Metabolic Clearance Rates of the Subunits of Human Chorionic Gonadotropin in Man

Highly purified preparations of the alpha- and beta-subunits of hCG (hCG alpha and hCG beta) were injected iv in normal subjects. After rapid injection, the disappearance of both subunits from serum was nonlinear when plotted on a semilog graph. A two-component exponential curve was fitted for each subject, and the curve parameters were used to estimate the MCR, apparent initial volume of distribution (Vd), and half-times of disappearance of the rapid and slow phase for each subunit. The Vd of hCG beta was indistinguishable from that of hCG alpha (1958 +/- 131 vs. 1729 +/- 99 ml/m2, respectively). The rapid phase half-time for hCG beta was significantly longer than that of hCG alpha (41.2 +/- 1.7 vs. 13.0 +/- 0.9 min; P less than 0.001), and the slow phase half-time of hCG beta was also significantly longer than that of hCG alpha (236 +/- 41 vs. 76 +/- 19 min; P less than 0.01). The estimated MCR of hCG alpha was 49.7 +/- 1.6 ml/min.m2; this value was significantly greater than that of hCG beta (19.0 +/- 0.7 ml/min.m2; P less than 0.001). No significant differences between sexes in parameters determined for the subunits were observed. Continuous infusion of subunits at a rate of 2.7 microgram/min achieved a steady state blood level of hCG beta that was significantly greater than that of hCG alpha (59.1 +/- 7.8 vs. 24.4 +/- 0.7 ng/ml; P less than 0.02) and gave a MCR of hCG alpha that was 3 times greater than the MCR of hCG beta (72.2 +/- 4.9 vs. 21.6 +/- 2.8 ml/min.m2; P less than 0.001). We conclude that the hCG subunits have similar Vds, but since hCG alpha has much shorter half-times of disappearance in both rapid and slow phases, the MCR of hCG alpha is much greater than that of hCG beta.