Hormonal Regulation of Hepatic Fatty Acid Synthetase in Chick Embryo

Hepatic fatty acid synthetase activity is low in chick embryo and precociously increases l&,20-, and 12-fold upon administration of insulin, glucagon, and dibutyryl adenosine 3”:5’-monophosphate (Bt&MP), respectively. Immunotitrations of different enzyme preparations from induced and uninduced embryos against anti-synthetase y-globulin gave identical equivalence points, indicating that the changes in enzyme activity after hormonal induction are due to changes in the amount of enzyme protein. The increase in hepatic synthetase content after administration of insulin, glucagon, and Bt,cAMP can be accounted for by a maximal increase in the rate of synthetase synthesis of 19-, 34-, and 16-fold, respectively. Several lines of evidence suggest that the induced enzyme in chick embryo is similar to the homogenous enzyme preparation from fasted-refed chickens. The enzyme preparations from induced embryos had similar chromatographic properties of DEAE-cellulose and Sepharose 6B as the authentic preparation and showed a first order thermal inactivation profile similar to the enzyme preparation from fasted-refed chickens. Double immunodiffusion and immunoelectrophoretic analyses demonstrated the similarity of the synthetase from induced embryos, uninduced embryos, and fasted-refed chickens. Furthermore, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of radioactive immunoprecipitate isolated from induced embryos pulsed with [“Hlleucine showed that all of the radioactivity in the immunoprecipitate was associated with a single band corresponding to the Jf, = 240,000 subunit of the synthetase. This result indicated that the newly synthesized fatty acid synthetase in the induced embryos has identical subunit size as the authentic enzyme. Administration of glucagon in chick embryos resulted in a IO-fold increase in the level of serum insulin followed by an increase in hepatic fatty acid synthetase, suggesting that elevated serum insulin level may be responsible for the induction of fatty acid synthetase in chick embryos receiving glucagon.