Cysteine sulfinate carboxylase in the visual pathway of adult chicken

Cysteine sulfinate (CSA) carboxylyase, the enzyme which synthesizes taurine through hypotaurine, shows a higher activity in the inner plexiform and nuclear layer of adult chick retina compared to the outer plexiform and nuclear layers whereas the outer segments of photoreceptors do not show any activity of this enzyme. These observations suggest an endogenous synthesis of taurine preferentially in certain layers of retina. Therefore, taurine fulfills one more criteria which is required by a substance to be accepted as a neurotransmitter in an organ. Studies on the distribution of CSA-carboxylyase in the visual pathway and other brain areas show a very high activity of this enzyme in optic tectum followed by cerebral cortex, cerebellum, retina, lateral geniculate body and optic nerve, taken with chiasma and tract in decreasing order. On the other hand, analysis of the free amino acid pool reveals a very high content of taurine in retina as compared to optic tectum. Cysteine sulfinate carboxylyase activity and the content of taurine therefore do not seem to bear a good correlation and other mechanisms of release, uptake and degradation might be involved in regulating the taurine content in these tissues.