Rat lens choline and ethanolamine kinases: independent kinetics in intact tissue-competition in homogenates

In the committed step of phosphatidylcholine and phosphatidylethanolamine synthesis, choline and ethanolamine are phosphorylated to form phosphocholine and phosphoethanolamine. Studies of rat tissues in several laboratories attribute these two kinase activities to a single enzyme for which choline and ethanolamine are competing alternative substrates. However, in this study of intact cultured lenses, choline and ethanolamine were phosphorylated independently, and neither compound inhibited phosphorylation of the other, even at high lenticular concentrations. In contrast, choline kinase in lens homogenates was competitively inhibited by ethanolamine (Ki = 2.3 mM), and choline strongly inhibited ethanolamine kinase activity. The results suggest a fragile metabolic compartmentation or organization of kinase enzyme(s) or substrates within the intact, physiologically integrated lens which results in striking changes in kinetic characteristics when the biological organization of the lens is disrupted.