| Abstract: | Beef heart mitochondrial beta-hydroxybutyrate dehydrogenase forms a catalytically active complex with lecithin and is inactive in the absence of lecithin. The specificity of the activation process was probed by studying the interaction of the enzyme with phospholipids and other compounds. The compounds were tested for their ability to form active complexes with the enzyme, for the stability of the complex formed, and for the correlation between the activator concentration and the level activation. The phospholipids tested were synthetic lecithins varying in the length (C2 to C18) and degree of unsaturation of the aliphatic chains and in the stereochemistry and type of linkage from the aliphatic chain to the glycerol moiety, synthetic and egg yolk lysolecithins, stearylphosphorylcholine, egg yolk phosphatidylethanolamine, egg yolk phosphatidyl-O-serine, and synthetic cardiolipins. Lecithins, lysolecithins, and stearylphosphoryl-choline form active complexes with the enzyme; the L-alpha-diC4:0 is the smallest lecithin forming an active complex and L-alpha-C12:0 is the smallest lysolecithin. Glycerophosphorycholine, mytistoylcholine, N-trimethyl-n-dodecylamine, decamethonium, sodium dodecyl sulfate, Triton X-100, and Lubrol do not activate the enzyme. A hydrophobic chain followed sequentially by a negative and a positive charge, as in stearylphosphorylcholine, is the minimal structural requirement of an activator. However, the stability of the enzyme-activator complex depends strongly on the aggregation state of the activators, complexes of appreciable stability being formed only with those phospholipids which exist in bilayer membrane-like structures. Thus, lecithins with long aliphatic chains (C9 to C18) form active and stable complexes with the enzyme. The maximal activity and the strength of the lipid-protein interactions depend on the nature of the aliphatic chains of the lipids. Lecithins with saturated and unsaturated fatty acid chains activate the enzyme, but the latter form somewhat more stable complexes. The enzyme-activator interactions in the bilayers can be qualitatively understood in terms of competition between lipid-lipid and lipid-protein interactions: the strength of the interaction between the protein and phosphatidylcholines decreases as the crystalline to amorphous phase transition temperature, which is a measure of the strength of lipid-lipid interactions, increases... |
