A range of cis- and trans-monoenoic fatty acids was tested as substrates for desaturation in microsomal preparations from rat liver. Trans-monoenoic acids were generally desaturated in the delta 9 position to the same extent as stearic acid. Acids with delta 7-trans- and delta 11-trans-olefinic unsaturation produced delta 7-trans, 9-cis- and delta 9-cis, 11-trans-conjugated dienoic acids, respectively, but the delta 8-trans-and delta 10-trans-monoenoic acids did not give delta 8,9- or delta 9,10-allenes. Of the cis-monoenoic acids examined, only those with double bonds at or beyond the delta 14 position gave any measurable delta 9 desaturation. When delta 9 desaturation of long chain saturated acids was inhibited by adding sterculic acid, these saturated acids were desaturated at the delta 5 and delta 6 positions. Many of the monoenoic acids tested were also desaturated at the delta 5 and/or delta 6 positions, although the percentage conversions were always low. delta 9-cis, 11-trans-, delta 9-cis, 12-trans- and delta 9-cis, 13-trans-dienoic acids, produced in situ by delta 9 desaturation of the corresponding monoenoic acids, were extensively desaturated in the delta 6 position. These results are discussed in terms of: (a) the various models proposed to explain the substrate specificities of the desaturases, and (b) the metabolism of unnatural fatty acids ingested from dietary sources.

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1. A study was made of the stereospecificity of hydrogen removal in the sequential desaturations performed by intact cells of Chlorella vulgaris in the biosynthesis of oleic acid, linoleic acid and alpha-linolenic acid. 2. By use of erythro- and threo-9,10-(2)H(2)-, -12,13-(2)H(2)- and -15,16-(2)H(2)-labelled precursors, it was demonstrated that the pair of hydrogen atoms removed from each of these positions had the cis relative configuration. 3. That the hydrogen atoms removed in oleic acid and linoleic acid formation were of the d absolute configuration was proved by use of d- and l-9-(3)H-and -12-(3)H-labelled precursors. 4. The presence of a substantial kinetic isotope effect of deuterium at both positions of the putative double bond was indicated, suggesting that the mechanism of desaturation involves simultaneous concerted removal of the pair of hydrogen atoms.

The oils from sclerotia or from suitable mycelial cultures ofClaviceps purpurea (ergot) contain up to 44% of ricinoleic acid but no free hydroxyl groups. This is due to the presence of, besides normal triglycerides, tetra-acid, penta-acid and hexa-acid triglycerides. These contain respectively one, two and three ricinoleic acids esterified to glycerol, these in turn being acylated at their hydroxy groups with normal long-chain fatty acids. By suitable complementary use of TLC, GLC and lipase hydrolysis techniques, the proportions, compositions and structures of these novel triglyceride classes were determined. Four types of positional specificities in fatty acid combinations could be shown by our procedures. These are discussed and, on the basis of our results, some tentative proposals as to possible biosynthetic mechanisms are advanced.