Role of vitamin B6 on leucine-induced metabolic changes

Administration of leucine at 3% level in the diet has been shown to increase tryptophan oxygenase activity and to decrease kynureninase activity in the liver and to increase quinolinic acid excretion in the urine of rats fed low amounts of vitamin B6 (0.5 mug/g diet). Vitamin B6 at 3 mug/g of diet was able to reverse the effects of leucine on enzyme activities, but not on quinolinic acid excretion. Isoleucine at 0.2% level could counteract the leucine effect on kynureninase but not on tryptophan oxygenase. Administration of leucine also decreased the 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the brain. Simultaneous administration of isoleucine could counteract the effects of leucine on brain 5-HT and 5-HIAA partially at low amount of vitamin B6 but completely at higher levels of vitamin B6 in the diet.