Studies in alkaptonuria reveal new roles beyond drug clearance for phase I and II biotransformations in tyrosine metabolism

Background and Purpose alkaptonuria (AKU) is an inherited disorder of tyrosine metabolism caused by lack of the enzyme homogentisate 1,2-dioxygenase (HGD). The primary biochemical consequence of HGD-deficiency is increased circulating homogentisic acid (HGA), which is central to AKU disease pathology. The aim of this study was to investigate the wider metabolic consequences of targeted Hgd disruption. Experimental Approach the first metabolomic analysis of the Hgd−/− AKU mouse model was performed. Urinary metabolites altered in Hgd−/− were further validated by showing that the HGA-lowering drug nitisinone reversed their direction of alteration in AKU Key Results comparison of Hgd−/− (AKU) versus Hgd+/− (heterozygous control) urine revealed increases in HGA and a group of 8 previously unreported HGA-derived transformation products from phase I and II metabolism. HGA biotransformation products HGA-sulfate, HGA-glucuronide, HGA-hydrate and hydroxymethyl-HGA were also decreased in urine from both mice and patients with AKU on the HGA-lowering agent nitisinone. Hgd knockout also revealed a host of previously unrecognised associations between tyrosine, purine and TCA cycle metabolic pathways. Conclusion and Implications AKU is rare, but our findings further what is currently understood about tyrosine metabolism more generally, and show for the first time that phase I and II detoxification is recruited to prevent accumulation of endogenously-produced metabolites in inborn errors of metabolism. The data highlight the misconception that phase I and II metabolic biotransformations are reserved solely for drug clearance; these are ancient mechanisms, which represent new potential treatment targets in inherited metabolic diseases. Bullet point summary What is already known Increased circulating homogentisic acid is central to disease pathology in the inherited metabolic disease alkaptonuria The Hgd knockout mouse, created in our laboratory, accurately models human alkaptonuria What this study adds Phase I and II biotransformations are recruited in alkaptonuria for detoxification of homogentisic acid These data challenge misconceptions that phase I and II metabolism is solely for drug clearance Clinical significance Phase I and II metabolic processes represent new treatment targets in inherited metabolic diseases The molecular pathology of AKU extends much further than the known alteration to tyrosine metabolism