Oestrogens tryptophan metabolism

Bender DA and Totoe L (1984b) Inhibition of tryptophan metabolism by oestrogens in the rat a factor in the aetiology of pellagra. British Journal of Nutrition 51,219-24. 

Rose DP (1966b) The influence of oestrogens on tryptophan metabolism in man. Clinical Science 31, 265-72. 

R9. Rose, D, P., and Adams, P. W., Oral contraceptives and tryptophan metabolism. Effects of oestrogen in low dose combined with a progestagen and of a low-dose progestagen (megestrol acetate) given alone. J. Clin. Pathol. 25, 252-258 (1972). 

R16. Rose, D. P., Strong, R., Adams, P. W., and Harding, P. E., Experimental vitamin Ba deficiency and the effect of oestrogen-containing oral contraceptives on tryptophan metabolism and vitamin Ba requirements. Clin. Sci. 42, 465-477 (1972). 

R12. Rose, D. P., Brown, R. R., and Price, J. M., Metabolism of tryptophan to nicotinic acid derivatives by women taking oestrogen-progestogen preparations. Nature (London) 219, 1250-1260 (1968). 

Kynureninase is a pyridoxal phosphate (vitamin Bg)-dependent enzyme, and its activity is extremely sensitive to vitamin depletion. Indeed, the ability to metabolize a test dose of tryptophan has been used to assess vitamin B nutritional status (section 11.9.5.1). Deficiency of vitamin B will lead to severe impairment of NAD synthesis from tryptophan. Kynureninase is also inhibited by oestrogen metabolites. 

Xanthurenic and kynurenic acids, and kynurenine and hydroxykynurenine, are easy to measure in urine, so the tryptophan load test (the ability to metabolize a test dose of 2—5 g of tryptophan) has been widely adopted as a convenient and very sensitive index of vitamin nutritional status. However, because glucocorticoid hormones increase tryptophan dioxygenase activity, abnormal results of the tryptophan load test must be regarded with caution, and cannot necessarily be interpreted as indicating vitamin B deficiency. Increased entry of tryptophan into the pathway will overwhelm the capacity of kynureninase, leading to increased formation of xanthurenic and kynurenic acids. Similarly, oestrogen metabolites inhibit kynureninase, leading to results that have been misinterpreted as vitamin B deficiency. 

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