The phytase RipBL1 enables the assignment of a specific inositol phosphate isomer as a structural component of human kidney stones

Abstract

Inositol phosphates (InsPs) are ubiquitous in all eukaryotes. However, since there are 63 possible different phosphate ester isomers, the analysis of InsPs is challenging. In particular, InsP₁, InsP_2, and InsP₃ already amass 41 different isomers, of which some occur as enantiomers. Profiling of these “lower” inositol phosphates in mammalian tissues requires powerful analytical methods and reference compounds. Here, we report an analysis of InsP₂ and InsP₃ with capillary electrophoresis coupled to electrospray ionization mass spectrometry (CE-ESI-MS). Using this method, the bacterial effector RipBL1 was analyzed and found to degrade InsP₆ to Ins(1,2,3)P₃, an understudied InsP₃ isomer. This new reference molecule then aided us in the assignment of the isomeric identity of an InsP₃ while profiling human samples: in urine and kidney stones, we describe for the first time the presence of defined and abundant InsP₃ isomers, namely Ins(1,2,3)P₃, Ins(1,2,6)P₃ and/or Ins(2,3,4)P₃.