Galactosemic cataractogenesis disrupts intracellular interactions and changes the substrate specificity of choline/ethanolamine kinase.

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.

Effects of xylose on monkey lenses in organ culture: a model for study of sugar cataracts in a primate.

Lenses exposed to high concentrations of xylose in organ culture produce xylitol, and they lose transparency and exhibit other changes characteristic of cataracts. Most previous studies of this model system for cataractogenesis have employed rat or rabbit lenses, where the activity of the enzyme aldose reductase has been definitely implicated as the initiating factor. Since lenses from this species have much higher aldose reductase activity and have other differences relative to human lenses, the relevance of these findings to the human lens is uncertain. To determine the effects of xylose on the lenses of a primate, lenses from the rhesus monkey (Macaca mulatta) were incubated 24-48 hr in control medium or in TC-199 medium containing 30 mM xylose. Xylose caused a general haziness, focal swelling of epithelial cells, and swollen peripheral fiber cells, but the changes were much less pronounced than in rat lenses under similar conditions. Monkey lenses exposed to 30 mM glucose, galactose or xylose accumulated measurable sorbitol, dulcitol or xylitol, respectively, but the amounts were much lower than in rat lenses, perhaps reflecting the lower aldose reductase and higher sorbitol dehydrogenase activities in monkey lenses. The damage to monkey lenses appeared to be limited to the outer layers. In monkey lenses, xylose caused little, if any, change in membrane transport of choline or alpha-aminoisobutyrate, but severely depressed synthesis of phosphorylcholine (P-choline), and increased leakage of P-choline into the culture medium, leading to a decrease in the P-choline concentration within 24-48 hr. In summary, xylose-induced damage to monkey lenses in organ culture is qualitatively similar to that seen in rat lenses, but the changes are much less rapid and severe. Culture of monkey lenses with xylose provides a model system to extend previous studies of sugar cataractogenesis in rats to a species more closely related to humans.