Effects of oestrogen administration on vitamin B6and tryptophan metabolism in the rat

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Abstract
1.In order to assess the effects of oestrogens on the metabolism of tryptophan and vitaminB6, ovariectomized rats have been maintained on diets providing known amounts of tryptophan, nicotinamide and vitaminB6. They received oestrone sulphate. 210μg/kg body-wt per d, either incorporated in the diet for 8 weeks, or by daily intraperitoneal injection for periods of 1–3 d.2. Oestrone sulphate administration caused a slight reduction in the concentration of pyridoxal phosphate in plasma. It had no effect on the concentration of pyridoxal phosphate in liver or kidney, the urinary excretion of 4-pyridoxic acid, the activation of erythrocyte aspartate aminotransferase (L-aspartate:2-oxo-glutarate aminotransferase,EC2.6.1.1)by incubation with added pyridoxal phosphate, or the activity of pyridoxal oxidase (aldehyde:oxygen oxido-reductase,EC1.2.3.1) in the liver.3. Oestrone sulphate administration caused an increase in the urinary excretion of kynurenine and a reduction in the activity of liver kynureninase (L-kynurenine hydrolase,EC3.7.1.3), It had no effect on the urinary excretion ofN1-methyl nicotinamide or the concentrations of nicotinamide nucleotides in blood, liver or kidney.4. There was a considerable excess of the apoenzyme of kynureninase in the liver. Incubation of liver homogenates with added pyridoxal phosphate led to a 4- to 5-fold increasein activity.5. We conclude that there is no evidence of any significant effect of oestrogens on vitaminB6. It is suggested that abnormalities of tryptophan metabolism in women receiving oestrogens. which have been widely attributedto drug-induced vitaminB6depletion, can be accounted for by inhibition of kynureninase by oestrogen metabolites.