Galactosemic Cataractogenesis Disrupts Intracellular Interactions and Changes the Substrate Specificity of Choline/Ethanolamine Kinase

Abstract

The functional characteristics of enzymes depends upon their environment, and within physiologically intact cells, many metabolic pathways are thought to involve multienzyme complexes and other enzyme–enzyme interactions that increase efficiency and specificity by mechanisms such as channeling of intermediates. A disease such as cataract may change the intracellular environment, but the effects of these changes on enzyme–enzyme interactions can be observed only in relatively intact cells, and in enzymes that have unambiguously different properties in different environments. In intact rat lenses, choline and ethanolamine are phosphorylated independently, with no competition between the two compounds, as the first step of phospholipid biosynthesis. However, disruption of lens structure and intracellular interactions by homogenization leads to a paradoxical change in enzymic properties, causing choline and ethanolamine to become competing alternative substrates of a single enzyme that resembles the purified choline/ethanolamine kinase from liver and other tissues. The properties of ethanolamine kinase in intact cataractous lenses from rats fed a 50% galactose diet for 7–14 days were intermediate between those of intact control lenses and those in lens homogenates. In monolayers of human and dog lens epithelial cells and human retinal pigment epithelial cells ethanolamine kinase was similar to that in intact tissue, showing that the kinetic differences between intact lenses and homogenates result from the intracellular environment, not from artifacts of diffusion, and that they are not exclusive to rats or to lens cells. Results with intact lenses from monkeys, rabbits, pigs and dogs showed some differences between species, but in every case, choline had little or no effect on the phosphorylation of radiolabeled ethanolamine. Further studies will be necessary to determine how the changes in intracellular environment during cataractogenesis affect other enzymes and whether other model systems for cataractogenesis cause similar changes.