Guanidine groups of arginine side chains were selectively chemically modified with 1,2-cyclohexanedione (CHD) in sturgeon (Acipenser güldenstädti Br.) gonadotropic hormone (GTH) and in its subunits. It was found that only two of the six guanidines were accessible for the reagent and each of the two modified groups was bound to an individual subunit. The results showed that both modified groups were located on the surface of the hormone dimeric molecule. CD-spectroscopy of the modified hormonal preparations did not indicate any considerable changes in their secondary structure. On the basis of the data obtained, a conclusion was made that the free guanidine groups are of exclusive importance for the hormone function at the receptor level as the bearers of the positive charge in the functionally important active sites or effector zones located on the surface of the hormone molecule. Also, it was shown that the guanidine groups played a certain role in sustaining the functionally effective spatial structure of the subunits and GTH.
