Stereochemistry at the Center of Squalene during Its Biosynthesis from Farnesyl Pyrophosphate and Subsequent Conversion to Cholesterol.

Recent studies on cholesterol biosynthesis have demonstrated that the biosynthesis of squalene from its immediate precursor, farnesyl pyrophosphate, is accompanied by the exchange of 1 hydrogen atom at C-l, in 1 of the 2 precursor molecules, for a hydride ion derived from reduced triphosphopyridine nucleotide (l-3). Thus, Popjak et al. obsrrved that the farnesol of farnesyl pyrophosphate biosynthesized from mevalonate-2J4C-5-Dz contained 6 atoms of deuterium (2), whereas squalene biosynthesized from the same presursor contained only 11 atoms of deuterium (1). Three of these deuterium atoms were attached to the central 2 carbon atoms of squalene (1). Furthermore, squalene formed from I-D2-2X-trans ,trans-farnesyl pyrophosphate contained only 3 atoms of deuterium, attached to the 2 central carbon atoms of the molecule (2). The fourth hydrogen present at the center of the squalene was derived entirely from TPNH, and not at all from the water of the incubation medium (1). Further studies from the same laboratories have indicated that this exchange of 1 hydrogen atom during biosynthesis of squalene is a stereospecific process. This was suggested by the absence of isotope discrimination during squalene formation from l-3H2-2%Xrans, truns-farnesyl pyrophosphate (2). Subsequently it was shown that the hydrogen transferred to the center of squalene is entirely transferred from the “P,’ position of TPNH (4). This report is concerned with the stereochemistry of this hydrogen transfer with respect to squalene. Squalene was biosynthesized from *4C-farnesyl pyrophosphate with rat liver microsomes and TPN3H, and then directly converted to cholesterol. The 2 central carbon atoms of squalene, which become labeled during squalene biosynthesis from farnesyl pyrophosphate with TPNSH, appear in cholesterol as carbon atoms 11 and 12. Determination of the stereochemistry and distribution of the tritium label in cholesterol confirmed the fact that the hydride ion transfer is a stereospecific reaction, and allowed assignment of the absolute configuration of the asymmetrical carbon atom formed by introduction of the tritium label.