The Biosynthesis of β-Hydroxy-β-methylglutaryl Coenzyme A in Yeast

When the β-hydroxy-β-methylglutaryl coenzyme A-condensing enzyme system of yeast is incubated with unlabeled acetoacetyl coenzyme A and acetyl coenzyme A is labeled with 14C in the coenzyme A portion of the molecule, no radioactivity can be detected in β-hydroxy-β-methylglutaryl coenzyme A. When unlabeled acetyl coenzyme A and acetoacetyl coenzyme A labeled with tritium in the coenzyme A portion are the substrates, the resultant β-hydroxy-β-methylglutaryl coenzyme A retains the radioactivity. These results show that the thioester linkage of acetoacetyl coenzyme A remains intact during the condensation reaction. When the enzyme is incubated with [1-14C] acetyl coenzyme A, a protein-bound form of acetate can be isolated. Protein-bound β-hydroxy-β-methylglutaric acid could not be detected when the enzyme was incubated with substrates.