ENZYMATIC-SYNTHESIS OF CAROTENES

The pathway of carotene biosynthesis from isopentenyl pyrophosphate in tomato fruit plastias has been established, except for minor details, through the conversion of radioactive substrates and intermediates to acyclic and cyclic carotenes in cell-free systems. After the pathway of carotene biosynthesis was established attention was turned to the isolation and purification of enzymes of carotene biosynthesis. An enzyme that converts isopentenyl pyrophosphate to phytoene has been isolated from an acetone powder of tomato fruit plastids and partially purified. This enzyme behaves as a complex on Biogel filtration. It dissociates, though, into two or more components on DEAE-cellulose chromatography. One of these synthesizes geranylgeranyl and prephytoene pyrophosphates from isopentenyl pyrophosphate. This enzyme has been purified to homogeneity and some of its characteristics and the reactions it catalyzes have been determined. Since this protein yields only one band on SDS gel electrophoresis, it is a polyfunctional peptide. Studies on the enzymes converting phytoene to lycopene have been minimal. It is known, from tracer studies, that one cis-trans isomerization and four dehydrogenations occur in these reactions. Whether five or fewer distinct enzymes occur is not known. Neither is it known whether the enzymes for this series of reactions are separable or whether they exist in a complex. Cofactor studies on the system converting phytoene to lycopene indiβcate that NADP+, FAD and Mn++ are required for these reactions. Studies on the conversion of lycopene to cyclic carotenes indicate that two separate enzymes are involved in the formation of α-ionone and β-ionone rings. This conclusion is based upon the finding of some tomato strains which have an enzyme system that forms only β-ionone ring-containing carotenes, whereas others have an enzyme for the formation of both of these compounds. It is not certain whether the introduction of a β-ionone ring into ϒ-and δ-carotenes to yield β- and α-carotenes requires two separate enzymes or whether this reaction is effected by a single enzyme. Cofactor studies have shown that FAD is necessary for the conversion of lycopene to cyclic carotenes. © 1979, Walter de Gruyter. All rights reserved.