Carboxymethylation of methionine residues in bovine pituitary luteinizing hormone and its subunits. Effects on the binding activity with receptor sites and interactions between subunits

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Abstract
The reaction of iodoacetic acid with bovine lutropin (luteinizing hormone [LH]) at pH 3.0 was specific for methionine residues; it was slow and reached its equilibrium after 12 h at 37.degree. C. The number of modified methionine residues increased proportionately with the amount of the alkylating reagent in the mixture. In the presence of a 20-fold molar excess of iodoacetic acid with respect to methionine, essentially all methionine residues in both subunits of bovine LH were carboxymethylated. Studies of various recombinations of modified and native .alpha. and .beta. subunits showed that methionine residues in bovine LH were not essential for interactions between subunits. Various recombinants were characterized by polyacrylamide-gel electrophoresis and gel filtration on Sephadex G-100. Immunological cross-reactivity by radioimmunoassay of the recombinants of modified .alpha. and .beta. subunits was relatively similar to that of the native subunits. The biological activity measured by receptor-site binding of the recombinants of .alpha. and .beta. chains with a total of 3 alkylated methionine residues was less than 5% of the activity of native LH. Recombinants of a modified subunit and a native counterpart subunit regenerated 20-30% of biological activity. At least 1-2 methionine residues in each subunit are probably involved in the hormone-receptor interaction for bovine LH. [Rabbit antiserum was used].

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