Effect of psoralen-induced photodermatitis on tryptophan metabolism in rats

Tryptophan Metabolism Along the Kynurenine Pathway in Rats

Enzyme activities along the kynurenine pathway, liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase, involved in the catabolism of tryptophan, were studied in male adult Wistar albino rats. Intestine superoxide dismutase and serum tryptophan were also determined. Hepatic tryptophan 2,3-dioxygenase is present both as holoenzyme and apoenzyme, but the total activity is inferior to that of intestine indole 2,3-dioxygenase which, therefore, actively oxidizes tryptophan in rats. However, this activity is inhibited by scavengers for the superoxide anion, such as superoxide dismutase, which also shows to be active in small intestine of rat. However, the more active enzymes appeared to be kynurenine 3-monooxygenase and 3-hydroxyanthranilate 3,4-dioxygenase. The former is equally active in both liver and kidney, the latter is more active in liver. Kynurenine-oxoglutarate transaminase is much more active in kidney than in liver, and much more active than kynureninase, which shows similar activities in both tissues. In contrast to the high activity of 3-hydroxyanthranilate 3,4-dioxygenase, aminocarboxymuconate-semialdehyde decarboxylase is 30-35 times less active, showing the efficiency of conversion of tryptophan to NAD. These data demonstrate that rat is a useful animal model for studying tryptophan metabolism along the kynurenine pathway. Serum tryptophan appeared to be 90% bound to proteins. Results demonstrate that, in rat, tryptophan is mainly metabolised along the kynurenine pathway. Therefore, rat is a suitable animal model for studying tryptophan metabolism in the pathological field.

Kynurenine Pathway Enzymes in Guinea pigs

In some animals, the administration of repeated doses of tryptophan can cause death. It has been reported that guinea pig does not survive repeated doses of tryptophan, due to the absence of the hormonal induction mechanism of liver tryptophan 2,3-dioxygenase (TDO). Therefore, it was of interest to investigate if guinea pig is an animal model suitable for studying tryptophan metabolism. The activities of the enzymes of the kynurenine pathway were determined. Liver TDO was present only as a holoenzyme; kynurenine 3-monooxygenase showed similar, but very high, activity in both liver and kidney. Liver and kidney kynureninase values were also similar, whereas kynurenine-oxoglutarate transaminase activity was higher in kidney than in liver. 3-Hydroxyanthranilate 3,4-dioxygenase gave similar, but very high, values in both liver and kidney, whereas aminocarboxymuconate-semialdehyde decarboxylase activity was double in kidney with respect to liver, but much lower than that of 3-hydroxyanthranilate 3,4-dioxygenase. Total and free tryptophan concentrations in serum were also determined. The free fraction was about 10% of total tryptophan.

Enzyme activities involved in tryptophan metabolism along the kynurenine pathway in rabbits

The following enzyme activities of the tryptophan-nicotinic acid pathway were studied in male New Zealand rabbits: liver tryptophan 2,3-dioxygenase, intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase. Intestine superoxide dismutase and serum tryptophan were also determined. Liver tryptophan 2,3-dioxygenase exists only as holoenzyme, but intestine indole 2,3-dioxygenase is very active and can be considered the key enzyme which determines how much tryptophan enters the kynurenine pathway also under physiological conditions. The elevated activity of indole 2,3-dioxygenase in the rabbit intestine could be related to the low activity of superoxide dismutase found in intestine. Kynurenine 3-monooxygenase appeared more active than kynurenine-oxoglutarate transaminase and kynureninase, suggesting that perhaps a major portion of kynurenine available from tryptophan may be metabolized to give 3-hydroxyanthranilic acid, the precursor of nicotinic acid. In fact, 3-hydroxyanthranilate 3,4-dioxygenase is much more active than the other previous enzymes of the kynurenine pathway. In the rabbit liver 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase show similar activities, but in the kidney 3-hydroxyanthranilate 3,4-dioxygenase activity is almost double. These data suggest that in rabbit tryptophan is mainly metabolized along the kynurenine pathway. Therefore, the rabbit can also be a suitable model for studying tryptophan metabolism in pathological conditions.

Kynurenic acid and xanthurenic acid excretion after tryptophan loading in actinic reticuloid

The metabolism of L-tryptophan (TRP) before and after an oral loading dose of 20 mg/kg body weight was studied in three patients with actinic reticuloid (AR) and four healthy male volunteers. The preload mean plasma TRP concentrations and mean 24-hour urinary excretions of TRP in the AR group were significantly lower than in the control group. After oral TRP administration, plasma TRP concentrations increased normally in AR, thereby excluding defective intestinal absorption. The mean preload and postload excretions of kynurenic acid (KA), xanthurenic acid (XA), 5-hydroxyindoleacetic acid (5HIAA), and 3-indoleacetic acid (3IAA) were not significantly different between the two groups. We conclude that KA, which has been shown to have phototoxic properties in vitro, is not altered primarily in AR, and that the reported abnormalities of TRP metabolism are most likely the consequence of the severe photosensitivity in this disorder.

Kynurenine metabolism in vitamin-B-6-deficient rat liver after tryptophan injection

Tryptophan contents of liver, serum and kidney were determined in normal and vitamin-B-6-deficient rats after tryptophan injection. Tryptophan contents of normal and B-6-deficient liver were different, but not those in serum and kidney. Both kynurenine and 3-hydroxykynurenine accumulated in B-6-deficient liver more than in the normal. The 3-hydroxykynurenine contents after tryptophan injection (30 mg/100 g body wt.) increased to 1380 nmol/g of liver at 1-1.5 h, a value sufficient to produce xanthurenate, in view of the Km value of kynurenine aminotransferase. The enzymes metabolizing kynurenine were assayed at various times after tryptophan injection. The activity of kynureninase holoenzyme in B-6-deficient liver was much decreased, but the activity of total enzyme was not changed. It appeared that a high dose of tryptophan in B-6-deficient rats could cause a greater deficiency of pyridoxal 5-phosphate. Tryptophan metabolism in B-6-deficient rat liver after tryptophan administration is discussed.