Targeted and untargeted urinary metabolomics of alkaptonuria patients using ultra high-performance liquid chromatography-tandem mass spectrometry

Abstract

Alkaptonuria (AKU) is a rare autosomal-recessive disease which is characterized through black urine and ochronosis. It is caused by deficiency of the enzyme Homogentisate 1,2-dioxygenase in the Phenylalanine/Tyrosine degradation pathway which leads to the accumulation of Homogentisic acid (HGA). Urine was provided by AKU patients and healthy controls. Several different methods were developed in this study each with a specific goal. Firstly, a simple and inexpensive RP-UHPLC-UV method for routine monitoring of HGA as a key metabolite employing a Phenylhexyl stationary phase chemistry. Validation was performed in accordance to FDA guidelines and method selectivity was further evaluated via on-line high-resolution sampling 2D-LC-QToF-MS, coupling the Phenylhexyl phase in the first dimension with a C18 phase in the second dimension. Secondly, a targeted and accurate RP-UHPLC-MRM-QTRAP assay, providing quantitative analysis of the relevant pathway metabolites based on a Phenylhexyl stationary phase, and lastly an untargeted HILIC-UHPLC-QToF-MS/MS method with SWATH (sequential window acquisition of all theoretical mass spectra) acquisition employing a sulfobetaine-type HILIC-Z superficially porous particle column, with the aim of uncovering more details about the metabolic profile of this genetic disorder. By untargeted analysis 204 metabolites could be detected and annotated in positive and negative ESI mode in total. Two separate LC methods were employed, differing in their conditions depending on the ionization mode (20 mM ammonium formate as buffer additive adjusted to a pH = 3.5 with formic acid in ESI⁺ mode and 20 mM ammonium acetate adjusted to a pH = 7.5 with acetic acid in ESI^- mode). By effectively combining the aforementioned methods, a comprehensive workflow was developed, allowing the effective analysis of both patient and control urine samples.