Stereospecific studies on carotenoid biosynthesis.

Various species of [2-14C] MVA stereospecifically labelled with tritium at C-2, C-4 and C-5 have been used to study details of carotenoid biosynthesis in a variety of preparations including chioroplasts, fruit and root slices, and fungi. Mechanisms have been proposed for the dimerization of geranylgeranyl pyrophosphate to form phytoene, for the cyclization of acyclic precursors to form aand a-carotene derivatives, for the formation of 3-hydroxy-carotenoids and for the formation of retro carotenoids. The stereochemistry of the desaturation of phytoene has been partly elucidated. INTRODUCTION General reviews of the biosynthesis of carotenoids have recently appeared'2'19 and, in the paper immediately preceding this one20, Porter has reviewed the enzymology of the various steps in detail and has described the important advances recently made in his laboratory. In this article the progress made in more clearly defining the details of carotenoid biosynthesis with the aid of various species of stereospecifically labelled mevalonic acid (MVA) will be discussed. This approach was initiated in the study of sterol biosynthesis by Popjàk and Cornforth'8. They synthesized [2-'4C-,4R,-3H1] MVA [1] and [2-14C-4S-3H,] MVA [2] and showed that in liver the isomerization of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP) occurs with the stereospecific loss of the pro-R CH OH CH OH HOOC\ ,C CH2OH HOOC /C CH2OH CH2 "c' "CH2 HI 1' H MVA 4-R-configuration MVA4-S-configurction [21 hydrogen from C-2 of IPP, that is the pro-S-hydrogen arising from C-4 of MVA (equation 1). The same stereochemical loss of hydrogen occurs when farnesyl pyrophosphate (C-15) is formed by the addition of two further IPP molecules. This is illustrated in equation 2 by the formation of geranyl pyrophosphate. Thus when two molecules of FPP react to form squalene 483