Mammalian metabolism of ingested cysteine is conducted
principally within the liver. The liver tightly regulates its
intracellular cysteine pool to keep levels high enough to meet the many
catabolic and anabolic pathways for which cysteine is needed, but low
enough to prevent toxicity.
One of the enzymes the liver uses to
regulate cysteine levels is CDO (cysteine dioxygenase). Catalysing the
irreversible oxidation of cysteine, CDO protein is up-regulated in the
liver in response to the dietary intake of cysteine.
In the present
study, we have evaluated the contribution of the ubiquitin–26 S
proteasome pathway to the diet-induced changes in CDO half-life.
In the
living rat, inhibition of the proteasome with PS1 (proteasome inhibitor
1) dramatically stabilized CDO in the liver under dietary conditions
that normally favour its degradation.
Ubiquitinated CDO intermediates
were also seen to accumulate in the liver. Metabolic analyses showed
that PS1 had a significant effect on sulphoxidation flux secondary to
the stabilization of CDO but no significant effect on the intracellular
cysteine pool.
Finally, by a combination of in vitro hepatocyte culture and in vivo
whole animal studies, we were able to attribute the changes in CDO
stability specifically to cysteine rather than the metabolite
2-mercaptoethylamine (cysteamine).
The present study represents the
first demonstration of regulated ubiquitination and degradation of a
protein in a living mammal, inhibition of which had dramatic effects on
cysteine catabolism.
Abbreviations:
CDO, cysteine dioxygenase;
CSA, cysteine sulphinic acid;
CSAD, CSA decarboxylase;
CYS, cysteine;
HP, diet, high-protein diet;
LP, diet, low-protein diet;
MEA, 2-mercaptoethylamine or cysteamine;
OPA, o-phthalaldehyde;
PS1, proteasome inhibitor 1;
SBD-F, ammonium 4-fluoro-7-sulphobenzofurazan;
THF, tetrahydrofuran
The human hereditary disorder pantothenate kinase-associated neurodegeneration (PKAN) has been associated with an accumulation of cysteine in the globus pallidus of the brain (41). Patients with PKAN show a pathological accumulation of iron in the basal ganglia and suffer from a gradual and steady deterioration of movement, speech, and cognition. The recent mapping of this disorder to mutations in the human PanK2 gene (42) suggested that impairment in coenzyme A synthesis could lead to cysteine accumulation in some tissues. Because PanK2 protein is widely expressed in tissues and is localized in the mitochondria, PKAN is thought to diminish mitochondrial function and adversely impact the globus pallidus and the retina, which are tissues with high metabolic requirements that are subject to oxidative damage. Iron accumulation in the brain is presumably caused by the lack of PanK2, which lowers the levels of 4′-phosphopantothenic acid and leads to the buildup of cysteine, which effectively binds iron. Cysteine is cytotoxic and, in the presence of iron, undergoes autooxidation, resulting in free radical formation. Free cysteine also enhances iron-induced lipid peroxidation. Thus, cysteine cytotoxicity as well as oxidative damage in the globus pallidus may contribute to the pathology of PKAN (42). From a metabolic point of view, the accumulation of cysteine suggests that its utilization for coenzyme A synthesis is important for regulation of its concentration in some tissues.
Even less is known about the regulation of coenzyme A degradation. Coenzyme A degradation involves sequential degradation of coenzyme A to dephospho-CoA + Pi, 4′-phosphopantetheine + AMP, and then pantetheine + Pi. In the final step of the degradation pathway, pantetheine is degraded to pantothenic acid and cysteamine (β-mercaptoethylamine) by an enzyme known as pantetheinase. Cysteamine can function as an antioxidant as well as a precursor for taurine biosynthesis. Little is known about the rate of cysteamine formation in mammalian tissues, but it is known that cysteamine can be converted to hypotaurine and, hence, to taurine.
Pitari et al. (43) recently reported that vanin-1–null mice were deficient in membrane-bound pantetheinase in liver and kidney and had negligible levels of cysteamine in their tissues. Vanin proteins were recently identified as pantetheinases on the basis of sequence similarity with pig pantetheinase (44). Vanin proteins are encoded by 2 genes in mice (vanin-1 and -3) and 3 in humans (vanin-1, -2, and -3) (45–48). The unanticipated observation of pantetheinase deficiency in the vanin-1–null mouse model may facilitate efforts to evaluate the quantitative significance of coenzyme A turnover in vivo. If further work confirms a substantial rate of pantetheine formation and hydrolysis in mammalian tissues, this would imply a substantial rate of coenzyme A turnover, leading to a substantial pool of cysteamine for taurine biosynthesis. In this regard, we have observed higher plasma cysteamine concentrations in rats fed a high-protein (low-carbohydrate) diet (13 ± 2 μmol/L, mean ± SD) than in rats fed a low-protein (high-carbohydrate) diet (2.3 ± 0.7 μmol/L) and also in rats that had been fasted overnight (23 ± 4 μmol/L) compared with rats in an absorptive state (13 ± 2 μmol/L).
When
the cysteinesulfinate intermediate itself was used as substrate, the
intact rat as well as isolated hepatocytes, renal cortical tubules, and
enterocytes all exhibited a high capacity for cysteinesulfinate
metabolism to CO2 or SO4, with rates of cysteinesulfinate oxidation far exceeding those for cysteine catabolism to CO2 or SO4 (53–56).
However, only hepatocytes had a high capacity for taurine synthesis
from cysteinesulfinate, which is consistent with their higher level of
cysteinesulfinate decarboxylase (CSD, EC 4.1.1.29)
activity. This tissue difference in taurine production from
cysteinesulfinate demonstrates that partitioning of cysteinesulfinate
between decarboxylation and transamination pathways can potentially be
regulated at the level of CSD activity. A modulatory role of CSD
activity on partitioning of cysteinesulfinate to taurine has, in fact,
been demonstrated in hepatocytes from rats fed high-protein or very high
sulfur amino acid–containing diets. The amount of hepatic CSD decreased
by up to 80% in rats fed a high-protein diet, and this was associated
with a decreased rate of taurine production from cysteinesulfinate in
studies done in vitro with hepatocytes from these rats (50).
Despite the existence of regulation at the level of the partitioning of
cysteinesulfinate between the decarboxylation (taurine) and
transamination (pyruvate + sulfate) pathways, however, the overall flux
of cysteine to taurine is largely driven by the dietary sulfur amino
acid level and the associated changes in hepatic cysteine dioxygenase
activity (i.e., by changes in rate of cysteinesulfinate production).
Thus, despite a modest decrease in CSD activity in response to an
increase in protein intake, the overall effect of an increase in protein
intake is an increase in substrate for CSD and a large increase in
taurine synthesis.
The high flux of cysteine through the cysteinesulfinate pathway under conditions of excess cysteine availability does not necessarily imply that the cysteamine pathway is a negligible contributor to taurine synthesis. On the contrary, there is ample indirect evidence for the synthesis of taurine via this route in the central nervous system and certain other tissues that express very low levels of cysteine dioxygenase and thus are unlikely to have substantial cysteine →taurine flux through the cysteinesulfinate pathway (57). Efforts to evaluate flux through the cysteamine pathway have been limited by incomplete data on the rate of coenzyme A turnover (58), by the technical difficulty in measuring cysteamine (59,60), and by the lack of definitive identification of cysteamine dioxygenase, the enzyme responsible for oxidation of cysteamine to hypotaurine (60).
We recently demonstrated that intact rats have a relatively large capacity for conversion of cysteamine to hypotaurine. Rats fed a basal low-protein diet (100 g casein/kg diet) supplemented with cysteamine or an equimolar amount of cysteine had markedly elevated levels of hypotaurine in liver, kidney, and brain at 6 and 10 h after introduction of the supplemented diet. As shown in Figure 8, tissue hypotaurine levels were higher in rats fed a diet supplemented with cysteine than in those fed a diet supplemented with an equimolar amount of cysteamine. This is consistent with cysteine being more readily converted to hypotaurine as a result of cysteine dioxygenase and cysteinesulfinate decarboxylase activities. Nevertheless, the increase (above basal, at 6 h) in hypotaurine level in liver and kidney of rats given supplemental cysteamine was 42 and 52% as much, respectively, as that observed in rats given an equimolar amount of supplemental cysteine, demonstrating that cysteamine is a good precursor of hypotaurine in vivo. This is even more striking because tissue cysteamine concentrations were not increased as much by cysteamine supplementation as cysteine concentrations were increased by cysteine supplementation. Supplementation of the diet with cysteine had no effect on tissue cysteamine concentrations, and supplementation of the diet with cysteamine had no effect on tissue cysteine concentrations. Thus, cysteamine can be converted to hypotaurine at a physiologically significant rate. Depending on the rate of cysteamine production via coenzyme A turnover, cysteamine could be a quantitatively important precursor of taurine.
CDO, cysteine dioxygenase;
CSA, cysteine sulphinic acid;
CSAD, CSA decarboxylase;
CYS, cysteine;
HP, diet, high-protein diet;
LP, diet, low-protein diet;
MEA, 2-mercaptoethylamine or cysteamine;
OPA, o-phthalaldehyde;
PS1, proteasome inhibitor 1;
SBD-F, ammonium 4-fluoro-7-sulphobenzofurazan;
THF, tetrahydrofuran
CDO (5q22.3) https://www.ncbi.nlm.nih.gov/gene/1036
- Also known as CDO-I
- Expression Biased expression in liver (RPKM 94.6), fat (RPKM 66.8) and 8 other tissues See more
- Orthologs mouse all
- Preferred Names
- cysteine dioxygenase type 1
- Names
- cysteine dioxygenase, type I
ORIGIN 1 meqtevlkpr tladlirilh qlfagdevnv eevqaimeay esdptewamy akfdqysrgr 61 glqfvvgggs gggwlwytrn lvdqgngkfn lmilcwgegh gssihdhtns hcflkmlqgn 121 lketlfawpd kksnemvkks ervlrenqca yindsiglhr venishtepa vslhlysppf 181 dtchafdqrt ghknkvtmtf hskfgirtpn atsgslenn //
- Methylated promoter DNA of CDO1 gene and preoperative serum CA19-9 are prognostic biomarkers in primary extrahepatic cholangiocarcinoma. Nakamoto S, et al. PLoS One, 2018. PMID 30325974, Free PMC Article
- Epigenetic Status of CDO1 Gene May Reflect Chemosensitivity in Colon Cancer with Postoperative Adjuvant Chemotherapy. Yokoi K, et al. Ann Surg Oncol, 2019 Feb. PMID 30311169
- Cysteine dioxygenase type 1 (CDO1) gene promoter methylation during the adenoma-carcinoma sequence in colorectal cancer. Kojima K, et al. PLoS One, 2018. PMID 29746493, Free PMC Article
- Prognostic significance of promoter DNA hypermethylation of the cysteine dioxygenase 1 (CDO1) gene in primary gallbladder cancer and gallbladder disease. Igarashi K, et al. PLoS One, 2017. PMID 29161283, Free PMC Article
- The clinical significance of cysteine dioxygenase type 1 methylation in Barrett esophagus adenocarcinoma. Kojima K, et al. Dis Esophagus, 2017 Mar 1. PMID 28184414
J Nutr. 2006 Jun;136(6 Suppl):1652S-1659S. doi: 10.1093/jn/136.6.1652S.
Mammalian cysteine metabolism: new insights into regulation of cysteine metabolism.
The mammalian liver tightly regulates its free cysteine pool, and intracellular cysteine
in rat liver is maintained between 20 and 100 nmol/g even when sulfur
amino acid intakes are deficient or excessive. By keeping cysteine levels within a narrow range and by regulating the synthesis of glutathione, which serves as a reservoir of cysteine, the liver addresses both the need to have adequate cysteine to support normal metabolism and the need to keep cysteine levels below the threshold of toxicity. Cysteine catabolism is tightly regulated via regulation of cysteine dioxygenase (CDO) levels in the liver, with the turnover of CDO protein being dramatically decreased when intracellular cysteine levels increase. This occurs in response to changes in the intracellular cysteine concentration via changes in the rate of CDO ubiquitination and degradation. Glutathione synthesis also increases when intracellular cysteine levels increase as a result of increased saturation of glutamate-cysteine ligase (GCL) with cysteine, and this contributes to removal of excess cysteine. When cysteine levels drop, GCL activity increases, and the increased capacity for glutathione synthesis facilitates conservation of cysteine
in the form of glutathione (although the absolute rate of glutathione
synthesis still decreases because of the lack of substrate). This
increase in GCL activity is dependent on up-regulation of expression of
both the catalytic and modifier subunits of GCL, resulting in an
increase in total catalytic subunit plus an increase in the catalytic
efficiency of the enzyme. An important role of cysteine utilization for coenzyme A synthesis in maintaining cellular cysteine levels in some tissues, and a possible connection between the necessity of controlling cellular cysteine
levels to regulate the rate of hydrogen sulfide production, have been
suggested by recent literature and are areas that deserve further study.
- PMID:
- 16702335
- DOI:
- 10.1093/jn/136.6.1652S
The mammalian liver tightly regulates its intracellular free cysteine pool. In rats, for instance, intracellular cysteine is narrowly maintained between 20 and 100 nmol/g even when dietary protein or sulfur amino acid intake is varied from subrequirement to above-requirement levels for this species (1). The effect of diet on plasma and hepatic cysteine levels is illustrated by the data shown in Figure 1. Rats that had been adapted to a high-protein diet and then fed a low-protein diet supplemented with cysteine had, at 6 h after the diet was introduced, a large increase in the portal plasma cysteine concentration but no increase above the fasting value for cysteine in the arterial plasma or in the liver. On the other hand, the plasma cysteine concentration was not significantly decreased, compared with fasting levels, in rats fed a low-protein diet, whereas the hepatic cysteine concentration was markedly decreased. Thus, in rats, the liver allows its own cysteine concentration to vary about 5-fold (from 20 to 100 nmol/g) while regulating cysteine degradation to maintain the plasma cysteine concentration within a 2.5-fold range (between 80 and 200 μmol/L). By keeping cysteine levels within a very narrow range, the liver addresses 2 opposing homeostatic requirements. Cysteine levels must be sufficiently high to meet the needs of protein synthesis and the production of other essential molecules that include glutathione, coenzyme A, taurine, and inorganic sulfur. At the same time, however, cysteine concentrations must also be kept below the threshold of cytotoxicity. The potent toxicity of excess cysteine has been demonstrated in several animal models (2–4), and chronically high levels of cysteine have been closely associated with rheumatoid arthritis (5), Parkinson's disease (6), Alzheimer's disease (6), systemic lupus erythematosus (7), increased risk of cardiovascular disease (8), and adverse pregnancy outcomes in humans (9).
The central role of hepatic cysteine dioxygenase in regulation of cysteine levels.
An important enzyme that contributes to the regulation of steady-state intracellular cysteine levels is cysteine dioxygenase (CDO,4 EC 1.13.11.20). Expressed at high levels in the liver with lower levels in the kidney, brain, and lung, this iron metalloenzyme catalyzes the addition of molecular oxygen to the sulfhydryl group of cysteine, yielding cysteinesulfinic acid. The oxidative catabolism of cysteine to cysteinesulfinate by CDO represents an irreversible loss of cysteine from the free amino acid pool; cysteinesulfinate is shuttled into several pathways including hypotaurine/taurine synthesis, sulfite/sulfate production, and the generation of pyruvate; a metabolic flow chart in Figure 2 highlights CDO's position within the context of cysteine's catabolic pathways. In vivo data suggest that the liver, the organ with the highest amount of CDO protein expression and activity, may use CDO as a means of disposing of excess cysteine obtained through the diet and in the process conveniently generates cysteinesulfinate, the biosynthetic precursor of the essential metabolites sulfate, hypotaurine, and taurine (10). These final endproducts of cysteine sulfoxidation, from a toxicity standpoint, are far more benign than cysteine.
....
Utilization of cysteine for coenzyme A synthesis: new insights from the roles of pantothenate kinase and pantetheinase in the pathway.
Although the pathway for coenzyme A synthesis is well established, the rate of coenzyme A turnover and the extent of cysteine consumption for coenzyme A turnover have not been quantified. The flux-generating step in coenzyme A biosynthesis is the first step in the pathway and is catalyzed by pantothenate kinase. Cysteine is condensed with pantothenate in the second step of the coenzyme A synthesis pathway to form 4′-phosphopantothenoylcysteine, and the cysteine moiety is decarboxylated in the third step of the pathway to form the cysteamine, or β-mercaptoethylamine, moiety of coenzyme A. The rate of coenzyme A synthesis is determined largely by the regulated pantothenate kinase step, which is highly regulated in response to factors that favor lipid oxidation. Multiple isoforms of mammalian pantothenate kinase (PanK) are encoded by 4 genes in humans and in mice, and the regulatory properties of the various pantothenate kinase isoforms allow the robust control of coenzyme A biosynthesis by coenzyme A and its thioesters (39,40).The human hereditary disorder pantothenate kinase-associated neurodegeneration (PKAN) has been associated with an accumulation of cysteine in the globus pallidus of the brain (41). Patients with PKAN show a pathological accumulation of iron in the basal ganglia and suffer from a gradual and steady deterioration of movement, speech, and cognition. The recent mapping of this disorder to mutations in the human PanK2 gene (42) suggested that impairment in coenzyme A synthesis could lead to cysteine accumulation in some tissues. Because PanK2 protein is widely expressed in tissues and is localized in the mitochondria, PKAN is thought to diminish mitochondrial function and adversely impact the globus pallidus and the retina, which are tissues with high metabolic requirements that are subject to oxidative damage. Iron accumulation in the brain is presumably caused by the lack of PanK2, which lowers the levels of 4′-phosphopantothenic acid and leads to the buildup of cysteine, which effectively binds iron. Cysteine is cytotoxic and, in the presence of iron, undergoes autooxidation, resulting in free radical formation. Free cysteine also enhances iron-induced lipid peroxidation. Thus, cysteine cytotoxicity as well as oxidative damage in the globus pallidus may contribute to the pathology of PKAN (42). From a metabolic point of view, the accumulation of cysteine suggests that its utilization for coenzyme A synthesis is important for regulation of its concentration in some tissues.
Even less is known about the regulation of coenzyme A degradation. Coenzyme A degradation involves sequential degradation of coenzyme A to dephospho-CoA + Pi, 4′-phosphopantetheine + AMP, and then pantetheine + Pi. In the final step of the degradation pathway, pantetheine is degraded to pantothenic acid and cysteamine (β-mercaptoethylamine) by an enzyme known as pantetheinase. Cysteamine can function as an antioxidant as well as a precursor for taurine biosynthesis. Little is known about the rate of cysteamine formation in mammalian tissues, but it is known that cysteamine can be converted to hypotaurine and, hence, to taurine.
Pitari et al. (43) recently reported that vanin-1–null mice were deficient in membrane-bound pantetheinase in liver and kidney and had negligible levels of cysteamine in their tissues. Vanin proteins were recently identified as pantetheinases on the basis of sequence similarity with pig pantetheinase (44). Vanin proteins are encoded by 2 genes in mice (vanin-1 and -3) and 3 in humans (vanin-1, -2, and -3) (45–48). The unanticipated observation of pantetheinase deficiency in the vanin-1–null mouse model may facilitate efforts to evaluate the quantitative significance of coenzyme A turnover in vivo. If further work confirms a substantial rate of pantetheine formation and hydrolysis in mammalian tissues, this would imply a substantial rate of coenzyme A turnover, leading to a substantial pool of cysteamine for taurine biosynthesis. In this regard, we have observed higher plasma cysteamine concentrations in rats fed a high-protein (low-carbohydrate) diet (13 ± 2 μmol/L, mean ± SD) than in rats fed a low-protein (high-carbohydrate) diet (2.3 ± 0.7 μmol/L) and also in rats that had been fasted overnight (23 ± 4 μmol/L) compared with rats in an absorptive state (13 ± 2 μmol/L).
Pathways of taurine synthesis: cysteinesulfinate- and cysteamine-dependent pathways.
The pathways for taurine synthesis from cysteine are shown in Figure 7. The relative contribution of the cysteinesulfinate-dependent pathway versus the cysteamine-dependent pathway to net taurine production is not clear, largely because the magnitude of flux through the cysteamine pool has not been assessed. Several research groups, including our own laboratory, have focused their efforts on the cysteinesulfinate pathway, and we have shown that flux through this pathway is highly responsive to cysteine concentration (49–53). Regulation of taurine biosynthesis via this pathway is mediated principally at the level of cysteine dioxygenase concentration, which is directly controlled by substrate concentration (17,18). Studies with isolated hepatocytes clearly show that changes in cysteine dioxygenase activity play a dominant role in determining the rates of both taurine and sulfate formation (50–52).
FIGURE 7
The high flux of cysteine through the cysteinesulfinate pathway under conditions of excess cysteine availability does not necessarily imply that the cysteamine pathway is a negligible contributor to taurine synthesis. On the contrary, there is ample indirect evidence for the synthesis of taurine via this route in the central nervous system and certain other tissues that express very low levels of cysteine dioxygenase and thus are unlikely to have substantial cysteine →taurine flux through the cysteinesulfinate pathway (57). Efforts to evaluate flux through the cysteamine pathway have been limited by incomplete data on the rate of coenzyme A turnover (58), by the technical difficulty in measuring cysteamine (59,60), and by the lack of definitive identification of cysteamine dioxygenase, the enzyme responsible for oxidation of cysteamine to hypotaurine (60).
We recently demonstrated that intact rats have a relatively large capacity for conversion of cysteamine to hypotaurine. Rats fed a basal low-protein diet (100 g casein/kg diet) supplemented with cysteamine or an equimolar amount of cysteine had markedly elevated levels of hypotaurine in liver, kidney, and brain at 6 and 10 h after introduction of the supplemented diet. As shown in Figure 8, tissue hypotaurine levels were higher in rats fed a diet supplemented with cysteine than in those fed a diet supplemented with an equimolar amount of cysteamine. This is consistent with cysteine being more readily converted to hypotaurine as a result of cysteine dioxygenase and cysteinesulfinate decarboxylase activities. Nevertheless, the increase (above basal, at 6 h) in hypotaurine level in liver and kidney of rats given supplemental cysteamine was 42 and 52% as much, respectively, as that observed in rats given an equimolar amount of supplemental cysteine, demonstrating that cysteamine is a good precursor of hypotaurine in vivo. This is even more striking because tissue cysteamine concentrations were not increased as much by cysteamine supplementation as cysteine concentrations were increased by cysteine supplementation. Supplementation of the diet with cysteine had no effect on tissue cysteamine concentrations, and supplementation of the diet with cysteamine had no effect on tissue cysteine concentrations. Thus, cysteamine can be converted to hypotaurine at a physiologically significant rate. Depending on the rate of cysteamine production via coenzyme A turnover, cysteamine could be a quantitatively important precursor of taurine.
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