Urobilin is a yellow linear tetrapyrrole, resulting from the breakdown of heme

Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

, a cyclic tetrapyrrole.

Urobilin is produced when Urobilinogen

Urobilinogen

Urobilinogen is a colourless product of bilirubin reduction. It is formed in the intestines by bacterial action. Some urobilinogen is reabsorbed, taken up into the circulation and excreted by the kidney. This constitutes the normal "enterohepatic urobilinogen cycle".Increased amounts of bilirubin...

 is oxidized by intestinal bacteria; it can also be produced when urobilinogen is exposed to the environment upon urination, resulting in its oxidation to urobilin.

Metabolism

A uroporphyrin; an acyclic tetrapyrrole that is one of the natural breakdown products of heme via choleglobin, verdohemochrome, biliverdin, bilirubin, and d-urobilinogen; a urinary pigment that gives a varying orange-red coloration to urine according to its degree of oxidation.

Importance

Many urine tests (urinalysis) monitor the amount of urobilin in urine because it is an important substance in metabolism, production of urine. Urobilin levels can give insight on the effectiveness of urinary tract function. Normally, urine would appear as either light yellow urine or colorless. The yellow in the urine is from the presence of urobilin. If there are other chemicals in the urine, the appearance of the urine can darken, or it can appear cloudy in case particles are present, or simply orange urine in case of dehydration.

Darker urine is not necessarily a sign of illness. 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. Of course, urine can appear amber or red when red blood cells are present in urine, which is a condition called hematuria

Hematuria

In medicine, hematuria, or haematuria, is the presence of red blood cells in the urine. It may be idiopathic and/or benign, or it can be a sign that there is a kidney stone or a tumor in the urinary tract , ranging from trivial to lethal...

. .