Carbohydrates

One of the most remarkable features of semen is that the predominant reducing sugar is not glucose, as in blood, but fructose (Mann 1946a,b), a sugar more usually found in plants. Small amounts of glucose are also present, and boar semen in particular contain large concentrations of inositol, but less fructose than semen from bulls or rams (Mann, 1951). Stallion semen also contains inositol and lower concentrations of fructose (Baronos, 1951; Mann et al., 1963), and other compounds of inositol are also present in some species (Seamark et al., 1968). Fructose in bulls and rams originates in the seminal vesicles, with some from the ampulla, but in the stallion, most comes from the ampulla. Inositol is secreted in the seminal vesicles (Mann and Lutwak-Mann, 1981).

Proteins, amino acids and other nitrogen-containing compounds

Seminal plasma contains a variety of proteins and peptides, the total concentration being somewhat less than that in blood plasma (Mann and Lutwak-Mann, 1981). Seminal plasma proteins are derived from the epididymis and the accessory glands, and are involved in several essential steps preceding fertilization, including capacitation, establishment of the oviductal sperm reservoir, modulation of the uterine immune response, sperm transport in the female tract and gamete interaction and fusion (Calvete et al., 1994; Topfer-Petersen et al., 2005; Karekoski et al., 2011).

Some proteins are higher in the semen of fertile bulls, whereas others are more abundant in the semen of bulls of lower fertility (Killian et al., 1993; Bellin et al., 1998; Brandon et al., 1999). In stallions, the abundance of some proteins (kallikrein-1E2, clusterin and seminal plasma proteins 1 and 2 (SP1 and 2) are negatively related to fertility, whereas cysteine-rich secretory protein 3 (CRISP3) is positively related (Novak et al., 2010).

Other proteins are involved in sperm–egg interactions and cell cycle regulation (Gaviraghi et al., 2010). Identified proteins include leptin and insulin-like growth factor I (IGF-I; Lackey et al., 2002) and phospholipidbinding proteins involved in sperm membrane lipid modification during capacitation (Manjunath and Therien, 2002). Seminal plasma from stallions contains SSP-7 (stallion seminal protein 7, also known as horse seminal protein 7 – HSP-7), a member of the spermadhesin protein family that is involved in the sperm binding to the zona pellucida of the oocyte (Reinert et al., 1997), and there are also heparin-binding proteins, which modulate capacitation (Miller et al., 1990; Nass et al., 1990; Bellin et al., 1994). Other proteins inhibit in vitro and cooling-induced capacitation (Vadnais and Roberts, 2010) and the ability of sperm to penetrate zona-free oocytes (Henault et al., 1995; Henault and Killian, 1996), as well as sperm transport and elimination (Troedsson et al., 2005), sperm longevity (Karekoski and Katila, 2008) and storage in the oviduct (Gwathmey et al., 2006). Seminal plasma from pigs contains high concentrations of transforming growth factor β (TGF-beta), an important immune deviating agent (Robertson et al., 2002).

Seminal plasma also contains considerable concentrations of free amino acids, particularly glutamic acid in rams and bulls (Setchell et al., 1967; Brown-Woodman and White, 1974) and hypotaurine in boars (Van der Horst and Grooten, 1966; Johnson et al., 1972). Hypotaurine may be important in preventing damage to sperm by reactive oxygen species (Alvarez and Storey, 1983; Bucak et al., 2009).

There are also appreciable concentrations of carnitine in the seminal plasma of rams (Brooks, 1979), bulls (Carter et al., 1980) and stallions (Stradaioli et al., 2004). This substance is involved in fatty acid transport in other tissues, but that present in semen is largely derived from the epididymis (Hinton et al., 1979). Boar semen also contains ergothioneine, the betaine of thiolhistidine, a sulfur-containing reducing base, which comes mainly from the seminal vesicle (Mann and Leone, 1953); it is also present in stallion semen, but in this species, it originates largely from the ampulla (Mann and Lutwak-Mann, 1963).

Semen and seminal plasma from rams, bulls, goats, boars and stallions were found to contain considerable amounts of glycerophosphorylcholine, which originates largely from the epididymis (Dawson et al., 1957; Brooks 1970), as well as glycerylphosphorylinositol.

Semen contains considerable amounts of lipid, both neutral lipids and phopholipids, most of which is in the spermatozoa (Hartree and Mann, 1959). In ram semen, the most abundant phospholipid is choline plasmalogen (also known as phosphatidalcholine), whereas in boars, it is lecithin (also known as phosphatidylcholine) and in bull sperm, the two phospholipids are present in approximately equal amounts (see Mann and Lutwak-Mann, 1981). One remarkable feature of these phospholipids is their high concentration of highly unsaturated fatty acids, 22 carbons in length, with six double bonds (22:6) in rams and bulls and five double bonds (22:5) in boars (Johnson et al., 1969; Poulos et al., 1973; Evans and Setchell, 1978). These constituent fatty acids are particularly susceptible to damage from reactive oxygen species. The phospholipids may also be important precursors of platelet activating factor (PAF), which is probably involved in sperm motility, the acrosome reaction and fertilization, and which is found in bull and boar sperm (Parks et al., 1990; Roudebush and Diehl, 2001). Seminal plasma from bulls and stallions contains an acetylhydrolase, which may play a role in regulating autocrine or paracrine functions of PAF (Parks and Hough, 1993; Hough and Parks, 1994).

Semen also contains appreciable concentrations of steroids. In bull semen, the concentrations of several steroids, including progesterone, dihydrotestosterone, androstanediols and oestrogens are much higher than in blood plasma. The oestrogens appear to come from the prostate, whereas the other steroids originate from the epididymis. Testosterone is present in seminal plasma at about the same concentration as in blood plasma, much less than in the rete testis fluid leaving the testis (Ganjam and Amann, 1976).