THE REGULATION OF FATTY-ACID CHAIN ELONGATION IN RAT-LIVER MICROSOMES- ROLE OF FASTING AND COASH

The elongation system for palmitic acid in rat liver microsomes was de creased to 1/3 by fasting, while the elongation of eicosapentaenoic ac id was not sensitive to fasting. The rate of eicosapentaenoic acid elo ngation in the fed state was 50% higher than using palmitic acid as a substrate. The saturated and polyunsaturated fatty acyl-CoA substrates exhibited positive cooperativity on the rate-limiting condensing step in the elongation system, with a Hill constant of approx. 2. An inhib ition by CoASH on the total elongation reaction as well as on the cond ensation step was demonstrated using acyl-CoA substrates, and followed a hyperbolic pattern. The concentrations giving a 50% inhibition (30- 70 mu M) were in the range found in rat hepatocyte cytosol, indicating that free CoASH has the potential to act as a physiological regulator .