Unsaturated fatty acid biosynthesis in Tetrahymena. Evidence for two pathways

The ciliate Tetrahymena pyriformis synthesizes a wide variety of saturated and unsaturated fatty acids. Variations in growth temperature or the addition of sterols such as cholesterol or ergosterol alter the proportions of a number of unsaturated fatty acids. The pattern of substitution is complex when examined on the basis of individual fatty acids. A straightforward explanation is possible, however, if biosynthetic groupings are considered. Stearic acid gives rise to oleic, linoleic, and gamma-linolenic acids as shown by others. The environmental parameters influence the amounts of the unsaturated derivatives of stearic acid as a group and result in an increase or decrease in all members of this biosynthetic family in a fashion that depends on a particular set of conditions. The replacement of the stearate derivatives by unsaturated components that are derived from palmitic acid has been demonstrated. The addition of radiolabeled palmitic, palmitoleic, or stearic acids to the cells when coupled with radioisotope distribution measurements, isolation, and characterization of the acids, and the refeeding of key intermediates substantiates a second biosynthetic scheme for the synthesis of unsaturated acids in this ciliate. This novel pathway yields an unusual fatty acid, 18:2 delta6,11, as a major terminal product. Plamitic acid is the precursor for the members of both sequences. Palmitoleic acid can be desaturated further to produce two hexadecadienoates and a hexadecatrienoate. Further, palmitoleic acid (16:1 delta9) is elongated to cis-vaccenic acid (18:1 delta11) which is then desaturated to 18:2 delta6,11. Linoleic and gamma-linolenic acids were essentially unlabeled when [14C]palmitoleic acid was provided. Refeeding radiolabeled 18:2 delta6,11 to the cells revealed the extensive incorporation of this acid into the polar lipids and the absence of additional metabolites. [14C]Stearic acid addition to the cells results in extensive labeling of linoleic and gamma-linolenic acids, but not of 18:2 delta6,11. These observations confirm the existence of alternative and separate pathways for the synthesis of unsaturated fatty acids.