Methionine-sparing effect of cystine in human subjects

Clinical characteristics of H-type hypertension and its relationship with the MTHFR C677T polymorphism in a Zhuang population from Guangxi, China

Objective

This study was designed to assess the clinical presentation of patients with H‐type hypertension who were of Zhuang nationality in Guangxi, China. The relationship between the C677T polymorphism in the MTHFR gene and H‐type hypertension was also assessed.

Methods

This was a case‐control study in which 185 Zhuang nationality patients with hypertension that had been hospitalized at the Wuming Hospital of Guangxi Medical University between February 2018 and December 2018 were assessed for plasma homocysteine (Hcy) levels. These levels were used to divide patients into H‐type (>15 μmol/L) and non‐H‐type (≤15 μmol/L) hypertension groups. Patient clinical data were then analyzed, and PCR was used to analyze samples from all patients for the presence of the C677T polymorphism in the MTHFR gene. Differences between these two groups of hypertension patients were then compared using appropriate statistical methods.

Results

We found that relative to patients in the non‐H‐type hypertension group, patients in the H‐type hypertension group exhibited significant differences in sex, age, urea nitrogen levels, creatinine levels, and uric acid levels. There were, however, no significant differences between these two groups with respect to interventricular septum thickness, left ventricular posterior wall thickness, or ejection fraction. We did not detect any association between the MTHFR gene C677T polymorphism and H‐type hypertension in Zhuang nationality individuals in Guangxi.

Conclusion

Risk of H‐type hypertension is not associated with the MTHFR C677T polymorphism in hypertensive individuals of Guangxi Zhuang nationality in China.

Mechanistic basis of hypermethioninemia

Hypermethioninemia is a condition defined as elevated plasma methionine levels and may be a consequence of different conditions that include non-genetic and genetic causes. In severe cases, hypermethioninemia may lead to development of neurological and hepatic impairments, but mechanisms are still not well elucidated. Therefore, this review aims to reunite the knowledge acquired about the methionine-induced brain and liver toxicity focusing on the results obtained by studies from patients, in vitro experiments, and in vivo animal models. In general, some studies have shown that methionine decreases Na⁺,K⁺-ATPase activity, induces oxidative stress, increases acetylcholinesterase activity, and leads to dendritic spine downregulation in brain. Concerning to liver, hypermethioninemia seems to provoke changes in cell morphology, lipid accumulation, oxidative stress, inflammation, and ATP depletion. It is possible to infer that oxidative damage is one of the most important mechanisms responsible for methionine toxicity, since different studies showed that this amino acid induces oxidative stress in brain and liver tissues. Besides, reactive oxygen species may mediate other alterations induced by methionine, such as the reduction in brain Na⁺,K⁺-ATPase activity, and liver inflammation.

Methyltransferase-inhibition interferes with neuronal differentiation of P19 embryonal carcinoma cells

We have analyzed the importance of substrate methylation by S-adenosylmethionine-dependent methyltransferases for neuronal differentiation of P19 embryonal carcinoma cells. We show that treatment of cells with methyltransferase inhibitor adenosine dialdehyde (AdOx) interferes with neuronal differentiation. Retinoic acid (RA) and AdOx co-treated cells had a decreased number of neurites and a flattened morphology compared with cells differentiated by RA. Also, the amount of neuronal class III tubulin (Tuj1) decreased from 76% to 9.6% with AdOx-treatment. Gene expression levels of wnt-1, brn-2, neuroD, and mash-1 were also down-regulated by AdOx-treatment. But AdOx-treatment did not up-regulate BMP-4 and GFAP genes. Treatment of RA decreased E-cadherin expression during neuronal differentiation. However, in AdOx/RA co-treated cells, E-cadherin expression was restored to the control level. Also, mRNA expression of N-cadherin decreased with AdOx-treatment. Taken together, these data show that methylation reactions might influence the cell-fate decision and neuronal differentiation of P19 cells.

The in vivo sparing of methionine by cysteine in sulfur amino acid requirements in animal models and adult humans

Sulfur amino acid metabolism has been receiving increased attention because of the link to chronic diseases such as cardiovascular disease, Alzheimer's disease, and diabetes. In addition, the role of cysteine and optimal intakes for physiological substrates such as glutathione are currently of considerable interest in human health. Although the dietary indispensability of methionine is not in question, the ability of cysteine to substitute for a portion of its requirement has been the topic of much debate. Methionine is often the most limiting amino acid in the diets of the developing world's population because of its low concentration in cereal grains. Therefore, the ability of cysteine to substitute for methionine requirement is not just biologically interesting; it is also of considerable economic and social importance. The primary goal of this review is to discuss the available evidence on the effect of cysteine substitution for methionine to meet the total sulfur amino acid requirement in adult humans, including an assessment of the methodological features of experiments with conflicting results. Assessment of the requirement experiments for amino acids with complex metabolism such as methionine and cysteine must begin with a careful definition of requirements and what substitution means. As a result of these definitions, a set of criteria for the intakes of methionine that will allow demonstration of the substitution effect have been developed. Some recent publications are assessed using these definitions and criteria, and a possible reason for the conflicting results in the literature is proposed. An approach to estimating tolerable upper intakes is also proposed. Research on in vivo sulfur amino acid metabolism in humans is tremendously difficult, and therefore, we do not wish to be overly critical of the high-quality work of the ambitious and highly intelligent men and women who have conducted various studies. Our goal is to objectively review the data for the reader in a logical and comprehensive manner and propose methods that may avoid difficulties in future studies.

Effect of cystine on the methionine requirement of healthy Indian men determined by using the 24-h indicator amino acid balance approach

BACKGROUND

The 1985 FAO/WHO/UNU requirement for methionine in healthy adults consuming a cystine-free diet is 13 mg.kg(-1).d(-1). It is unclear whether this daily requirement is influenced by dietary cystine.

OBJECTIVE

We assessed the effect of 2 intakes of cystine (5 and 12 mg.kg(-1).d(-1)) on methionine requirements in well-nourished Indian men by using 7 test methionine intakes (3, 6, 9, 13, 18, 21 and 24 mg.kg(-1).d(-1)) and the 24-h indicator amino acid oxidation (24-h IAAO) and balance (24-h IAAB) methods. We combined these data with those from an experiment with zero cystine intake and in which the exact same method was used.

DESIGN

Two studies were performed in which a diet containing either 5 or 12 mg cystine.kg(-1).d(-1) was fed to 21 well-nourished Indian men over three 7-d periods. The 24-h IAAO and 24-h IAAB values were measured on day 7 with the use of a 24-h intravenous [13C]leucine tracer infusion. The breakpoints in the relation between these values and methionine intake in each study were assessed by two-phase linear regression.

RESULTS

Breakpoints in the response curve were obtained at methionine intakes of 20 (95% Fiellers CI: 17, 26) and 10 (95% Fiellers CI: 8, 16) mg.kg(-1).d(-1) with cystine intakes of 5 and 12 mg.kg(-1).d(-1) intakes, respectively, which suggested a sparing effect of cystine. Although the 5- and 12-mg cystine breakpoints differed from one another, they did not differ significantly from that estimated previously with 0 mg cystine.

CONCLUSION

Cystine may spare the methionine requirement in healthy men, although the amount of sparing is difficult to quantify.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

Analysis of cell-cycle kinetics and sulfur amino acid metabolism in methionine-dependent tumor cell lines; the effect of homocysteine supplementation

Methionine dependence is a feature unique to cancer cells, exhibited as inability to grow in a methionine-depleted environment supplemented with homocysteine, the immediate metabolic precursor of methionine. This study explores the effect of methionine depletion and homocysteine supplementation on the viability, sulfur amino acid metabolism and cell-cycle kinetics of normal and cancer cells, as well as their ability to recover from the treatments. An array of cells including hepatomas (HTC, Phi-1), prostate adenocarcinomas (PC-3) and transformed (3T3) and normal (HS-27) fibroblasts, has been used aiming to evaluate the importance of tissue specificity. All cell lines proliferated well in methionine-complete media (M+H-), whilst only the normal fibroblasts HS-27 grew in methionine-depleted homocysteine-supplemented media (M-H+). None of the tested cell lines were able to grow in media without methionine or homocysteine (M-H-). HTC was the only cell line that did not recover from the M-H+ treatment whilst PC-3 did not recover from the M-H- treatment. Methionine and homocysteine depletion (M-H+ and M-H-) were found to induce arrest at different phases of the cell cycle, depending on the cell line: the methionine-dependent HTC, PC-3 and 3T3 arrested at the S and G2/M phase, whilst Phi-1 and the methionine-independent HS-27 accumulated in the G1 phase. The cell-cycle kinetics showed that the observed blockades were reversible. The information resulting from these studies is important for not only the behavior of cancer cells, but also for appreciating the potential of developing cancer therapies based on methionine-depletion strategies.

Methionine restriction selectively targets thymidylate synthase in prostate cancer cells

Tumor cells are more sensitive to methionine restriction than normal tissues, a phenomenon known as methionine auxotrophy. Previous studies showed that 5-fluorouracil and methionine restriction act synergistically against a variety of tumors. The purpose of the current studies was to determine the molecular mechanism(s) underlying this synergy. 5-Fluorouracil is known to inhibit thymidylate synthase (TS), a key enzyme that transfers a methyl group from 5,10-methylene-tetrahydrofolate to dUMP during nucleotide biosynthesis. We found that methionine restriction reduced 5,10-methylene-tetrahydrofolate levels by 75% and selectively inhibited TS activity in PC-3 human prostate cancer cells within 24hr, whereas it did not in normal prostate epithelial cells. The observed fall in TS activity was accompanied by a commensurate reduction in TS protein levels as determined by western blot analysis. In contrast, 5-fluorouracil inhibited TS activity by >90% but increased TS protein levels. This increase was abrogated by methionine restriction. Surprisingly, methionine restriction increased 3H-leucine incorporation in PC-3 cells over the first 24hr, suggesting that reduction of TS levels was not simply due to global protein synthesis inhibition. Methionine restriction also significantly reduced the ratio of dUMP to dTTP in PC-3 cells, creating an imbalanced nucleotide pool. These results suggest that synergy between methionine restriction and 5-fluorouracil is attributable to multiple factors, including depletion of reduced folates, selective inhibition of TS, and creation of an imbalanced nucleotide pool. Dietary and/or enzymatic methionine restriction combined with 5-fluoruracil has great promise as a novel treatment for advanced cancer.

The influence of the albumin fraction on the bioavailability and postprandial utilization of pea protein given selectively to humans

Pulse seed proteins such as those found in peas (Pisum sativum) contain fractions of very dissimilar composition and properties, which may therefore be differently utilized by the human body. To analyze the nutritional value of the soluble protein fractions of pea seed, human volunteers ingested a mixed meal of 30 g of raw purified pea protein either as [15N]-globulins (G, n = 9) or as a mix of [15N]-globulins and [15N]-albumins (GA, n = 7) in their natural proportions (22:8). Dietary and endogenous nitrogen fluxes at the terminal ileum were assessed using a tube perfusion technique with an isotopic dilution method. Systemic dietary amino acid availability and the retention of dietary amino acids were determined using 15N enrichment in plasma amino acids and deamination products in blood and urine for 8 h postprandially. The results showed that the pea albumin fraction had the following effects: 1) significantly lowered the real ileal digestibility of pea protein (94 +/- 2.5% for G vs. 89.9 +/- 4% for GA), probably because of a direct effect of trypsin inhibitors; 2) did not promote acute intestinal losses of endogenous nitrogen; and 3) did not significantly improve the postprandial biological value of pea protein (76.5 +/- 3.9% for G vs. 78.7 +/- 3.6% for GA), despite the fact that it corrected the globulin deficiency in sulfur amino acids. We conclude that both G and GA are of good nutritional value for humans and show that cysteine-rich albumins have a far more modest effect on the efficiency of postprandial dietary protein utilization than would be expected from the amino acid scores.

Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults

BACKGROUND

We previously studied methionine kinetics and oxidation with the tracer L-[1-(13)C, methyl-(2)H(3)]methionine.

OBJECTIVES

We sought to explore methionine-cysteine interrelations in adults by using L-[1-(13)C]cysteine under different dietary conditions.

DESIGN

In experiment 1, 12 adults consumed a protein-free diet for 6 d. On day 7, methionine (n = 6) or cysteine (n = 6) oxidation rates were measured during an 8-h continuous infusion of L-[1-(13)C, methyl-(2)H(3)]methionine or L-[1-(13)C]cysteine, respectively. In experiment 2, 6 young men consumed 3 diets for 6 d each before a tracer study on day 7 with L-[1-(13)C]cysteine. The amounts (in mg*kg(-)(1)*d(-)(1)) of methionine and cysteine, respectively, were: high-methionine (HM) diet, 13 and 0; low-methionine (LM) diet, 6.5 and 0; and methionine-plus-cystine (MC) diet, 6.5 and 5.6. Cysteine flux and oxidation rates were determined and sulfur amino acid (SAA, methionine plus cysteine) balances were estimated.

RESULTS

In experiment 1, rates of methionine and cysteine oxidation were similar to losses predicted from obligatory nitrogen losses. In experiment 2, SAA balance was less negative when subjects consumed the HM diet than the LM and MC diets (interaction, P = 0.034), largely because of a difference in fed-state balance (HM compared with LM, P < 0.01; HM compared with MC, P < 0.05). There was no evidence of a sparing effect of dietary cystine on the methionine requirement.

CONCLUSION

These studies support use of [1-(13)C]cysteine for studying whole-body SAA oxidation and conclusions that maintenance of SAA balance is best achieved by supplying methionine at approximately the FAO/WHO/UNU recommendations for total SAA intake (13 mg*kg(-)(1)*d(-)(1)).