Biochemical Properties of Equine Chorionic Gonadotropin from Two Different Pools of Pregnant Mare Sera

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Abstract
Equine chorionic gonadotropin (eCG, PMSG) has been isolated from two different pools (high and low titer) of pregnant mare sera using the methodology previously employed for the isolation of eCG. Gonadotropin from both pools of sera exhibit a similar apparent molecular weight (Ve/Vo = 1.2–1.3) on gel exclusion chromatography. Chemically, the two types of eCG are different in both amino acid and carbohydrate composition. Low titer serum eCG (eCGLT) has significantly less carbohydrate (25%) than does high titer gonadotropin (eCGHT) (46%). Phenylalanine and serine were detected as major amino termini in high titer eCG in comparison to phenylalanine and leucine in low titer gonadotropin. Immunologically, high titer eCG is considerably more active (9750 IU/mg) than low titer eCG (2850 IU/mg). Both in vitro and in vivo bioassays for LH and FSH also show that purified low titer eCG has a lower specific activity (1500–5700 IU/mg) than does high titer serum gonadotropin (3450–13,050 IU/mg). Both biologically and chemically, low titer eCG more closely resembles eCG isolated from trophoblastic tissue or from equine chorionic cell culture medium, whereas high titer eCG resembles eCG previously isolated from serum. Our results demonstrate the natural existence in pregnant mare serum of different forms of eCG, which differ not only chemically but in biological and immunological potency.