Carboxyterminal peptide fragments of the beta subunit are urinary products of the metabolism of desialylated human choriogonadotropin.

Previous investigations of patients with gestational trophoblastic neoplasia have shown that their urines often contain carboxyterminal peptide (CTP) fragments of the choriogonadotropin (hCG) beta-subunit as well as forms of hCG deficient in sialic acid. In order to determine whether beta-CTP fragments are among the urinary products of the peripheral degradation of desialylated hCG (as-hCG), using a continuous infusion technique, we gave highly purified as-hCG to humans. Six healthy subjects were given a loading dose of 0.8 mg of as-hCG followed by an infusion of the same preparation. An overall mean infusion rate of 62.9 micrograms/min was maintained for 6 h, and the mean serum concentration of as-hCG achieved during the infusion was 72.1 ng/ml. In all six subjects, beta-CTP fragments were the predominant immunoreactive forms of as-hCG in urine obtained during the infusion. In contrast, the urine of subjects infused with hCG has been shown to contain hCG itself, but nil beta-CTP fragments or as-hCG. After the as-hCG infusion was stopped, the excretion of the beta-CTP fragments in urine declined rapidly. There were no beta-CTP fragments detectable in sera obtained during the infusion or in sera incubated with as-hCG at 37 degrees C. After incubation with as-hCG for 4 h, the urine of normal subjects contained small amounts of beta-CTP fragments; however, the apparent proteolytic activity was too low to account for either the quantity of beta-CTP fragments produced during the infusion or the extremely low levels of as-hCG in the urine. These data demonstrate the existence in humans of a peripheral metabolic pathway that cleaves beta-CTP fragments from as-hCG and allows their excretion in urine. Thus, the frequent presence of beta-CTP fragments in the urines of patients with gestational trophoblastic neoplasia can be accounted for in part by the metabolism of the forms of hCG that bear an altered carbohydrate structure, which are prevalent in this disease.