Cystine neurotoxicity is increased by taurine deficiency.

In an attempt to increase taurine biosynthesis in cats fed a taurine-free diet we supplied an excess of the precursor, cystine, in the diet. All nine cats exhibited extreme signs of neurotoxicity including lethargy, inability to stand, rigidity of the neck and lower limbs, absence and epileptic seizures, severe retinal damage and death. In a similar group of cats fed 0.05% taurine in addition to an excess of cystine, four cats died after showing minimal symptoms of lethargy and unsteadiness and the remainder showed no adverse effects. Biochemical measurements, tissue concentrations of cystine, cysteine, bound cysteine, glutamate and taurine and activities of enzymes involved in taurine biosynthesis, revealed significant differences only in taurine concentrations.