DELTA(6) HEXADECENOIC ACID IS SYNTHESIZED BY THE ACTIVITY OF A SOLUBLE DELTA(6) PALMITOYL-ACYL CARRIER PROTEIN DESATURASE IN THUNBERGIA-ALATA ENDOSPERM

Delta(6) Hexadecenoic acid (16:1 Delta(6)) composes more than 80% of t he seed oil of Thunbergia alata. Studies were conducted to determine t he biosynthetic origin of the double bond of this unusual fatty acid. Assays of fractions of developing T. alata seed endosperm with [1-C-14 ]palmitoyl (16:0)-acyl carrier protein (ACP) revealed the presence of a soluble Delta(6) desaturase activity. This activity was greatest whe n 16:0-ACP was provided as a substrate, whereas no desaturation of the coenzyme A ester of this fatty acid was detected. In addition, Delta( 6)16:0-ACP desaturase activity in T. alata endosperm extracts was depe ndent on the presence of ferredoxin and molecular oxygen and was stimu lated by catalase. To further characterize this enzyme, a cDNA encodin g a diverged acyl-ACP desaturase was isolated from a T. alata endosper m cDNA library using polymerase chain reaction with degenerate oligonu cleotides corresponding to conserved amino acid sequences in Delta(9)s tearoyl (18:0) and Delta(4)16:0-ACP desaturases. The primary structure of the mature peptide encoded by this cDNA shares 66% identity with t he mature castor Delta(9)18:0-ACP desaturase and 57% identity with the mature coriander Delta(4)16:0-ACP desaturase. Extracts of Escherichia coli that express the T. alata cDNA catalyzed the Delta(6) desaturati on of 16:0-ACP. These results demonstrate that 16:1 Delta(6) in T. ala ta endosperm is formed by the activity of a soluble Delta(6)16:0-ACP d esaturase that is structurally related to the Delta(9)18:0- and Delta( 4)16:0-ACP desaturases. Implications of this work to an understanding of active site structures of acyl-ACP desaturases are discussed.