Bile is a route of excretion for homogentisic acid in alkaptonuria

Altered activity of specific enzymes in phenylalanine-tyrosine (phe-tyr) metabolism results in incomplete breakdown of various metabolite substrates in this pathway. Increased biofluid concentration and tissue accumulation of the phe-tyr pathway metabolite homogentisic acid (HGA) is central to pathophysiology in the inherited disorder alkaptonuria (AKU). Accumulation of the metabolites upstream of HGA, including tyrosine, occurs in patients on nitisinone, a licenced drug for AKU and hereditary tyrosinaemia type-1, which inhibits the enzyme responsible for HGA production. The aim of this study was to investigate the phe-tyr metabolite content of key biofluids and tissues in AKU mice on and off nitisinone to gain new insights into the biodistribution of metabolites in these altered metabolic states. The data show for the first time that HGA is present in the bile in AKU (mean [±SD] = 1003[±410] μmol/L; nitisinone-treated AKU mean [±SD] = 45[±23] μmol/L). Biliary tyrosine, HPPA and HPLA are also increased on nitisinone. Urine was confirmed as the dominant elimination route of HGA in untreated AKU, but with indication of biliary excretion and possible metabolism of HGA by the gut microbiome. These data provide new insights into the pathways of phe-tyr metabolite biodistribution and metabolism, showing for the first time that hepatobiliary excretion contributes to the total pool of metabolites in this pathway. Our data suggest that biliary elimination of organic acids and other metabolites may play an underappreciated role in disorders of metabolism. Take-home message This paper presents the first observation of elevated hepatobiliary circulation of metabolites associated with disease in alkaptonuria, including homogentisic acid in addition to tyrosine and the tyrosine metabolites 4-hydroxyphenylpyruvic acid and 4-hydroxyphenyllactic acid on nitisinone treatment. ### Competing Interest Statement LRR received fees for lectures and consultations from Swedish Orphan Biovitrum. All other authors declare no competing interests.