Purification and Structural Characterization of In Vitro Synthesized (γ-Glutamyl) Spermidine Conjugates of a Major Protein Secreted from the Rat Seminal Vesicles

Very little is known about the physiological meaning of spermidine (Spd) and spermine (Spm) occurrence in mammal seminal plasma where polyamines are secreted in large amounts by the prostate (1–3). One postulated role for semen polyamines is to serve as acyl acceptor substrates for transglutaminase (TGase; EC 2.3.2.13). In fact, it has been demonstrated that rat seminal clot proteins have covalently bound polyamines and upon proteolytic digestion released (γ-glutamyl)polyamine derivatives (4). MDreover, N-mono- and N, N-bis-(γ-glutamyl)polyamine derivatives were found after proteolysis of rat vesicular secretion proteins that have been incubated with coagulating gland extracts (5). Some of us have recently shown the ability of one of the major proteins secreted from the rat seminal vesicle epithelium to covalently bind radioactive Spd in the presence of either purified guinea pig liver TGase or crude secretory fluid produced by the rat coagulating gland (6). Such a protein, named SV-IV, is the fourth major band observed on the SDS-PAGE pattern of the proteins present in the rat seminal vesicle secretion. The sequence of its 90 amino acids (Mr= 9, 758) and the main features of the gene coding for it are known (6–9). We have demonstrated that SV-IV possesses immunosuppressive and anti-inflanmatory activities (6, 10, 11). Microgram amounts of SV-IV are able to inhibit both the mitogen-induced lymphocyte blastogenesis and the mixed lymphocyte reaction; the observed anti-inflaitmatory activity, comparable to that of dexamethasone, seems to be due to the block of arachidonic acid cascade at level of the enzyme phospholipase A2.