Human seminal plasma inhibits brain nitric oxide synthase activity

Nitric oxide is a chemical messenger which functions as a neurotransmitter or as a cytotoxic agent. Nitric oxide synthase (NOS) has been isolated from various mammalian reproductive tissues. The presence or absence of NOS in spermatozoa has not yet been reported. We therefore tested human and murine spermatozoa for NOS activity by measuring the conversion of arginine to citrulline. No activity was found either in human or in murine spermatozoa. Human native semen and human seminal plasma exerted an inhibition on brain NOS activity, as assayed on rat brain cytosolic fractions. This inhibitory effect was dependent on the amount of protein present in the human seminal plasma. No inhibitory effect was observed when homogenates of washed spermatozoa were tested. The human seminal plasma did not affect the Michaelis constant (Km) of NOS for L-arginine (endogenous NOS substrate) whereas the maximal velocity (Vmax) was reduced, suggesting that it contains a non-competitive inhibitor of brain NOS. This inhibitory component was virtually insensitive to heat; a 10 min treatment to 95 degrees C only slightly reduced its ability to inhibit brain NOS. The physiological relevance of our observations remains to be elucidated. Human seminal plasma may exert an inhibition of nitric oxide synthesis on cells other than spermatozoa or on cells from the male or female genital tract, modulating directly or indirectly (via modulation of reactive oxygen species formation) the functional state of the spermatozoa.