Evaluating metal ion salts as acid hydrolase mimics: metal-assisted hydrolysis of phospholipids at lysosomal pH

Abstract

Niemann-Pick disease and drug-induced phospholipidosis are lysosomal storage disorders in which there is an excessive accumulation of sphingomyelin in cellular lysosomes. Here we have explored the possibility of developing metal-based therapeutic agents to reverse phospholipid build-up through phosphate ester bond hydrolysis at lysosomal pH (~4.8). Towards this end, we have utilized a malachite green/molybdate-based colorimetric assay to quantitate the inorganic phosphate released upon the hydrolysis of sphingomyelin by twelve d- and f-block metal ion salts. In reactions conducted at 60 °C, the yields produced by the cerium(IV) complex Ce(NH₄)₂(NO₃)₆ were superior. An Amplex^® Red-based colorimetric assay and mass spectrometry were then employed to detect choline. The data consistently showed that Ce(IV) hydrolyzed sphingomyelin more efficiently at lysosomal pH: i.e., yields of choline and phosphate were 54 ± 4 and 22 ± 5 % at pH ~ 4.8, compared to 8 ± 1 and 5 ± 2 % at pH ~ 7.2. Hydrolysis at 60 °C could be significantly increased by converting sphingomyelin vesicles to mixed lipid vesicles and mixed micelles of Triton X-100. We then utilized cerium(IV) to cleave sphingomyelin at 37 °C (no Triton X-100). Although choline and phosphate levels were relatively low, hydrolysis continued to be considerably more efficient at lysosomal pH. A side by side comparison to phosphatidylcholine was then made. While the yields of choline and phosphate produced by phosphatylcholine were higher, the ratio of pH ~ 4.8 hydrolysis to pH ~ 7.2 hydrolysis was usually more favorable for sphingomyelin (37 and 60 °C).