Induction of Tyrosine-α-Ketoglutarate Transaminase in Fetal Rat Liver

Abstract

Abstract Decapitation of fetuses in utero rendered rat liver tyrosine transaminase (l-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) occasionally inducible by hydrocortisone. By placing fetal rat liver in organ culture, reproducible induction by the steroid could be obtained. Histological and biochemical studies supported the conclusion that the hepatocytes remained healthy over a 3-day period in culture. The response to hormone was very low in fresh explants for the first 12 hours, but thereafter a 4- to 8-fold elevation in enzyme activity was obtained within 4 to 6 hours. The induced level of the transaminase was maintained unless steroid was removed, and then the activity fell rapidly back toward the control level. The optimal effective concentration of hydrocortisone was 10-6 m, but levels as low as 10-8 m produced a limited response. Only the glucocorticoids and the protein hormones insulin and glucagon were active inducers among a number of hormones tested. Cycloheximide and actinomycin D prevented enzyme induction, but hydrocortisone did not affect either general RNA or protein synthesis. Lactic dehydrogenase activity in explants of fetal liver was unaffected by hydrocortisone. Titrations against antibody showed that the tyrosine transaminases in fetal and adult liver are immunologically identical and proved that the increase in enzyme activity in explants exposed to hydrocortisone was due to an increase in enzyme protein. Immunochemical labeling experiments confirmed that synthesis de novo of enzyme protein is stimulated by hydrocortisone. The results are consistent with the hypothesis that the synthesis of this enzyme is repressed during gestation.