of seminal fluid and expressed prostatic secretion (EPS). A constant ratio between citrate and isocitrate of about 33:1 was found (r = 0.93, P < 0.0001) despite the wide variation in concentrations. Citrate ranged from 1 to 180 mM in EPS and from 13 to 50 mM in seminal fluid while isocitrate varied between 0 to 4.8 mM in EPS and from 0.4 to 1.5 mM in seminal fluid. Isocitrate is present in EPS and semen at much higher levels than found in most other animal or plant tissues or fluids and may be actively secreted by the same mechanism as citrate. The high citrate to isocitrate ratio of about 33:1, compared to the expected value of about 10:1, supports suggestions that citrate to isocitrate oxidation by aconitase is a rate limiting step in prostatic citrate metabolism. A low aconitase activity will therefore play a significant role in enabling accumulation of high citrate levels in prostatic epithelia and acini.