Acylation of amino groups in human chorionic gonadotropin with acid anhydrides

Abstract

7 of the 10 ϵ-amino groups of human chorionic gonadotropin (hCG) were acylated with maleic anhydride, whereas the number of modified ϵ-amino groups with citraconic anhydride varied from 3 to 5. Maleylation of isolated α- and β-subunits of hCG modified 5 and 4 ϵ-amino groups, respectively, but citraconylation was incomplete. In these reactions α-amino groups were completely blocked. Either maleylation or citraconylation reduced the biological activity markedly but the complement-fixing activity of hCG to variable extents. Deblocking of the modified amino groups in the maleylated hCG product restored both biological and immunological activities nearly completely. The modified hCG-α's combined with native hCG-β to yield less amounts of product than obtained with native recombination. However, the yield of recombination products of modified hCG-β's with native hCG-α was about the same as that of the native recombinant. Recombination also took place between the modified hCG-α's and the modified hCGβ's but with lower yields than obtained with the recombination products of native with the modified subunits. The biological activity of these various recombination products varied considerably, ranging from 5% to 53% of that of the native recombinant. There was evidence that spontaneous deblocking of the acylated groups occurred in some of the citraconylated subunits. These results suggest that some amino groups in hCG are essential for the biological activity of this hormone and that amino groups of hCG-α, but not hCG-β, play an important role in subunit—subunit interaction.