Tryptophan Metabolism in the Isolated Perfused Liver of the Rat: Effects of Tryptophan Concentration, Hydrocortisone and Allopurinol on Tryptophan Pyrrolase Activity and Kynurenine Formation

Abstract

1 The effect of tryptophan concentration on the rate of kynurenine appearance and tryptophan disappearance in the medium perfused through the isolated liver of the rat has been investigated. The effect of pretreatment of the rat with hydrocortisone or allopurinol was also examined, together with the effects of these treatments on liver tryptophan pyrrolase activity measured in vitro at the beginning and end of perfusion. 2 Hydrocortisone (5 mg/kg) injection 3 h before perfusion resulted in a four-fold increase in kynurenine production by the liver during perfusion with a medium containing either 0.1 mmol/l or 1.0 mmol/l tryptophan. Injection of allopurinol (20 mg/kg) together with hydrocortisone and addition of allopurinol (4 mg/100 ml) to the medium abolished the hydrocortisone-induced rise of kynurenine in the 0.1 mmol/l tryptophan medium but not the 1.0 mmol/l tryptophan medium. 3 Injection of cycloheximide (30 mg/kg) with hydrocortisone (5 mg/kg) 3 h before perfusion inhibited the hydrocortisone-induced rise of kynurenine production and the increase in pyrrolase activity measured in vitro both before and at the end of perfusion with 1.0 mmol/l tryptophan. This last result suggests that protein synthesis is involved not only in hydrocortisone induction of pyrrolase but also in substrate induction. 4 Kynurenine production in the 1.0 mmol/l tryptophan medium was less in both saline-and hydrocortisone-treated older rats (335-450 g) compared to younger rats (180-220 g). In agreement with a previous study, pyrrolase activity in vitro was also lower in both saline-and hydrocortisonetreated older rats at the beginning of the perfusion although activity had risen equally in both young and older rats at the end of perfusion.5 There was little correlation between the rate of tryptophan disappearance from the medium and the activity of tryptophan pyrrolase either as measured in vitro or as indicated by the rate of kynurenine production. 6 In general, the production of kynurenine in the medium at the end of the 60 min perfusion was indicative of in vitro pyrrolase activity at the start of the perfusion. 7 It is concluded that while in vitro pyrrolase assay does not give a quantitative index of kynurenine production, it does provide a qualitative index. Furthermore, if kynurenine production in the isolated perfused liver of the rat is indicative of in vivo pyrrolase activity, then hydrocortisone must induce pyrrolase activity in vivo.

Metabolism of an oral tryptophan load. II: Effect of pretreatment with the putative tryptophan pyrrolase inhibitors nicotinamide or allopurinol.

“…This present study may partly explain these seemingly contradictory findings. In agreement with the previous rat liver perfusion data (Green et al, 1976) and rat in vivo data , allopurinol did not apparently alter the metabolism of a tryptophan load in human subjects. Nevertheless it did increase the volume of distribution of the amino acid.…”

“…Badawy & Evans (1975), however, found that nicotinamide was also effective in producing an enhanced tryptophan accumulation. The greater increase in brain tryptophan seen in animals pretreated with allopurinol before a tryptophan load was, however, difficult to explain, since studies on the metabolism of a tryptophan load by a perfused liver preparation suggested that allopurinol did not, in fact, inhibit pyrrolase activity in the presence of high tryptophan concentrations (Green, Joseph & Woods, 1976). Furthermore, allopurinol did not inhibit tryptophan catabolism in the rat liver following a tryptophan load although the previously observed enhancement of the brain tryptophan concentration was confirmed (Joseph, Young & Curzon, 1976).…”

L-Kynurenine/Aryl Hydrocarbon Receptor Pathway Mediates Brain Damage After Experimental Stroke

“…We cannot disregard that some cerebral L-Kyn may derive from the periphery because large amounts of L-Kyn are produced in the liver through L-Trp degradation by TDO. 38 In addition, local synthesis of L-Kyn by microglia, astrocytes, or macrophages in the ischemic brain could also occur in an inflammatory microenvironment. 39 Of note, both AhR and TDO are coexpressed by neurons placed in the infarct border (data not shown), suggesting both a paracrine and an autocrine model for AhR activation by L-Kyn.…”

Kynurenine pathway metabolism following prenatal KMO inhibition and in Mecp2+/− mice, using liquid chromatography-tandem mass spectrometry

“…The alterations of development are thought to underlie some neurological or psychiatric disorders in the offspring (Brown, 2006, Brown, 2011, Hornig et al., 1999, Meyer and Feldon, 2010). Interferons generated during infection, are potent inducers of indoleamine-2,3-dioxygenase and KMO (Carlin et al., 1989, Alberati-Giani et al., 1996, Silva et al., 2002, Brooks et al., 2016), and corticosteroids produced during stress activate TDO (Green and Curzon, 1975, Green et al., 1976, Nakamura et al., 1987, Young, 1981, Zunszain et al., 2012). These compounds could, therefore, mediate the effects of infection and stress on kynurenine metabolism.…”