Fatty acid synthetase of mammalian brain, liver and adipose tissue regulation by prosthetic group turnover

Abstract

The role of prosthetic group turnover in the regulation of the fatty acid synthetase of mammalian brain, adipose tissue and liver has been studied. The fatty acid synthetase was labeled in the 4'-phosphopantetheine prosthetic group of the acyl carrier protein by administration to rats of calcium [ 3H]pantothenate. By determining the time course of incorporation of radioactivity into both CoA, the precursor for the 4'-phosphopantetheine moiety of acyl carrier protein, and the fatty acid synthetase, it was possible to estimate the rate of prosthetic group turnover. The turnover rate was found to be extremely rapid in all three tissues. In brain, the prosthetic group is replaced completely in 10–11 h and this is an order of magnitude faster than the turnover of the enzyme complex, previously shown to occur with a t 1 4 of 6.4 days. In liver and adipose tissue, the prosthetic group turnover is slightly faster than in brain. Thus, the rapid turnover of the prosthetic group, first shown in Escherichia coli, also occurs in three different mammalian tissues and may represent a general biological phenomenon. Since turnover of the protein of fatty acid synthetase of brain does not change at all in various nutritional states when turnover of liver synthetase changes dramatically, we considered the question of whether the striking turnover of the prosthetic group in these tissues also responded differently to those states. Distinct changes in the efficiency of the exchange of the 4'-phosphopantetheine between CoA and fatty acid synthetase were observed only in the liver but not in the brain. Increased exchange in liver during fat-free feeding was discovered and the earlier observation of diminished exchange during starvation was confirmed. We also evaluated by immunochemical methods the possibility that rapid regulation of fatty acid synthetase is mediated by changes in catalytic efficiency. Quantitative precipitin analyses of liver extracts from animals during the first hours after onset of starvation or upon refeeding after starvation demonstrated that changes in enzyme activity were related entirely to changes in enzyme content. Thus, the data did not support the possibility that fatty acid synthetase activity is decreased by changes in catalytic efficiency due to removal of the 4'-phosphopantetheine prosthetic group or to inhibition by metabolites.