Reaction of ovine interstitial cell stimulating hormone with citraconic and maleic anhydrides

Structure and structure-function relationships in glycoprotein hormones

Relationships between the sequences of thyrotropin (thyroid-stimulating hormone, TSH), lutropin (luteinizing hormone, LH), human choriogonadotropin (chorionic gonadotropin, hCG) and follitropin (follicle-stimulating hormone, FSH) are now well established. Each beta-subunit contains six disulphide bonds and considerable homology is seen when all four linear sequences are aligned with half-cystine residues in juxtaposition. Major questions about the tertiary structures of the subunits and their interactions to form active hormone remain. Determination of the disulphide bridges in both alpha- and beta-subunits has not yielded to usual methods and conflicting data about the alpha-subunit have been reported. Partial reduction of the beta-subunits of LH and TSH with subsequent labelling of the cysteines formed has shown that a single bond is first reduced. This bond is between positions 93 and 100 in LH-beta and the corresponding positions 88-95 in TSH-beta. Thus, as would be expected from the fact that interhormone hybrids can be made with the common alpha-subunits, the chemical data, though still limited, indicate similar tertiary structures for the different beta-subunits. To investigate whether other useful intermediates can be obtained after partial reduction, we have studied reduction and derivative formation in various conditions. Intact LH is more resistant to reduction than either its alpha- or beta-subunit but no intermediates have been observed which are not present after partial reduction of individual subunits. Preliminary experiments on the reoxidation of fully reduced alpha-subunit show that the reoxidized material will recombine with native beta-subunits to yield biologically active TSH or LH. Studies from this and other laboratories on chemical modifications of several amino acid residues of glycoprotein hormones and their subunits are also summarized.

Effect of amino group modification of ovine luteinizing hormone (oLH) by N-succinimidyl 6-[3-(2-pyridyldithio)propionate]hexanoate, a long chain N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP) on immunological and biological properties: a comparative study with SPDP modified oLH

The epsilon-NH2 groups of ovine luteinizing hormone has been modified with the long chain N-succinimidyl-3-(2-pyridyl dithiopropionate (LC-SPDP). The LC-SPDP modification primarily occurs in -NH2 groups of the alpha-subunit. Although, the sequential modification of lysine residue in alpha-subunit led to progressive reduction in the receptor binding and immunological properties but the steroidogenic activity was relatively unaffected. The immunoreactivity and receptor binding properties of LC-SPDP modified oLH molecule were more affected comparative to SPDP modified derivatives. This suggested that the increase in hydrophobic carbon chain in LC-SPDP-oLH molecules resulted into the drastic inhibition in the immunological and biological properties. However, the steroidogenic potential of LC-SPDP/or SPDP-oLH derivative was comparable. The present study clearly demonstrate that a single -NH2 group modification with LC-SPDP would generate the site for the conjugation to the toxin/carrier proteins and the resultant oLH-S-S-toxin conjugate would retain significant immunological and biological properties of the hormone molecule.

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.

Hormonotoxins. I. Strategy for synthesis of ovine luteinizing hormone--gelonin conjugate bearing the toxin in the beta-subunit

The amino groups in the beta-subunit of ovine luteinizing hormone (oLH) were modified by thiolation using N-succinimidyl-3-(2-pyridyldithio) propionate so that it may be coupled in a disulfide linkage to similarly modified ribosome inactivating protein, gelonin. The modified beta-subunit was able to hybridize with free LH alpha-subunit and the complex retained full biological activity. However, when gelonin was coupled to the beta-subunit, the resulting conformational changes masked or eliminated the sites necessary for intersubunit recognition of the free alpha-subunit. This has important implications for the design in the synthesis of gonadotropin-toxin/drug conjugates.

Human chorionic gonadotropin: acetylation of tyrosyls with N-acetylimidazole

When human chorionic gonadotropin (hCG) was treated with a 20-fold molar excess of N-acetylimidazole in aqueous solution, two tyrosyls were acetylated, resulting in a 50% reduction in in vivo biological activity. With increased quantities of reagent, the number of tyrosyls acetylated increased but with no further decrease in the biological activity. In vitro biological activity (binding to hCG receptors) of the hormone was not affected at all. Acetylation in urea increased tyrosyl modification, accompanied by acetylation of some lysyls, yielding a product in which all seven tyrosyls were acetylated and both in vivo and in in vitro biological activities were completely abolished. Deacylation with hydroxylamine partially restored biological activity of some but not all of the modified products. When individual hCG subunits were treated with the same reagent, the number of tyrosyls acetylated in each subunit again increased with increasing amounts of reagent, up to three in the alpha subunit and two in the beta subunit in the absence of urea. The tyrosyls in the beta subunit appeared less reactive to the reagent than those in the alpha subunit. Subunits modified to these extents retained ability to recombine as examined by gel electrophoresis, but the recombined products varied considerably in both in vivo and in vitro biological activities. A completely tyrosyl-acetylated product of hCG-alpha did not combine with intact hCG-beta, while fully modified hCG-beta did with intact hCG-alpha.

Preparation of lutropin with acetyl or acetimidinyl substituents on the amino groups of the beta-subunit

The free amino groups in ovine lutropin beta subunit were acylated with acetic anhydride and methyl acetimidinate-HCl to produce the corresponding acetyl and acetimidinyl ovine lutropin beta derivative. These two derivatives recombined with ovine lutropin alpha as well as native ovine lutropin beta, but produced lutropin derivatives which were 33-50% less active than the ovine lutropin alpha + ovine lutropin beta in biological assays.