Avian liver 3-hydroxy-3-methylglutaryl-CoA synthase: distinct genes encode the cholesterogenic and ketogenic isozymes.

Full length cDNA (1.85 kb) coding for an avian liver 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase has been isolated and sequenced. The cDNA isolation relied on hybridization to a 32P-labeled oligonucleotide coding for a portion of the active site of HMG-CoA synthase. The identity of the avian liver cDNA was confirmed by comparison of the deduced amino acid sequence with experimentally determined protein sequence data generated upon isolation and analysis of several cysteine-containing tryptic peptides prepared from the purified ketogenic avian liver enzyme. Structural comparisons with the hamster enzyme also support this assignment. In liver, two distinct forms of HMG-CoA synthase exist to support cholesterogenic and ketogenic pathways, although this latter pathway accounts for most of the enzyme activity. In order to determine which isozyme is encoded by the isolated avian liver cDNA, the deduced amino acid composition, protein sequence, and pI have been compared with the corresponding protein chemistry data that were experimentally determined using the ketogenic enzyme. Results of these comparisons unambiguously indicate that the cDNA encodes the avian liver cholesterogenic enzyme. Observed differences between deduced and empirically determined sequence data rule out the possibility that differential splicing of a primary transcript derived from one gene can account for both isozymes.