Mechanism of Sulfate Segregation during Glass Melting

ABSTRACTSulfate retention in glass during the vitrification process can be as low as 1/3 of the solubility limit, or can exceed the solubility limit if suspended in the glass in the form of droplets. This study is focused on the mechanism of incorporating and segregating sodium sulfate during the melting of an alkali-alumino-borosilicate glass batch. Batches were ramp heated at 4°C/min to temperatures ranging from 600°C to 1050°C and fractured for examination. Observation of the melts showed that as the batch temperature increases and the primary oxo-anionic, predominantly nitrate melt decomposes, the sulfate residue accumulates inside gas bubbles and is transported in them to the melt surface, where it remains segregated. The degree of sulfate incorporation into the final glass depends on the relative rates of sulfate dissolution in the borosilicate melt and sulfate lifting inside bubbles.

Proteoglycan synthesis is increased in cells with impaired clathrin-dependent endocytosis

Overexpression of a GTPase deficient dynamin mutant in HeLa dynK44A cells causes a block in clathrin-dependent endocytosis. When endocytosis is inhibited, these cells incorporate higher levels of [(35)S]sulfate into both cellular and secreted macromolecules and larger amounts of proteoglycans such as syndecan and perlecan are immunoprecipitated from [(35)S]sulfate-labelled lysates. Gel filtration and ion-exchange chromatography revealed that the increased [(35)S]sulfate incorporation into proteoglycans was not due to significant differences in size or density of negative charge of glycosaminoglycan chains attached to proteoglycan core proteins. On the other hand, measurements of the syndecan-1 mRNA level and of [(3)H]leucine-labelled perlecan after immunoprecipitation supported the idea that the increased [(35)S]sulfate incorporation into proteoglycans was due to a selective increase in the synthesis of proteoglycan core proteins. Interestingly, the activity of protein kinase C was increased in cells expressing mutant dynamin and inhibition of protein kinase C with BIM reduced the differences in [(35)S]sulfate incorporation between cells with normal and impaired clathrin-dependent endocytosis. Thus, the activation of protein kinase C observed upon inhibition of clathrin-dependent endocytosis may be responsible for the increased synthesis of proteoglycans.