The regulation of tryptophan-niacin metabolism was studied in vivo with regard to the activity of liver ƒ¿-amino-ƒÀcarboxymuconate-c-semialdehyde decarboxylase (picolinic carboxylase) using non-diabetic and diabetic rats. Non-diabetic rats and rats with diabetes induced by alloxan were fed on a commercial diet for 80 days,
The effects of dietary protein and soybean oil on the me tabolism of L-tryptophan through the NAD pathway and the activity of ƒ¿-amino-ƒÀ-carboxymuconate-ƒÃ-semialdehyde decarboxylase (ACMSD) [EC 4.1.1.45] in rat liver and kidney were investigated. The animals were fed on the following niacin-free diets for 11 days: group 1, a fat-free 20% casein diet (20% Cas); group 2, a fat-free 40% casein diet (40% Cas); group 3, a 40% casein diet containing 20% soybean oil (40% Cas+20% F). After feeding on each diet for 7 days, the urine was collected during 48h. On day 9, L-tryptophan was force-fed and the urine was collected during 48h. Niacin metabolites such as N-methylnicotinamide (MNA), N-methyl-2-pyridone-5-carboxamide (2-Py) and N-methyl-4-pyridone-3 carboxamide (4-Py) in the urine samples were analyzed by chromatog raphy. The activity of ACMSD in the liver and kidney was assayed immediately after the last urine collection. The results indicate that the urinary excretion of the niacin metabolites in the 40% Cas group was lower than that of the 20% Cas group in spite of the more intake of tryptophan in the former group. On the other hand, in the 40% Cas+ 20% F group, tryptophan intake was lower and the excretion of the metabolites was significantly higher than that in the 40% Cas group. The hepatic ACMSD activity in the 40% Cas group was 5.8 times that of the 20% Cas group and that in the 40% Cas+20%F group was one tenth that of the 20% Cas group. These results indicate that the ratio of the excreted metabolites to the tryptophan intake was reduced by the high dietary protein level, but increased by the addition of high soybean oil. The data analysis shows that the amount of urinary total niacin metab olites ([TNM]: MNA+2-Py+4-Py) could be expressed in the following equation in the rats fed on each diet:
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