Presence of a 16 kd protein in human seminal plasma counteracts the effects of the anti-fertility agent, gossypol

Gossypol inhibited lactate dehydrogenase (LDH) non-competitively in human spermatozoa. The inhibitory effect of gossypol on LDH was cancelled by the addition of human serum albumin, human γ-globulin, bovine serum albumin or human seminal plasma. Seminal plasma was at least 10 times more effective than the other three proteins, when expressed on a per mg protein basis. Attempts were made to purify the active fraction from human seminal plasma. The purification steps included gel filtration, ammoniun sulphate precipitation, centrifugal microconcentration and fast-performance liquid chromatography. A single active protein of M_r = 16 000 was purified to a final yield of 0.18%. The 16 kd protein was not observed in male blood plasma. The protein was found to be heat-stable and leucine-rich (16% of the molecule), and has been designated 'gossact'. The inhibitory effect of gossypol on the LDH reaction was completely blocked by the addition of gossact (5 μg/ml); human blood plasma (25 μg/ml) and human serum albumin (200 μg/ml) were far less potent in this assay. In addition, gossact bound 1.4 mol of gossypol/mol of protein with the dissociation constant (K_d = 3.06 × 10^-5 M. The role of gossact in the protection of LDH from gossypol is discussed.