Compartmentation of ATP:Citrate Lyase in Plants

Extracts prepared from young leaves of Pea (Pisum sativum), tobacco (Nicotiana tabacum), rape (Brassica napus), and spinach (Spinacia oleracea) all contained ATP:citrate lyase (ACL) activity, which was most active in rape leaflets (130 nmol min(−1) g fresh weight). In rape and spinach, ACL activity was predominantly localized in the plastids (between about 78% and 90% of the total activity), whereas in pea and tobacco, distribution was mainly cytosolic (about 85% and 78%, respectively, of the total). These distributions were calculated from the relative distributions of plastid and cytosol marker enzymes. Cross-reactivity between plant and rat ACL antibody was carried out by immunoblot analysis and, in rape and spinach, showed that a 120-kD protein, presumably indicating homomeric ACL proteins, was present in both cytosolic and plastidic fractions. In pea, two cross-reacting proteins were detected, the major material being in the cytosol fraction. Therefore, ACL occurs both in the cytosol and plastids of higher plants, but the distribution of activity changes according to the species. The plastidic ACL is proposed to function for the supply of acetyl-coenzyme A for lipid biosynthesis de novo, whereas the cytosolic ACL may provide acetyl-coenzyme A for the mevalonate pathway or fatty acid elongation.