The effects of a dietary excess of leucine on the synthesis of nicotinamide nucleotides in the rat

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Abstract
1. In order to test the suggestion that a dietary excess of leucine may be a precipitating factor in pellagra, rats were fed on diets that provided 15 g leucine/kg in excess of requirements for 7 weeks from weaning. This led to a significant reduction in the concentrations of nicotinamide nucleotides in liver and blood. The effect was only apparent when the diets provided less than a minimally adequate amount of nicotinamide, so that the animals were dependent on the synthesis of nicotinamide nucleotides from tryptophan to meet all or part of their requirements.2. Urinary excretion of N1-methyl nicotinamide was not a useful indicator of tissue concentrations of nicotinamide nucleotides, and seemed not to be adequately sensitive to differentiate between minimal adequacy and marginal deficiency, as demonstrated by changes in concentrations of nicotinamide nucleotides in liver and blood.3. The addition of leucine to incubation media for the measurement of enzyme activity in tissue homogenates at concentrations within the physiological range, led to a significant activation of tryptophan oxygenase (L-tryptophan: oxygen oxidoreductase (decyclizing),EC1.13.11.11) and significant inhibition of kynureninase (L-kynurenine hydrolase,EC3.7.1.3). The effect on tryptophan oxygenase may not be physiologically significant, in view of the considerable range of activity of this enzyme under normal conditions. However, the inhibition of kynureninase, which was primarily competitive with respect to the substrate, probably is physiologically significant, and was enough for this enzyme to become a probable rate-limiting step in tryptophan metabolism and nicotinamide nucleotide synthesis. Other enzymes of the tryptophan – nicotinamide nucleotide pathway were not affected by the addition of leucine to the incubation medium.4. Feeding 15 g leucine/kg diet in excess of minimum requirements had no effect on the activities of tryptophan oxygenase or kynureninase in liver homogenates. This may reflect the reversible competitive nature of the inhibition of kynureninase by leucine, and hence be an artefact of the incubation procedure. Rats fed on the high-leucine diets excreted significantly more kynurenine than did control animals, which is evidence of inhibition of kynureninase in vivo.5. It appears that a dietary excess of leucine, of the order of 15 g/kg above requirements, may be a precipitating factor in pellagra when there is reliance on the synthesis of nicotinamide nucleotides from tryptophan to meet a part or all of the requirements, but not when minimally adequate niacin is available from the diet.