Regulation of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene expression in liver and intestine from the rat.

The enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase catalyses the condensation of acetoacetyl-CoA and acetyl-CoA to form HMGCoA plus free CoA. The activity of HMG-CoA synthase is located in two different compartments: the cytosol and the mitochondria. The HMG-CoA produced by cytosolic HMG-CoA synthase is transformed into mevalonate by the action of HMG-CoA reductase, thus starting the isoprenoid pathway which, in addition to cholesterol as the main end product, produces dolichol, isopentenyl adenosine, ubiquinone and farnesylated proteins. The HMGCoA produced inside the mitochondria is transformed into acetoacetate by the action of HMG-CoA lyase. Acetoacetate is transformed into u-3-hydroxybutyrate and acetone; all of these are known as ketone bodies. In 1986 Gil et al. reported the cloning and sequencing, first of the cDNA [ l ] and then of the gene for hamster cytosolic HMG-CoA synthase [2]. These authors speculated about whether the gene for the cytosolic synthase would also code for the mitochondrial HMG-CoA synthase. The comparison between the amino acid sequence of the catalytic site for chicken mitochondrial HMG-CoA synthase, obtained by direct sequencing by Miziorko and Behnke [3], with the sequence deduced from the cDNA for hamster cytosolic HMG-CoA synthase suggested that both sequences could be the same in mammals. In addition, Gil et al. observed that the cytosolic HMG-CoA synthase gene had an exon (exon 2) whose appearance in mature transcripts was optional and speculated whether the functional role of this exon could be the targeting of the mRNA to a specific cellular site, most likely the mitochondria [4]. However, the fact that between the two enzymes there were many differences in kinetic and immunological properties and that it was difficult to explain how a gene could regulate the cholesterol and ketone body pathways in opposite directions at the same time under starvation, moved us to test whether this gene really existed, beginning with its cloning.