Structure of asteroid bodies in the vitreous of galactose-fed dogs

Purpose: Asteroid hyalosis is a condition in which white spherical particles (asteroid bodies) are suspended in the vitreous, usually in the dependent part of the vitreous. These particles seldom cause serious visual symptoms; however, their presence can be a source of irritation. It has been suggested, but not confirmed, that asteroid hyalosis may be associated with systemic diseases such as diabetes, hyperlipidemia, or hypertension. Studies indicate that these particles are composed of lipid material and calcium; however, the specific composition and structure of asteroid bodies remains unknown. We have observed that asteroid hyalosis occurs in galactose-fed dogs, and this represents the first animal model which consistently forms this vitreal condition. The purpose of this study was to identify the main structural component of the asteroid bodies present in the vitreous of these dogs. Methods: Vitreous humor containing asteroid bodies was collected and frozen from long-term galactose-fed beagles and from age-matched normal controls where asteroid bodies were absent. A portion of the frozen vitreous was sent out for elemental analysis. Thawed vitreous samples were sonicated with HPLC grade water and the aqueous layer was extracted three times with chloroform and then three times with n-butanol. The three organic layers from each extraction were combined and the solvents removed in vacuo. The residue from each extraction was re-dissolved in methanol and analyzed by electrospray ionization mass spectrometry (ESI-MS). Results: Vitreous-containing asteroid bodies had significantly higher levels of calcium and phosphorus. Negative mode ESI-MS analysis of the n-butanol extracts from vitreous samples with and without asteroid bodies were similar with both containing a predominant peak with a mass to charge ratio (m/z) of 538.4. However, similar analyses of the chloroform extracts indicated that three peaks with m/z values of 547.1, 690.5, and 1430.6 were present only in vitreous samples containing asteroid bodies. Subtraction analysis indicated that the m/z of 690.5 peak corresponded to the main component present. This peak was identified and confirmed to be the quasimolecular ion of 1,2-dipalmitoyl-glycero-3-phosphoethanolamine (DPPE). Conclusions: Based on the current belief that asteroid bodies are composed of lipid-calcium complexes, we propose that the main component of asteroid hyalosis in the galactose-fed dog is a quasimolecular ion of DPPE in which two molecules of DPPE are complexed through their phosphates groups with calcium.