Most mammals have a keen sense of smell. That's why urinary odorants and pheromones are used to attract mates and mark territory. In most cases we can't detect these odors but there's one major exception.
The urine of cats contains chemicals that are easily detectable by humans. The smell is not pleasant. Mature cats will spay almost anything to stake out their territory, especially males). This isn't a problem if it's outdoors but it can be a major problem for indoor cats because carpets and spraying are not a good mix.
The urine of mature cats contains a chemical known as felinine (2-amino-7 -hydrozxy-5, 5-dimethyl-4-thiaheptanoic avid). Felinine excretion is stimulated by the hormone testosterone, which isn't produced until the cat reaches maturity. Both male and female cats excrete felinine in their urine but males typically excrete twice as much.
Felinine is odorless, to us. A recent paper (October 2006) describes how it is made and how it is converted to more pungent compounds.
The biochemical pathway leading to felinine begins with 3-methylbutanol- glutathionine (3-MBG) (compound A in the figure below). 3-MBG is a normal precursor in the synthesis of cholesterol but in cats some of it is converted to 3-methylbutanol-cysteinylglycine (3-MBCG) (compound B) by a pepdidase activity that removes glutamate. This reaction takes place in the bloodstream and 3-MBCG is excreted in the urine in cats of from the time they are born.
Mature cat urine contains high levels of protein, 90% of which is a medium-sized protein (70kDa) called cauxin. Cauxin levels rise as the cats reach maturity because transcription of the gene is stimulated by sex hormones. Cauxin is produced only in kidney cells and is secreted directly into the urine. The novel finding is that cauxin is a peptidase that cleaves 3-MBCG producing felinine (compound C). What this means is that production of felinine from 3-MBCG takes place in urine, probably in the nephrons before urine is released into the bladder.
Felinine breaks down into a number of smaller compounds that give rise to the characteristic smell of cat urine. The main breakdown product is 3-mercapto-3-methy-1-butanol formed by splitting felinine at the sulfur atom. Other breakdown products are formed. The complex mixture of derivatives is probably produced by a combination of unknown enzymatic act ivies and spontaneous reactions. The characteristic odor of domestic cats differs from that of lynx and bobcats and the differences are due to the concentrations of the various breakdown products of felinine.
Miyazaki, M., Yamashita, T., Suzuki, T., Saito, Y., Soeta, S., Taira, H., and Suzuki, A. (2006) A Major Urinary Protein of the Domestic Cat Regulates the Production of Felinine, a Putative Pheromone Precursor. Chemistry & Biology 10: 1071-1079.