Probing a membrane matrix regulating hormone action: I. The molecular length of effective lipids

Auxin-induced pea (Pisum sativum) stem section elongation is enhanced at levels of 3 to 40 micromolar by six new classes of alkane derivatives additional to those described earlier, providing that length of their molecules exceeds 20 A. Increasingly longer homologous series of alkyl chlorides, bromides and iodides, alkyl benzenes, dialkyl ethers, and sulfides show a logarithmically linear increase in specific activity above this length, reaching an optimum near 28 to 30 A. Longer dialkyl ethers and sulfides are less effective, while steroids, or alkanes with substituents at both ends, are ineffective.Neither common metabolism nor common physical properties, other than over-all length of active molecules, seem to explain these results. However, the dimensions of the most abundant phospholipid of etiolated peas, 1-palmitoyl-2-linoleoyl-3-sn-phosphatidyl choline are such that a monolayer of this lecithin would contain cavities 20 A long. It is postulated that lipids of this length or longer are active in the pea assay by forcing apart lecithin molecules, changing the charge distribution or chelating properties of a regulatory membrane.Steroid dimensions also match the cavity in lecithin monolayers and steroids would be maintained within such a cavity by hydrogen bonds. The name lipometrin for lipids whose physiological activity varies with length is proposed, among these, substances active in the pea bioassay could be termed oleanimins.