The structural specificity of lecithin for activation of purified D-beta-hydroxybutyrate apodehydrogenase.

n-/3-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme which has an absolute requirement for lecithin for enzymic activity. We have studied the activation of the purified apodehydrogenase by a number of lecithin analogues with modifications in either the hydrophobic or polar regions of the molecule in order to map the structural specificity for the lecithin molecule. The apodehydrogenase is activated by all lecithin analogues tested which contain a phosphorylcholine moiety but with modifications in the hydrophobic or glycerol portions of the molecule: 1) D and L stereoisomers of lecithin activate the enzyme equally well; 2) octadecyleicosylphosphorylcholine, a branched alkylphosphorylcholine, activates the enzyme to the same extent as mitochondrial lecithin; the diacylglycerol moiety is therefore not essential to obtain activation; 3) the apodehydrogenase is activated by dialkoxyphosphatidylcholines in which the hydrocarbon chains are ether-linked rather that ester-linked to the glycerol moiety, confirming the lack of specificity of the apodehydrogenase for this part of the lecithin molecule; 4) the apodehydrogenase is activated by phosphono analogues of lecithin in which the glycerol to phosphorus oxygen is missing or replaced isosterically with a methylene group; 5) lysolecithin and sphingomyelin, both of which contain the phosphorylcholine polar moiety, activate the apodehydrogenase, although to a lesser extent than the activation by mitochondrial lecithin. Therefore, although the hydrophobic region is necessary for activation of o-fi-hydroxybutyrate apodehydrogenase, the enzyme does not exhibit specificity for this part of the lecithin molecule, The phosphinate analogue of lecithin, in which both oxygens of the phosphate esters are missing, i.e., with glycerol and with choline, did not activate the apoenzyme. Thus, a decreased separation of the phosphoryl and quaternary ammonium groups or a specific requirement for the oxygen in the phosphorylcholine moiety prevents the activation of the apodehydrogenase by this analogue. The specificity of o-fi-hydroxybutyrate apodehydrogenase for the polar region of the lecithin molecule has