Effect of xylose on the synthesis of phosphorylcholine and phosphorylethanolamine in rat lenses

Choline, an essential phospholipid precursor, enters the lens by a facilitated transport system and is phosphorylated to form phosphorylcholine (P-choline). Intact lenses incubated with [³H]choline accumulate both [³H]choline and P-[³H]choline. The rate and extent of this accumulation have been used to study the effects of osmotic or oxidative cataractogenic stress, and also to test the ability of compounds to protect lenses from stress-related damage. The initial effect of oxidative stress on choline metabolism is decreased choline transport, but the mechanism by which osmotic stress affects the accumulation of [³H]choline is not understood. The effects of osmotic and oxidative stress on choline influx and metabolism were compared in rat lenses incubated in TC-199 medium. After osmotic stress by incubation with 30 mM xylos for up to 24 hr, lenses accumulated the same amount of radiolabel as controls during a 30 min pulse with [³H]choline. However, if the lenses were exposed to [³H]choline for 6 hr so that accumulation of radiolabel in the lenses was limited by the rate of P-choline synthesis, xylose treated lenses accumulated less choline than controls. Separation of the lenticular radiolabel into [³H]choline and P-[³H]choline confirmed that xylose decreased synthesis of P-choline, although adequate unphosphorylated [³H]choline was availabile in the lenses. A decrease in P-[³H]ethanolamine synthesis was also seen in xylose-treated lenses incubated with [³H]ethanolamine. Ethanolamine can enter lenses by a non-saturable process which is not dependent upon a transporter. Although xylose decreased P-choline synthesis in intact lenses, neither xylose nor xylitol inhibited choline kinase in lens homogenates. Xylose also decreased transport of α-aminoisobutyric acid, which enters lenses by a different transporter than choline. In contrast to osmotic stress from xylose, oxidative stress from singlet oxygen decreased choline influx. Thus, the superficially similar effects of osmotic and oxidative stresses on accumulation of radiolabeled choline by cultured rat lenses actually occur by different mechanisms and reflect different types of damage to the lens.