Abstract
Serum has long been known to enhance sperm motility, and the major factor that mediates this activation has been shown to be a macromolecule complex with a MW of about 250 kD.
This macromolecule, named Sperm Activating Protein (SPAP) has now been purified by a four step protocol. Starting with 100 mL of serum 20 to 200 μg of purified SPAP was recovered (N=40). Studies on the identification and structure of SPAP revealed that the macromolecule is formed by an IgG molecule to which an apolipoprotein A-I molecule is bound.
Further studies on the structure of the complex was performed by partial proteolytic cleavage of SPAP by pepsin and papain and interaction with poly- and monoclonal antibodies. Based on these results the most plausible structure of SPAP places one molecule of apo A-I between the two arms of the Fab portion of an IgG4 molecule.
Human spermatozoa incubated with polyclonal anti-SPAP serum and a second FITC-labelled antiserum showed fluorescence at a distinct band at the lower part of the sperm head, indicating that SPAP acts by direct interaction with the spermatozoa.
The SPAP molecule shows a new mechanism mediated by the immunoglobulins, that may be very important for male fecundity.