The cell-free synthesis of the alpha subunit of human chorionic gonadotropin.

The synthesis of the alpha subunit of human chorionic gonadotropin (hCG) was demonstrated in a cell-free system composed of polysomes derived from first trimester placenta and cell sap prepared from ascites tumor cells. The in vitro synthesized proteins labeled with [35S]methionine were shown to have at least 4 tryptic peptides that co-migrated with the same peptides from authentic hCG. Sodium dodecylsulfate-polyacrylamide gel electrophoresis revealed the synthesis of a discrete protein, migrating with an apparent molecular weight of about 17,000, which contained methionine-labeled tryptic peptides found in the alpha subunit. The level of radioactivity in these tryptic peptides was five times greater with polysomes from first trimester placentae than with those from term placentae. The efficiency of total protein synthesis in both cases was about the same. These data strongly suggest that the decrease in blood levels of hCG after the first trimester is caused by a selective decrease in the rate of synthesis of the hormone.