The heterogeneity of human chorionic gonadotropin (hCG). II. Characteristics and origins of nicks in hCG reference standards.

hCG, the hormone produced by the trophoblast throughout pregnancy, has peptide bond cleavages, or nicks, in the beta-subunit. We sought to compare the nature of these nicks in standard reference preparations of hCG, to determine the enzymes that may be responsible for generating the peptide bond cleavages, and to devise means of separating nicked from intact hormone. The standard reference preparations of hCG, which are purified from a commercial product made from large pools of pregnancy urine, were found to have varying concentrations of nicked hormone. The preceding report showed that 11 of 13 hCG preparations isolated from individual pregnancy urine samples were nicked at the beta 47-48 bond, with 2 of 13 having a second nick at beta 44-45. As shown here, all of the hCG reference standards are nicked to similar extents at both the beta 47-48 bond and the beta 44-45 bond. The percentage of peptide bond nicking in the various hCG standard preparations ranged from 10-20% and appeared higher in the more recent preparations. We showed that human leukocyte elastase is capable of specifically cleaving the beta 44-45 bond, and in extended digests it can also cleave the beta 48-49 and beta 51-52 peptide bonds. Thus, human leukocyte elastase may be the origin of some of these cleavages in the individual samples and the reference standards. Furthermore, we report that a monoclonal antibody directed to hCG alpha-beta dimer binds preferentially to nonnicked hCG and much less to nicked hCG.