The Partial Purification, Properties, and Mechanism of Action of Pig Liver Isopentenyl Pyrophosphate Isomerase.

Numerous reports on the intermediate reactions in the biosynthesis of terpenoids were published after the characterization of mevalonic acid (1, 2) and the establishment of its conversion to squalene (3-8), cholesterol (9, lo), carotenes (ll-14), and rubber (15-17). These reports established the conversion of mevalonic acid to mevalonic 5-phosphate, mevalonic pyrophosphate, and A3-isopentenyl pyrophosphate (18-26). They also established isopentenyl pyrophosphate as the biologically active isoprene unit, as suggested by Ruzicka (27). Proof of the isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate has provided a substrate for the initial condensation reaction in the formation of terpenyl pyrophosphates (28). The condensation of these Cg units yields geranyl pyrophosphate. Further condensations of terpenyl pyrophosphates then result in the formation of farnesyl and geranylgeranyl pyrophosphates. The isopentenyl pyrophosphate isomerase of yeast has been isolated by Agranoff et al. (29), and some of the properties of this enzyme have been reported. A mechanism for the isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate has also been proposed. However, experimental proof for this proposal is still lacking. The present paper reports the partial purification of the isopentenyl pyrophosphate isomerase from a mammalian source (pig liver) and some of the properties of this enzyme. Experiments with tritiated water and 14C-labeled isopentenyl pyrophosphate are also reported. These experiments provide proof of the mechanism of the isomerization reaction. In this reaction a proton is accepted by carbon atom 4 of isopentenyl pyrophosphate and the resultant carbonium ion is then stabilized, possibly through reaction with a thiol group of the enzyme. Subsequent deprotonation at carbon atom 2, and breakage of the thioether bond, result in the formation of dimethylallyl pyrophosphate.