Effect of yohimbine on 14CO2 production from 14C-pyrrole-ring-labeIed DL-tryptophan in rats treated with L-tryptophan, DL-α-methyltryptophan, and Cortisol

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Abstract
Yohimbine, previously shown to inhibit tryptophan pyrrolase in vitro and in vivo, was tested in rats whose pyrrolase activity had been elevated by prior treatment with cortisol, tryptophan, or α-methyltryptophan. The alkaloid exerted an inhibitory effect on the oxidation of tryptophan-2-(pyrrole ring)-14C in rats given Cortisol or α-methyltryptophan, but had no effect on those given tryptophan loads. Consideration of the respective modes of action of the three stimulators of tryptophan pyrrolase activity and the observed results suggests that yohimbine acts at the stage of reduction of holoenzyme to its active form, a process requiring L-tryptophan.The oxidation of 14C-tryptophan in vivo seems to involve a progressive increase in the activity of pyrrolase during the first 1–1.5 h. By prior treatment of rats with tryptophan loads, the oxidation approaches a linear, but submaximal, rate during this period. The size of the air-pool in the metabolic cages (dead space) does not introduce, within the limits studied, an artifact into the measurements of the rate of tryptophan oxidation in vivo.

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