Response of Phosphopyruvate Carboxylase to Tryptophan Metabolites and Metal Ions

Abstract

Tryptophan, administered to rats, inhibits gluconeogenesis, but increases the activity of hepatic phosphopyruvate carboxylase when assayed in vitro. Quinolinate, a metabolite of tryptophan, inhibits gluconeogenesis in perfused livers, and inhibits phosphopyruvate carboxylase in liver cytosol from tryptophan-treated rats but not from normal rats.Metal The elevated phosphopyruvate carboxylase activity in cytosol of tryptophan-treated rats was lowered in vitro by quinolinate to the same extent as was the activity of the Fe2+-treated enzyme from normal rats. Ferrous iron is proposed as the physiologically important cation and quinolinate the important metabolite responsible for altering the activity of phoshopyruvate carboxylase in vivo following tryptophan administration.Tryptophan administered to rats enhances the activity of several hepatic enzymes that participate in gluconeogenesis. Among these, phosphopyruvate carboxylase responds in an unusual fashion ; the measurable activity is rapidly increased but the reaction catalyzed by this enzyme appears to be blocked in the intact rat 11-31. The observed elevation of P-pyruvate carboxylase activity is the result of two processes, first a rapid activation of existing enzyme and then a slower increase in activity as new enzyme is synthesized [3]. Tryptophan, at levels as high as 1.7 mM, did not affect the enzymic activity in vitro. However, 50 to 100pM MnCl, or FeCl, added to reaction mixtures containing 6 mM MgCI, activated P-pyruvate carboxylase to levels seen with the enzyme from livers of tryptophantreated rats [3]. The P-pyruvate carboxylase from the latter source was not activated by Mn2+ [3].Ray et al. [2] demonstrated that inhibition of gluconeogenesis after tryptophan administration to fasted rats occurred at the P-pyruvate carboxylase step. Metabolite crossover-type plots showed that the concentrations of gluconeogenic intermediates before the P-pyruvate carboxylase step were higher, while metabolites after that step were lower than the Enzyme. Phosphopyruvate carboxylase or GTP : oxaloacetate carboxy-lyase (transphosphorylating) (EC 4.1.1.32).concentrations of corresponding metabolites in livers of control animals. Veneziale et al.[a] perfused isolated livers from fasted rats with various metabolites of tryptophan and found that compounds in the pathway to nicotinic acid ribonucleotide also inhibited glucose production. Quinolinate, the precursor of nicotinic acid ribonucleotide, blocked gluconeogenesis and activated P-pyruvate carboxylase and was suggested to be the agent responsible for the effect of tryptophan.The key role that P-pyruvate carboxylase plays in gluconeogenesis and the need for information on the regulation of this enzyme led to further investigation of the interaction of metal ions and tryptophan metabolites on enzyme activity in vitro. MATERIALS AND METHODSMale albino rats weighing 160 to 200g (Badger Research Corporation, Madison, Wisconsin) were maintained on water and Rockland chow ad libitum. Animals were fasted for 24 h before sacrit...

Suppressive effect of dietary unsaturated fatty acids on .ALPHA.-amino-.BETA.-carboxymuconate-.EPSILON.-semialdehyde decarboxylase, a key enzyme of tryptophan-niacin metabolism in rat liver.

“…It has been shown that administration of tryptophan to fasting rats results in the suppression of gluconeogenesis and enhancement of fatty acid synthesis in the liver (24). These phenomena have been partially interpreted as being caused by the inhibitory effect of quinolinate, which is produced from tryptophan, on phosphoenolpyruvate carboxykinase activity (25). Therefore it is likely that dietary fat would alter lipid and carbohydrate metabolism by increased quinolinate production.…”

The Inhibition of PhospoEnol Pyruvate Carboxykinase Following in Vivo Chronic Phenobarbital Treatment in the Rat is Due to a Post‐Translational Event

“…In addition to hormones, divalent cations have been reported to stimulate Ppymvate carboxykinase activity (Snoke et al, 1971 ;Colombo and Lardy, 1981 ;Schramm et al, 1981;Schramm 1986;Lee et al, 1981 ;Nowak, 1986) by modifying transcription, although this subject is a matter of controversy (Maggini et al, 1993). Lardy and coworkers proposed a catalytic effect mediated by a metal-enzyme complex dependent upon a protein activator found in the hepatocyte cytosol (Bentle, 1976;Bentle and Lardy, 1977).…”

Inhibition by Hydrazine Sulfate and Various Hydrazides, of <i>in vivo</i> Growth of Walker 256 Intramuscular Carcinoma, B-16 Melanoma, Murphy-Sturm Lymphosarcoma and L-1210 Solid Leukemia

“…V en ezia le et al [1967] have shown that the metabolite of L-tryptophan largely (but not completely) responsible for the anti-gluconeogenic action (PEP CK inhibition) of L-tryptophan, is quino linic acid -so that the compound quinolinic acid dihydrazide has actually two chemical moieties in the same molecule (quinolinic acid and hydrazine), each of which can separately inhibit PEP CK; this may very well account for its augmented action against tumors. Regarding the mechanism of action against PEP CK itself, there is evidence that this metal enzyme is activated in the intact liver by ferrous ion [Snoke et al, 1972] and it is known that quino linic acid in sufficient quantities will bind ferrous ion, forming a ferrous coor dination complex which inhibits the enzyme [Veneziale et al, 1967]. It has not been established whether the same iron complexing mechanism pertains to the hydrazides as a class, although it is known that sebacic dihydrazidean exceedingly good tumor inhibitor in the present study -is also an excellent chelating agent [Olin Corporation, personal commun.…”