Cystine metabolism in vitamin B 6 deficiency: Evidence of muliple taurine pools

Abstract

The role of cystine in the in vivo metabolism of vitamin B 6-deficient and pair-fed control rats has been studied by the use of dietary supplements of cystine and by intraperitoneal injection of l- 35S-cystine. The radioactive cystine was metabolized differently in vitamin B 6-deficient rats: They excreted more radioactivity than pair-fed control rats, which contained a higher proportion of 35S-inorganic sulfate, and a lower proportion of 35S-taurine. The urinary taurine from vitamin B 6-deficient rats had a higher specific activity than that from pair-fed control animals. Less 35S-taurine was present in the TCA-soluble extracts of organs from vitamin B 6-deficient rats than from pair-fed control rats, and the specific activity also was generally lower, in contrast to urinary taurine. No 35S-taurine could be detected in plasma of vitamin B 6-deficient or pair-fed control rats despite easily measurable unlabeled taurine. These findings suggest the presence of separate taurine pools: a “storage pool” with a slow turnover rate, and a “newly formed pool” which is only slowly transferred to the storage pool. They further suggest that the newly formed pool of the kidney is the major source of urinary 35S-taurine. Incoporation of 35S-cystine into protein was relatively un-affected in vitamin B 6 deficiency, except into hair, where it was reduced dramatically. Supplementation of the pyridoxine-deficient diet with cystine failed to alleviate hair thinning, indicating that the decreased synthesis of this protein is not a function of cystine deficiency.