Hormone-specific inhibitory influence of alpha-subunit Asn56 oligosaccharide on in vitro subunit association and follicle-stimulating hormone receptor binding of equine gonadotropins

Abstract

Hybrid hormones were created using combinations of equine (e) LH, eFSH, and eCG alpha- and beta-subunit preparations. The efficiency of eFSH beta association was highest with eLH alpha (64-72%) and was lowest with eCG alpha (37-50%). Selective removal of alphaAsn56 oligosaccharide increased heterodimerization efficiency by 9-20% for eLH alpha, by 21-28% for eFSH alpha, and by 28-41% for eCG alpha. Both alpha and beta subunits contributed significantly to FSH receptor-binding activities of the hybrids. Purified hybrid hormone preparations consisting of either eFSH beta or eLH beta combined with eLH alpha, eFSH alpha, or eCG alpha were prepared. Equine FSH beta hybrids were more active in the FSH radioreceptor assay than eLH beta hybrids; within each beta-subunit group the eLH alpha hybrids were the most active, followed by eFSH alpha hybrids, while the least active were eCG alpha hybrids. A truncated, des(121-149) eLH beta derivative (eLH beta t) combined with native alpha-subunit preparations exhibited the same effect of alpha-subunit type on FSH receptor binding. Hybrids combining the eLH beta t derivative with Asn56-deglycosylated (N56dg-)eLH alpha, N56dg-eFSH alpha, and N56dg-eCG alpha preparations possessed 2.2- to 4.3-fold increased FSH receptor-binding activities as compared with the same hybrid preparations possessing the Asn56 carbohydrate. Granulosa cell bioassay of purified native eFSH beta and eLH beta hybrid hormones indicated no significant effect of the alpha-subunit carbohydrate differences on progesterone production. The alpha-subunit Asn56 oligosaccharide exerts a hormone-specific inhibitory influence on in vitro subunit reassociation and FSH receptor binding related to the size of its Man(alpha1-6)Man antenna.