What substance imparts an orange-brown color to urine that is not fresh?

The substance that imparts an orange-brown color to urine that is not fresh is urobilin. Urobilin is a derivative of bilirubin, formed primarily in the liver and excreted in urine after it undergoes a series of enzymatic conversions and oxidations. When urine ages or is exposed to light and air, urobilin can form from the breakdown of urobilinogen, leading to the characteristic orange-brown coloration.

This coloration is significant as it indicates the oxidation process of waste products in the urine. It is important to differentiate urobilin from other components such as uroerythrin, which contributes a pinkish hue to fresh urine, urobilinogen, which is colorless or faintly yellow and is an intermediate in the conversion to urobilin, and creatinine, which does not significantly affect urine color. Therefore, urobilin serves as a key indicator of urine aging and the biochemical activity occurring within the urinary system.