Urobilin or urochrome is the chemical primarily responsible for the yellow color of
urine. It is a linear
tetrapyrrole compound that, along with the related colorless compound
urobilinogen, are
degradation products of the cyclic tetrapyrrole
heme.
Metabolism
Urobilin is generated from the degradation of
heme, which is first degraded through
biliverdin to
bilirubin. Bilirubin is then excreted as bile, which is further degraded by microbes present in the large intestine to urobilinogen. The enzyme responsible for the degradation is
bilirubin reductase, which was identified in 2024.[1][2] Some of this remains in the large intestine, and its conversion to
stercobilin gives
feces their brown color. Some is reabsorbed into the bloodstream and then delivered to the kidneys. When urobilinogen is exposed to air, it is oxidized to urobilin, which has a yellow color.[3]
Importance
Many urine tests (urinalysis) monitor the amount of urobilin in urine, as its levels can give insight on the effectiveness of urinary tract function. Normally, urine would appear as either light yellow or colorless. A lack of water intake, for example following sleep or dehydration, reduces the water content of urine, thereby concentrating urobilin and producing a darker color of urine.
Obstructive jaundice reduces biliary bilirubin excretion, which is then excreted directly from the blood stream into the urine, giving a dark-colored urine but with a paradoxically low urobilin concentration, no urobilinogen, and usually with correspondingly pale faeces. Darker urine can also be due to other chemicals, such as various ingested dietary components or drugs,
porphyrins in patients with
porphyria, and
homogentisate in patients with
alkaptonuria.
^John E. Hall (2016). "The liver as an organ". Guyton and Hall Textbook of Medical Physiology, 13th edition. Elsevier. p. 885.
ISBN978-1455770052.
Further reading
Bishop, Michael; Duben-Engelkirk, Janet L., and Fody, Edward P. (1992). "Chapter 19, Liver Function, Clinical Chemistry Principles, Procedures, Correlations, 2nd Ed." Philadelphia, J.B. Lippincott Company.
Miyabara, Yuichi; Tabata, Masako; Suzuki, Junzo; Suzuki, Shizuo (1992). "Separation and sensitive determination of i-urobilin and 1-stercobilin by high-performance liquid chromatography with fluorimetric detection". Journal of Chromatography B: Biomedical Sciences and Applications. 574 (2): 261–265.
doi:
10.1016/0378-4347(92)80038-R.
PMID1618958.
Miyabara, Y.; Sakata, Y.; Suzuki, J.; Suzuki, S. (1994). "Estimation of faecal pollution based on the amounts of urobilins in urban rivers". Environmental Pollution. 84 (2): 117–122.
doi:
10.1016/0269-7491(94)90093-0.
PMID15091706.
Munson-Ringsrud, Karen and Jorgenson-Linné, Jean (1995). "Urinalysis and Body Fluids, a ColorText and Atlas." St. Louis, Mosby.
Nelson, L.; David, Cox M.M. (2005). “Chapter 22 – Biosynthesis of Amino Acids, Nucleotides, and Related Molecules”, pp. 856, In Lehninger Principles of Biochemistry. Freeman, New York. pp. 856.