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el-Zoghby SM, el-Shafei AK, Abdel-Tawab GA, Kelada FS. 
“Studies on the effect of reserpine therapy on the functional capacity of the tryptophan-niacin pathway in smoker and non-smoker males”. 
Biochem Pharmacol. 1970 May 28;19(5):1661-7.
Abstract—The functional capacity of the tryptophan-niacin pathway was tested by analysis of eleven metabolites excreted in the urine of smoker and non-smoker males after loading with L-tryptophan (2 g) with and without vitamin B₆ supplement (12mg). Furthermore, the excretion pattern was re-investigated after depletion of the catechol amines by applying reserpine therapy to both groups.The results obtained were compatible with the suggestion that the modification of tryptophan metabolism by smoking may be through the catecholamines discharged by nicotine that man absorbs from smoking. Thereafter, the catecholamines react with the coenzyme (i.e. pyridoxal phosphate) to form inactive tetrahydroisoqumoline derivatives causing a rapid inactivation of the coenzyme, and thereby inhibit preferably the B₆-dependent quinolinic acid decarboxylase enzyme. It is striking, however, that this type of inhibition could not be completely overcome by a large dose of vitamin B₆ orally supplemented. It was only after reserpine therapy that both smokers and nonsmokers gave excretion patterns of tryptophan metabolites which were qualitatively and quantitatively similar.
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