The Necessity of Niacin in Rats Fed on a High Protein Diet

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1 Biosci. Biotechnol. Biochem., 69 (2), , 2005 The Necessity of Niacin in Rats Fed on a High Protein Diet Naoko KIMURA, Tsutomu FUKUWATARI, Ryuzo SASAKI, and Katsumi SHIBATA y Laboratories of Food Science and Nutrition, Department of Life Style Studies, School of Human Cultures, The University of Shiga Prefecture, 2500 Hassakacho, Hikone, Shiga , Japan Received June 29, 2004; Accepted November 4, 2004 It is known that niacin itself is not necessary in rats when tryptophan is given in adequate amounts, because rats can biosynthesize niacin from tryptophan. In our experiment, young rats were fed on a 20%, 40%, 60%, or 70% casein diet with or without niacin. The rats fed on the 20%, 40%, and 60% casein diets did not require niacin for growth, but the rats fed on the 70% casein diet needed it. This phenomenon was attributed to the supposition that liver aminocarboxymuconate semialdehyde decarboxylase activities increased according with the dietary casein levels. The conversion ratio of tryptophan niacin in rats fed on the 70% casein diet became extremely low, and then the rats needed niacin. Key words: tryptophan; necessity of niacin; high protein diet; conversion ratio of tryptophan niacin Because more than 500 enzymes need niacin coenzymes, it is important to determine the control mechanisms of the coenzyme supply in vivo. Mammals including humans can biosynthesize niacin from an indispensable amino acid Trp. Therefore, many nutritionists including our group claim that niacin itself is not necessary when Trp is taken suitably. In fact, it lacks any influence on the growth of young rats even if they are given niacin-free diets containing a suitable amount of protein, such as 20% casein diets. 1) However, we found that rats needed niacin for maximum growth when they are fed a 70% casein diet. Our paper explains our methods and results. 1) Materials and Methods Chemicals. Vitamin-free milk casein, sucrose, L- methionine, Nam, and L-Trp were purchased from Wako Pure Chemical Industries (Osaka, Japan). Kynurenine sulfate, KA, and MNA chloride were purchased from Tokyo Kasei Kogyo (Tokyo, Japan). 2-Py and 4-Py were synthesized by the methods of Pullman and Colowick 2) and Shibata et al. 3) respectively. Corn oil was purchased from Ajinomoto (Tokyo, Japan). The mineral (AIN- 93M-MX) and vitamin (AIN-93-VX) mixtures were obtained from Oriental Yeast Kogyo (Tokyo, Japan), all the other chemicals used being of the highest purity available from commercial sources. Animals. The care and treatment of the experimental animals conformed to The University of Shiga Prefecture guidelines for the ethical treatment of laboratory animals. Experiment 1 (70% casein diets with or without NiA in the presence of vitamin B 6 ). Male rats of the Wistar strain (4 weeks old with a body weight of around 60 g) were obtained from CLEA Japan (Tokyo, Japan) and immediately placed in individual metabolic cages (CT- 10; CLEA Japan). To acclimatize the rats to these conditions, they were initially fed ad libitum for 7 d with a complete 20% casein diet 1) and water. They were then divided into the two groups and fed ad libitum for 19 d, with a 70% casein diet with or without NiA in the presence of vitamin B 6 (Table 1). The room temperature was kept at 22 2 C at about 60% humidity, and a 12-h light/12-h dark cycle was maintained. Body weight and food intake were measured periodically, usually every other day at 9:00 10:00 a.m. Urine samples (24-h; 9:00 a.m. 9:00 a.m.) were collected for the last day of the experiment in amber bottles containing 1 ml of 1 M HCl, and were stored at 25 C until needed. The rats were killed by decapitation after the collection of urine samples. The liver of each animal was removed, and a portion of it (approximately 1 g) was treated as described in the literature 4,5) to measure the enzyme activities involved in the metabolism of Trp to niacin. Experiment 2 (70% casein diets with or without NiA in the absence of vitamin B 6 ). The same procedure was performed as with Experiment 1 except for the diet, from which was removed only vitamin B 6, as shown in Table 1. y To whom correspondence should be addressed. Tel: ; Fax: ; kshibata@shc.usp.ac.jp Abbreviations: Trp, L-tryptophan; XA, xanthurenic acid; KA, kynurenic acid; 3-HK, 3-hydroxyanthranilic acid; Nam, nicotinamide; MNA, N 1 - methylnicotinamide; 2-Py, N 1 -methyl-2-pyridone-5-carboxamide; 4-Py, N 1 -methyl-4-pyridone-3-carboxamide; ACMSDase, aminocarboxymuconate-semialdehyde decarboxylase

2 274 N. KIMURA et al. Table 1. Composition of the 70% Casein Diets Experiment 1 Experiment 2 þnia & þb 6 NiA & þb 6 þnia & B 6 NiA & B 6 % % % % Vitamin-free milk casein L-Methionine Sucrose Corn oil Mineral mixture* Vitamin mixture* NiA-free vitamin mixture* B 6 -free vitamin mixture* NiA and B 6 -free vitamin mixture* *AIN 93 was used (Reeves, P.G., Components of the AIN-93 diets as improvements in AIN-76A diet. J. Nutr., 127, 838S 841S (1997)). The diet (þnia & þb 6 ) contained 6 mg NiA and 0.8 mg of pyridoxine-hcl per 100 g of diet. Table 2. Composition of the 40% and 60% Casein Diets 40% Casein diet 60% Casein diet þnia NiA þnia NiA % % % % Vitamin-free milk casein L-Methionine Sucrose Corn oil Mineral mixture* Vitamin mixture* NiA-free vitamin mixture *AIN 93 was used (Reeves, P.G., Components of the AIN-93 diets as improvements in AIN-76A diet. J. Nutr., 127, 838S 841S (1997)). The diet (þnia & þb 6 ) contained 6 mg NiA and 0.8 mg of pyridoxine-hcl per 100 g of diet. Experiment 3 (40% and 60% casein diets with or without NiA in the presence of vitamin B 6 ). The same procedure was performed as with Experiment 1 except for the diets was done. The composition of the diets used in Experiment 3 is shown in Table 2. Analyses. To measure the conversion ratio of Trp to niacin, the urinary contents of Nam and its metabolites MNA, 2-Py, and 4-Py were measured. This method does not take account of the increased body store of Nam during growth, and the value does not, therefore, represent the net conversion ratio. However, this value is useful for the assessment of the apparent conversion ratio. The conversion ratio was calculated as the sum of the urinary excretions of fnam þ MNA þ 2-Py þ 4-Py(mmol/day)g100/Trp intake during urine collection (mmol/day). The contents of Nam, 2-Py, and 4-Py in the urine were simultaneously measured by the HPLC method of Shibata et al., 3) while the content of MNA in the urine was measured by the HPLC method of Shibata. 6) The contents of KA 7) and XA 8) in the urine were measured by HPLC. Trp oxygenase (EC ), 9) kynureninase (EC : the reaction was done in the absence of added pyridoxal 5 0 -phosphate), 7) kynurenine aminotransferase (EC : the reaction was done in the absence of added pyridoxal 5 0 -phosphate), 10) 3-HA oxygenase (EC ), 9) kynurenine 3-hydroxylase (EC : the reaction was done in the presence of added NADPH), 11) ACMSDase (EC ), 12) NMN adenylyltransferase (EC ), 13) Nam methyltransferase (EC ), 14) 2-Py-forming MNA oxidase (EC ), 14) and 4-Pyforming MNA oxidase (EC number not identified) 12) were measured as described in the literature. Results Experiment 1 (70% casein diets with or without NiA in the presence of vitamin B 6 ) Table 3 shows the effects of feeding the 70% casein diet with or without NiA on the body weight gain, food intake, and food efficiency ratio. The food intake was almost the same between the two groups, but the body weight gain was significantly lower in the group fed on the NiA-free diet, as shown in Fig. 1. As a result, the food efficiency ratio was significantly lower in the NiA group than in the þnia group. That is, the necessity of niacin itself was observed in the 70% casein diet, even when a sufficient amount of Trp was taken. The urinary excretion of KA and XA in terms of nmol/g of diet is shown in Table 4. The urinary Table 3. Effects of Feeding the 70% Casein Diet with or without NiA on Body Weight Gain, Food Intake, and Food Efficiency Ratio (Experiment 1) þnia & þb 6 NiA & þb 6 Initial body weight (g) Final body weight (g) * Body weight gain (g/19 days) * Food intake (g/19 days) FER 1 0:45 0:02 0:33 0:01* 1 FER, Food Efficiency Ratio.

3 Niacin Necessity in Rats Fed on a High Protein Diet 275 Table 5. Effects of Feeding the 70% Casein Diets with or without Niacin on the Enzyme Activities Involved in the Metabolism of Trp to Niacin (Experiment 1) þnia & þb 6 NiA & þb 6 Trp oxygenase 2:03 0:22 1:77 0:22 Kynureninase 1:57 0:05 1:55 0:04 Kynurenine aminotransferase 1:14 0:10 1:03 0:02 Kynurenine 3-hydroxylase 1:54 0:15 1:96 0:40 3-HA oxygenase ACMSDase 11:3 1:4 12:1 1:1 NMN adenylyltransferase 8:97 0:71 8:31 0:46 NAD þ synthetase 0:59 0:12 0:61 0:04 Nam methyltransferase 1:85 0:03 1:89 0:04 2-Py-forming MNA oxidase 0:68 0:07 0:71 0:06 4-Py-forming MNA oxidase 1:70 0:08 1:59 0:03 Fig. 1. Effects of Feeding the 70% Casein Diets with or without NiA on Body Weight Gain (Experiment 1)., þnia & þb 6 ;, NiA & þb 6. Each point represents the mean SEM for five rats. Values with different superscript letters are statistically significantly different at p < 0:05, as calculated by the Student Newman Keuls Multiple Comparisons test. Table 4. Effects of Feeding the 70% Casein Diets with or without NiA on the Urinary Excretion of KA and XA, Nam and Its Metabolites, the Excretion Ratio of (2-Py + 4-Py)/MNA, and the Conversion Ratio of Trp to Niacin (Experiment 1) þnia & þb 6 NiA & þb 6 Food intake (g/day) 14:3 0:3 15:0 0:7 KA XA Nam * MNA * 2-Py * 4-Py * Sum * (2-Py + 4-Py)/MNA 4:7 0:3 7:1 0:8 NiA intake Trp intake Conversion ratio of Trp to Niacin (%) Not calculated 0:32 0:03 1 Sum, Nam + MNA + 2-Py + 4-Py. Values are means SEM for five rats, expressed as nmol/g of food, except for (2-Py + 4-Py)/MNA and the conversion ratio of Trp to niacin and means SEM for five rats. excretion of KA and XA was almost the same between the two groups. The urinary excretion of Nam, MNA, 2- Py, 4-Py, and the sum of Nam þ MNA þ 2-Py þ 4-Py in terms of g of diet respectively is also shown in Table 4. The higher values of each of these in the þnia group than in the NiA group was attributed to the intake of dietary NiA. The conversion ratio could not be calculated for the group fed on the þnia diet, but it could be done on the group fed on NiA diet by comparison with Trp intake during urine collection and the urinary Values are expressed as mmol/h/g of liver and means SEM for five rats. Table 6. Effects of Feeding the Vitamin B 6 -Free, and 70% Casein Diets with or without Niacin on Body Weight Gain, Food Intake, and Food Efficiency Ratio (Experiment 2) þnia & B 6 NiA & B 6 Initial body weight (g) Final body weight (g) * Body weight gain (g/19 days) * Food intake (g/19 days) * FER 1 0:20 0:02 0:09 0:01* 1 FER, Food Efficiency Ratio. Values are means SEM for five rats, expressed as mmol/h/g of liver and means SEM for five rats. excretion of sum. The value was 0:32 0:03% (mean SEM for 5 rats), as shown in Table 4. The next step was done to investigate the effects of the 70% casein diet with or without NiA on the enzyme activity of Trp to niacin. As Table 5 shows, none of the enzyme activities showed a difference between the two groups. Experiment 2 (70% casein diets with or without NiA in the absence of vitamin B 6 ) Table 6 shows the effects of feeding the vitamin B 6 - free, 70% casein diet with or without NiA on the body weight gain, food intake, and food efficiency ratio. The food intake was significantly lower in the NiA group than in the þnia group and the body weight gain was greatly lower in the group fed on the NiA-free diet, as shown in Fig. 2. As a result, the food efficiency ratio was significantly lower in the NiA group than in the þnia group. That is, the necessity of niacin itself was also ascertained in the vitamin B 6 -free and 70% casein diet. The urinary excretion of KA and XA in terms of nmol/g of diet is shown in Table 7. The urinary excretion of KA and XA was almost the same between the two groups. But that of XA was much higher in Experiment 2 than in Experiment 1 (Tables 4 and 7).

4 276 N. KIMURA et al. Table 8. Effects of Feeding Vitamin B 6 -Free, and 70% Casein Diet with or without NiA on the Enzyme Activities Involved in the Metabolism of Trp to Niacin (Experiment 2) þnia & B 6 NiA & B 6 Trp oxygenase 1:82 0:09 1:81 0:14 Kynureninase 0:39 0:09 0:32 0:03 Kynurenine aminotransferase 0:38 0:04 0:35 0:04 Kynurenine 3-hydroxylase 3:07 0:17 2:79 0:18 3-HA oxygenase ACMSDase 11:1 1:6 12:6 1:5 NMN adenylyltransferase 8:16 0:27 8:75 0:57 NAD þ synthetase 0:56 0:07 0:56 0:06 Nam methyltransferase 1:79 0:02 1:82 0:03 2-Py-forming MNA oxidase 0:05 0:02 0:10 0:04 4-Py-forming MNA oxidase 0:18 0:09 0:74 0:10* Fig. 2. Effects of Feeding the Vitamin B 6 -Free and 70% Casein Diets with or without NiA on Body Weight Gain (Experiment 2)., þnia & B 6 ;, NiA & B 6. Each point represents the mean SEM for five rats. Values with different superscript letters are statistically significantly different at p < 0:05, as calculated by the Student Newman Keuls Multiple Comparisons test. Table 7. Effects of Feeding the Vitamin B 6 -Free, and 70% Casein Diet with or without NiA on the Urinary Excretion of KA and XA, Nam and Its Metabolites, the Excretion Ratio of (2-Py + 4-Py)/MNA, and the Conversion Ratio of Trp to Niacin (Experiment 2) þnia & B 6 NiA & B 6 Food intake (g/day) 7:4 1:3 5:8 1:1 KA XA Nam MNA * 2-Py * 4-Py * Sum * (2-Py + 4-Py)/MNA 0:54 0:04 1:2 0:06* NiA intake Trp intake Conversion ratio of Trp to Niacin (%) Not calculated 0:39 0:05 1 Sum, Nam + MNA + 2-Py + 4-Py. Values are means SEM for five rats, expressed as nmol/g of food, except for (2-Py + 4-Py)/MNA and conversion ratio of Trp to niacin and means SEM for five rats. The abnormal increase in XA means that the rats were in a vitamin B 6 -deficient state. The urinary excretion of Nam, MNA, 2-Py, 4-Py, and the sum of Nam þ MNA þ 2-Py þ 4-Py in terms of g of diet respectively is also shown in Table 7. The higher values of each of these in the þnia group than in the NiA group was attributed the intake of dietary NiA. The conversion ratio could not be calculated on the group fed on þnia diet, but it could be done on the group fed on NiA group by Values are expressed as mmol/h/g of liver and means SEM for five rats. * Statistically significant difference at p < 0:05, compared with the þnia comparison with Trp intake during urine collection and the urinary excretion of sum. The value was 0:39 0:05%, as shown in Table 7. The next step was done to investigate the effects of the 70% casein diet with or without NiA on the enzyme activity of Trp to niacin. As Table 8 shows, none of the enzyme activities except for 2-Py- and 4-Py-forming MNA oxidases showed a difference between the two groups. Experiment 3 (40% and 60% casein diets with or without NiA in the presence of vitamin B 6 ) The body weight gain, food intake, and food efficiency ratio in the rats fed the 20%, 1) 40%, and 60% casein diets with or without NiA are shown in Table 9. These values are almost the same among all of the groups irrespective of dietary protein levels and NiA intake. Table 10 shows the urinary excretions of Trp niacin metabolites in the groups of 20%, 1) 40%, and 60% casein diets with and without NiA. The urinary excretions of KA and XA in terms of nmol/g of diet increased with dietary protein intake irrespective of the presence or absence of vitamin B 6. The higher values of Nam, MNA, 2-Py, and 4-Py in the þnia group than in the NiA group between each protein level were attributed the intake of dietary NiA. The conversion ratio could not be calculated on the group fed on þnia diet, but it could be done on the group fed on NiA diet by comparison with Trp intake during urine collection and the urinary excretion of sum. The value was 1:90 0:25% for the 20% casein diet, 1) 1:13 0:07 for the 40% casein diet, and 0:60 0:08% for the 60% casein diet. Table 11 shows the activities of ACMSDase in the liver. The activities were not different irrespective of presence or absence of vitamin B 6 between the same protein levels, while the activities increased with dietary protein levels.

5 Table 9. Effects of NiA Addition to the 20%, 40%, and 60% Casein Diets on Body Weight Gain, Food Intake, and Food Efficiency Ratio (Experiment 3) 20% Casein diet 1 40% Casein diet 60% Casein diet þnia NiA þnia NiA þnia NiA Initial body weight (g) Final body weight (g) Body weight gain (g/19 d) Food intake (g/19 d) Food efficiency ratio* 0:41 0:01 0:38 0:01 0:40 0:01 0:41 0:01 0:42 0:01 0:42 0:02 *FER, body weight gain (g/19 d)/food intake (g/19 d). Each value is the mean SEM for five rats. 1 Data were drawn from reference 1. Niacin Necessity in Rats Fed on a High Protein Diet 277 Table 10. Effects of Feeding the 20%, 40%, and 60% Casein Diets with or without NiA on the Urinary Excretion of KA and XA, Nam and Its Metabolites, the Excretion Ratio of (2-Py + 4-Py)/MNA, and the Conversion Ratio of Trp to Niacin (Experiment 3) 20% Casein diet 1 40% Casein diet 60% Casein diet þnia NiA þnia NiA þnia NiA Food intake (g/day) 16:7 0:6 18:0 0:8 17:9 0:6 18:5 0:7 17:9 0:6 16:5 0:4 KA 35 2:1 40 5: : : : :7 XA 28 2:5 36 4:7 55 6:4 51 4:2 67 7:8 81 3:1 Nam 16 0:8 13 1:3 17 1:4 9 0:4 9 1:4 7 0:3 MNA 55 1:1 18 1:8* 57 4:9 22 2:2* 84 8:1 24 1:1* 2-Py 61 2:4 16 2:7* 55 5:3 20 1:9 66 4:1 21 0:9* 4-Py * * * Sum * * * (2-Py + 4-Py)/MNA 9:9 0:4 9:8 0:5 8:8 1:5 10:0 1:1 3:8 0:6 7:1 0:7 NiA intake Trp intake Conversion ratio of Trp to niacin (%) Not calculated 1:90 0:25 Not calculated 1:13 0:07 Not calculated 0:60 0:08 1 Sum, Nam + MNA + 2-Py + 4-Py. Values are means SEM for five rats, expressed as nmol/g of food, except for (2-Py + 4-Py)/MNA and conversion ratio of Trp to niacin and means SEM for five rats. *Statistically significant difference at p < 0:05, compared with the respective the þnia 1 Data were drawn from reference 1. Table 11. Effects of Feeding the 20%, 40%, and 60% Casein Diets with or without NiA on the ACMSDase Activity in the Liver (Experiment 3) ACMSDase (mmol/h/g of liver) 20% Casein diet 1 40% Casein diet 60% Casein diet þnia NiA þnia NiA þnia NiA 2:4 0:6 2:3 0:6 3:81 0:37 3:69 0:57 8:85 0:36 7:90 1:16 1 Data were drawn from reference 1. Discussion Sanada 15) and our group 5,16) have reported that the ACMSDase activity increases with dietary protein levels. We also found that the conversion ratio of Trp to niacin decreases with dietary protein levels. 5,16) Hence we thought that the ACMSDase controls niacin formation to shunt the excessive niacin supply. However, from the present experimental findings, we learned that the view of the Trp niacin relationship that the ACMSDase controls the niacin formation is not right. In a previous report, 1) we found that the body weight gains between the rats fed on a diet containing NiA and those fed on the diet minus only NiA were exactly the same. That is, when rats were fed on the 20% casein diet, they do not need niacin. Vitamin B 6 is important in the Trp niacin metabolism, especially in the metabolism of kynurenine. 17) The body weight gain in young rats was significantly lower in the group fed on the 20% casein diet without vitamin B 6 than in the group fed on the 20% casein diet with vitamin B 6. 1) The lower body weight gain in the vitamin B 6 -free group was due to a

6 278 N. KIMURA et al. deficiency of vitamin B 6. 1) In fact, the urinary excretion of XA, which is an indicator of vitamin B 6 deficiency, 18) was significantly higher in the vitamin B 6 -free group than in the vitamin B 6 -containing group. 1) However, no necessity of niacin on the 20% casein diet was observed in the diet without vitamin B 6. 1) Feeding the NiA-free 70% casein diets rats caused a decrease in body weight gain as compared with the 70% casein diets containing NiA (Fig. 1). In the groups fed on the NiA-free 20%, 40%, and 60% casein diets, the urinary excretion of the sum were nmol/g of diet (Table 10), while it was around 130 nmol/g of diet in the groups on the NiA-free 70% casein diet (Tables 4 and 7). These results indicate that the rats fed on the 70% casein diet without NiA were niacin deficient. So the supplementation of NiA to the rats fed on the 70% casein diet caused the growth promoting action. These findings are very curious, since niacin is believed to be supplied from dietary Trp. 19) Under the 20% casein diet conditions, about 2% of Trp is converted to niacin. 1) On the contrary, when rats were fed on 70% protein diets, the conversion ratio was very low, about 0.3% (Table 4), and the rats, therefore, needed niacin for normal growth. The intake of Trp in the group fed on the 70% protein diet increased by 7/2 in comparison with the 20% casein diet, while the conversion decreased by 2/0.3. Therefore, the absolute formation of niacin was about half (7=2 0:3=2 ¼ 0:525) that of the rats fed on the 20% casein diet. The conversion ratio was not affected by the presence or absence of vitamin B 6 on the 70% casein diets (Tables 4 and 7), although it was severely affected by the presence or absence of vitamin B 6 on the 20% casein diets. 1) In the experiment with the 70% casein diets, the urinary excretion of XA was much more increased by feeding the vitamin B 6 -free diets (Table 7) than by feeding the B 6 -containing diets (Table 4). That is, in Experiment 1, the rats were not vitamin B 6 deficient even when they were fed the 70% casein diet. Therefore, the necessity of niacin in the 70% casein diet was not associated with the nutritional state of vitamin B 6. 20) The excretion of KA was almost the same among the four groups (Tables 4 and 7). The formations of KA and XA are catalyzed by the same enzyme, kynurenine aminotransferase, which is a PLP-dependent enzyme. This enzyme activity was much lower in the groups fed on the vitamin B 6 -free diets (Table 8) than in those fed on the vitamin B 6 -containing diets (Table 5). Nevertheless, the flux of Trp to XA increased greatly (Tables 4 and 7). The mechanism in the case of increased XA only can be explained as follows: Kynurenine was more efficiently converted to 3- hydroxykynurenine in the vitamin B 6 -deficient rats than in the normal rats because the activity of kynurenine 3- hydroxylase was increased on the vitamin B 6 -free diets (Tables 5 and 8), and 3-hydroxykynurenine, therefore, accumulates because the activity of kynureninase, which catalyzes the reaction of 3-hydroxykynurenine to 3- hydroxyanthranilic acid, decreased on the vitamin B 6 - free diets (Tables 5 and 8). The accumulated 3-hydroxykynurenine in the group fed on two vitamin B 6 -free diets in Experiment 2 was converted to XA by kynurenine aminotransferase. The reason the urinary excretion of KA did not increase might be increased kynurenine 3- hydroxylase. The side flux of Trp increased in the groups fed on the two vitamin B 6 -free diets in experiment 2, but the conversion ratio of Trp to niacin did not change between the groups fed on the diets with or without vitamin B 6 (Tables 4 and 7). This phenomenon has not been explained. The decreased conversion ratio in the high protein diets was due to the increased activity of ACMSDase as compared with that of the 20% protein diets (Tables 5 and 8 and Reference 1). It is a question why the activity of ACMSDase so increased on the high protein diets. High protein diets mean low carbohydrate diets, so that under the conditions, amino acids can be catabolized into energy formation pathways but not into protein synthesis and other biofactors. But in the present experiments it was clearly shown that the conversion ratio of Trp to niacin is subjected to the reaction - amino--carboxymuconate-"-semialdehyde! -amino muconate-"-semialdehyde, which is catalyzed by ACMSDase, and -aminomuconate-"-semialdehyde is then catabolized into acetyl-coa, but not into niacin. On the contrary, the reaction -amino--carboxymuconate- "-semialdehyde! quinolinic acid is non-enzymatic, and quinolinic acid is then metabolized into niacin. Accordingly, quinolinic acid formation from Trp is subjected to the activity of ACMSDase. The administration of an inhibitor of ACMSDase causes the greatly increased conversion ratio of Trp to niacin. 21) In conclusion, we found that rats need dietary niacin when they are fed a 70% casein diet for maximum growth, while they do not need it when they are fed 20%, 40%, or 60% diets. This phenomenon is attributed to changes in the Trp niacin conversion ratio due to the amount of protein intake. Therefore, when evaluating niacin requirements or status, protein intake must be considered. Acknowledgment This investigation was supported by a Grant-in-Aid for Scientific Research (Comprehensive Research on Cardiovascular Diseases), from The Ministry of Health, Labor and Welfare. References 1) Shibata, K., Mushiage, M., Kondo, T., Hayakawa, T., and Tsuge, H., Effects of vitamin B 6 deficiency on the conversion ratio of tryptophan to niacin. Biosci. Biotechnol. Biochem., 59, (1995). 2) Pullman, M. E., and Colowick, S. P., Preparation of 2- and 6-pyridones of N 1 -methylnicotinamide. J. Biol.

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