A study of norepinephrine metabolism in rat brain using a double label technique

Using a double-label procedure, the incorporation of endogenously-derived ³⁵SO₄ into phenylethylene glycol sulfates (PGS) was estimated. When ³H]norepinephrine and ³⁵S]cysteine were injected concomitantly into the brain, about 30–80 percent of the tritium and about 4 percent of the ³⁵S retained in the brain 1 hr later were in PGS. In B₆-deficient rats, the proportion of ³⁵S was increased as was the ${}^{35}\text{S}{}^3\text{H}$ ratio. Probenecid caused a significant increase in the amount of PGS found in the brain, but a minimal enrichment of ³⁵S was observed, suggesting that there was little or no effect on sulfotransferases. Cysteine in high concentrations inhibits the incorporation of tritium into PGS.