Effects of thiolation of amino groups of ovine lutropin on immunoreactivity, receptor binding and bioactivity.

We have studied the reaction of epsilon-amino groups of ovine lutropin with N-succinimidyl-3-(2-pyridyl-dithio) propionate (SPDP) and observed that modification of up to 1/3 of all available residues primarily occurs in the alpha-subunit. The 12 lysines available could be classified into three categories, as highly, moderately and less accessible. The three lysine residues of the hormone that appeared to resist modification may be in the intersubunit contact region. Sequential modifications of lysine residue in the alpha-subunit led to progressive reduction in receptor binding and immunological activities, but not in full steroidogenic potential. The discrepancy between reduced receptor binding and full steroidogenic activity may be related to the introduction of addition hydrophobicity to the ovine lutropin molecule. Total inactivation resulted only when the majority (9/12) of available amino groups were altered by this reaction. Thus, our study shows that by a limited reaction it is possible to create conjugation site(s) in ovine lutropin with significant retention of hormonal activity.