Precipitin-Reactions of Human Seminal Plasma
Authors: Ross Victor 
Published: January 2023

Discussion and Summary
The experimental data collected appear to show that the almost equally large precipitates which form on adding ASr and ASp antisera are produced by different proteins in this fluid. In the case of the ASr antisera the largest part of the precipitate is due to the relatively small quantity of serum-albumin present in semen. Chemical and electrophoretic studies previously reported which showed that this protein was present to the extent of a few hundredths of one per cent are supported by the serological data. Because of this low concentration ASp antisera contain correspondingly small amounts of precipitins for albumin, and when added to blood-serum or serum-albumin solutions produce very small precipitates. Part of the small precipitate with blood-serum results from the serological relationship between serum-globulins and two specific antigenic proteins present in seminal plasma. These have been referred to in an earlier publication as P2 and P3 with electrophoretic mobilities of −2.9 and −4.6 respectively. There is probably a third (P2a) with a mobility of −3.8. It is these components that contribute to the formation of the greatest part of the precipitate which forms when ASp antisera are added to seminal plasma. The P2 and P3 components resemble blood-serum globulins as evidenced by the cross-precipitation. Chemically also there is a resemblance. P2 and P3 are soluble in dilute acid and dilute alkali and in NaCl solution although a greater concentration of this salt is required than in the case of the blood-proteins. They have water-soluble and water-insoluble fractions. Two-thirds saturation with ammonium sulfate is necessary to precipitate them out of seminal plasma. They differ from serum-globulins in not being heat-coagulable. Heating seminal plasma from which the glycoprotein has been removed by the addition of acetic acid in the cold produces little or no precipitate. They are probably not readily denatured, since heating a suspension of the freshly precipitated protein at 100 C for ½ hour left it soluble in N/10 HCl and N/10 NaOH. They also have different sedimentative rates (ca 2.5 and 5) from those of the serum-globulins. Their electrophoretic mobilities are somewhat different from those of the serum globulins. Some of these chemical and physical data suggest that at least P2, and possibly P3 as well, may be a proteose of larger molecular size than P1 proteose.
The considerable proportion of all the protein of seminal plasma which passes through a viscose-membrane with an average pore-size of 25Å appears to be non-precipitinogenic, reacting neither with ASr nor ASp antisera. It has a low molecular weight, evidenced by the observation that there was no noticeable sedimentation in the ultracentrifuge (165,000 × g) after 2 hours.
Another component of seminal plasma, the glycoprotein, likewise was not precipitated by the antisera. This protein possesses a very large molecular weight since its sedimentation constant S20 was found to be about 7 (S units) and its diffusion-constant about 0.4 (D20 × 107). Its ultraviolet spectrum (7) shows little selective absorption, although there is a slight inflection at 285 mμ. This, and the positive Millon and xanthoproteic reactions which we have previously reported, indicate the presence of tyrosine or tryptophane.