Role of sulfur amino acids in controlling nutrient metabolism and cell functions: implications for nutrition.

Protein synthesis is affected when an insufficient level of sulfur amino acids is available. This defect may originate from dietary amino acid deficiency and/or excessive amino acid utilisation for other purposes such as the synthesis of glutathione and acute-phase proteins during catabolic stress. Sulfur amino acids are recognised to exert other significant functions since they are precursors of essential molecules, are involved in the methylation process, participate in the control of oxidative status, and may act as mediators affecting metabolism and cell functions. Despite this increased understanding of the role of sulfur amino acids, many questions still remain unanswered due to the complexity of the mechanisms involved. Moreover, surprising effects of dietary sulfur amino acids have been reported, with the development of disorders in cases of both deficiency and excess. These findings indicate the importance of defining adequate levels of intake and providing a rationale for nutritional advice. The aim of the present review is to provide an overview on the roles of sulfur amino acids as regulators of nutrient metabolism and cell functions, with emphasis placed on the implications for nutrition.

Restriction of sulfur-containing amino acids alters claudin composition and improves tight junction barrier function.

Restriction of sulfur-containing amino acids (SCAA) has been shown to elicit a similar increase in life span and decrease in age-related morbidity as caloric restriction. The singular importance of epithelial barrier function in both physiological homeostasis and prevention of inflammation raised the issue of examining the effect of SCAA restriction on epithelial tight junction structure and permeability. Using a well-described in vitro, epithelial model, the LLC-PK(1) renal epithelial cell line, we studied the effects of SCAA restriction in culture medium. Reduction of methionine by 90%, cysteine by 50%, and total elimination of cystine resulted in dramatically lower intracellular pools of these amino acids and their metabolite, taurine, but the intracellular pools of the non-SCAA were all elevated. Cell growth and differentiation were maintained, and both confluent cell density and transepithelial short circuit current were unaffected. Certain tight junctional proteins, such as occludin and claudins-1 and -2 were not altered. However, claudins-3 and -7 were significantly decreased in abundance, whereas claudins-4 and -5 were markedly increased in abundance. The functional result of these structural changes was improved barrier function, as evidenced by increased transepithelial electrical resistance and decreased transepithelial (paracellular) diffusion of D-mannitol.

Functions of sulfur-containing amino acids in lipid metabolism.

It is known that plasma lipid levels are controlled not only by dietary fat and carbohydrate but also by dietary protein and amino acids. Although it used to be thought that the source of protein was important, it is known that amino acid composition, amino acids themselves, and peptides from digested protein are more important than the protein source. Sulfur-containing amino acids (SAAs) are recognized to be some of the most potent modulators of lipid metabolism among amino acids. It has been demonstrated that SAAs have an increasing effect on HDL (high-density lipoprotein)-cholesterol and a decreasing effect on VLDL (very low-density lipoprotein)-cholesterol. These data lead us to propose that SAAs have some beneficial functions against atherosclerotic diseases and metabolic syndrome. Relative availability of SAAs (RASAA) as well as the amount of SAAs in dietary protein would determine lipid metabolism. Therefore, we propose RASAA as a feasible index for improvement of lipid metabolism by amino acids. Although it is not clear how SAAs influence gene expression and lipid metabolism at a molecular level, SAAs change the metabolic pathway through transcriptional stimulation and posttranslational modification of regulatory proteins.

Sulfur-containing amino acid metabolism in parasitic protozoa.

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Whey components: millennia of evolution create functionalities for mammalian nutrition: what we know and what we may be overlooking.

Nutrition is undergoing a revolution owing to the recognition that some foods contain trophic, health-promoting factors distinct from essential nutrients. In this revolution, whey is increasingly being viewed as more than a source of proteins with a particularly nutritious composition of essential amino acids. Milk evolved under continuous Darwinian selection pressure to nourish mammalian neonates. Evolutionary pressure appears to have led to the elaboration of a complex food that contains proteins, peptides, complex lipids, and oligosaccharides that act as growth factors, toxin-binding factors, antimicrobial peptides, prebiotics, and immune regulatory factors within the mammalian intestine. Importantly, these trophic macromolecules are not essential, although the health benefits that their biological activities within the intestine provide likely contributed to neonatal survival. Human and bovine milks contain many homologous components, and bovine whey may prove to be a source for molecules capable of providing biological activities to humans when consumed as food ingredients. To approach this potential, food and nutrition research must move beyond the description of food ingredients as delivering only essential nutrients and develop a mechanistic understanding of the interactions between dietary components and the metabolic and physiological properties of the intestine.

Plasma sulfur amino acids in relation to cardiovascular disease, nutritional status, and diabetes mellitus in patients with chronic renal failure at start of dialysis therapy.

Sulfur amino acids (sAAs) are potential candidates as risk factors for cardiovascular disease (CVD). However, we recently reported that chronic hemodialysis patients with CVD had a greater prevalence of malnutrition, hypoalbuminemia, and lower plasma total homocysteine (tHcy) levels than those without CVD. In this cross-sectional study, we examined the relationship of plasma sAAs to CVD and nutritional status in 151 patients with chronic renal failure (CRF) close to the start of regular dialysis treatment (33 +/- 7 days before the first dialysis treatment). Clinical signs of CVD were present in 32% of patients with CRF, 41% had malnutrition assessed by subjective global nutritional assessment (SGNA) score, and 26% had diabetes mellitus (DM). Plasma tHcy levels were high in 91% of patients, as were plasma total cysteine (tCys) levels, whereas plasma methionine (Met) and taurine (Tau) levels were normal. Patients with CRF who had CVD were older, more often malnourished, and had lower tHcy and serum albumin (s-albumin) levels and a greater frequency of DM than those without CVD. Plasma tCys, Met, and Tau levels did not differ between patients with CRF with and without CVD. The tCys-tHcy ratio was higher in patients with CVD and related to SGNA score and DM. Moreover, this ratio, but not tHcy or tCys level, correlated with age and triglyceride, total cholesterol, and apolipoprotein B levels. Malnutrition and hypoalbuminemia were associated with low plasma sAA levels (tHcy, Met, and Tau); tCys was related to s-albumin level, but not SGNA score. Among patients with diabetes, sAA levels did not differ between patients with and without CVD or between malnourished and well-nourished patients. In conclusion, patients with CRF at the start of dialysis treatment with CVD were more often diabetic, malnourished, and had lower s-albumin and tHcy levels and a higher tCys-tHcy ratio than patients with no CVD. tCys-tHcy ratio, but not tHcy or tCys levels per se, was related to cardiovascular risk factors, suggesting that cysteine may have a role in the development of CVD. Malnutrition, hypoalbuminemia, and DM in patients with CRF influence sAA levels, mainly plasma tHcy, which should be considered when evaluating hyperhomocysteinemia as a cardiovascular risk factor.

Endogenous sulphur-containing amino acids: potent agonists at presynaptic metabotropic glutamate autoreceptors in the rat central nervous system.

We have recently demonstrated that presynaptically located metabotropic glutamate (mGlu) autoreceptors regulate synaptic glutamate release both in vitro and in vivo. We now report a positive modulatory action of the sulphur-containing amino acids (SCAAs), L-cysteic acid (CA) and L-cysteine sulphinic acid (CSA), at presynaptic group I mGlu receptors, specifically of the mGlu5 subtype, acting to enhance synaptic glutamate release from the rat forebrain in vitro. Neuronal glutamate release was monitored using electrically-evoked efflux of preloaded [(3)H]-D-aspartate from rat forebrain hemisections. Both CA (3 - 100 muM) and CSA (1 - 100 microM), in addition to the selective group I mGlu receptor agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG), concentration-dependently enhanced electrically-stimulated efflux of [(3)H]-D-aspartate from the rat forebrain slices. Basal efflux of label remained unchanged. The inhibitory activity of the broad spectrum mGlu receptor antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG; 200 microM), coupled with the inactivity of the selective mGlu1 receptor antagonists, (R,S)-1-aminoindan-1,5-dicarboxylic acid ((R,S)-AIDA; 100 - 500 microM) and the more potent (+)-2-methyl-4-carboxyphenylglycine (LY367385; 10 microM) against these responses, indicates an action of the SCAAs at the mGlu5 receptor subtype. This proposal is supported by the potent inhibition of these responses by the selective, non-competitive mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 microM). The observed enhancement of the responses to high concentrations of CA by the selective mGlu5 receptor desensitization inhibitor, cyclothiazide (CYZ; 10 microM), is also consistent with this concept. Administration of the agonists in the presence of bovine serum albumin (BSA; 5 - 15 mg ml(-1)) markedly attenuated the positive modulatory responses observed, strongly supporting a role for arachidonic acid in the expression of these mGlu5 receptor-mediated responses. The regulatory actions of SCAAs on synaptic glutamate release demonstrated in the present study may provide a physiological function for these putative neurotransmitter amino acids in the mammalian brain. These central actions of the SCAAs may have wide-ranging implications for a range of neurological and neuropsychiatric disease states and their treatment.

Immunonutrition: role of sulfur amino acids, related amino acids, and polyamines.

Pro-inflammatory cytokines mediate widespread changes in protein metabolism. Amino acids released from peripheral tissues fulfill a number of functions. They act as substrate for acute phase protein and immunoglobulin synthesis and, together with polyamines, in the replication of immune cells. Demands for specific amino acids may outstrip the supply from endogenous sources. A number of strands of evidence suggest that sulphur amino acids, and amino acids that are metabolically related to them, may be required in increased amounts. Protein deficiency impairs the acute phase response. However, sulfur amino acid insufficiency compromises glutathione synthesis, to a greater extent than hepatic protein synthesis, in the presence and absence of an inflammatory stimulus. The resulting effect may be compromised antioxidant defences. Functioning of T cells is dependent on intracellular glutathione concentrations and may also be affected by sulphur amino acid insufficiency. It has been suggested that the increased N excretion, which occurs during the immune response, is a reflection of a relative imbalance in the profile of amino acids released from peripheral tissues and the requirements imposed by the synthesis of substances involved in the acute phase response. Phenylalanine, tyrosine, tryptophan serine, and cysteine are released in amounts closest to requirements. Polyamine synthesis may be important for the fidelity of the enhanced level DNA transcription and RNA translation that occurs in response to infection and during tissue repair, gut growth after surgery, and in gut barrier functions. Although synthesized de novo from ornithine, arginine and S-adenosyl methionine (SAM), substantial recycling is a key feature of polyamine metabolism. The recycling may be a reflection of the need to maintain adequate tissue SAM during periods of rapid cell growth. During an immune/inflammatory response the combination of enhanced utilization of cysteine for glutathione synthesis and cell replication may lead to depletion of cellular SAM. A relatively small addition of polyamines to the diet may improve gut-associated aspects of the hosts' antibacterial defenses.

Identification of mechanisms that may modulate the role of lipoprotein(a) in thrombosis and atherogenesis.

In this report, we review recent findings concerning the identification of mechanisms that may modulate the role of lipoprotein(a), or Lp(a), in thrombosis and atherogenesis. Lp(a) binds to surface-immobilized plasmin-modified fibrin, thus providing a mechanism for incorporating Lp(a) into the vessel wall. We found that homocysteine and other sulfhydryl-containing amino acids markedly increase the binding of Lp(a) to plasmin-modified fibrin. Our results suggest that homocysteine alters the structure of Lp(a) to expose lysine-binding sites on the apolipoprotein(a) portion of the molecule, and thus provide a potential biochemical link between thrombosis and atherogenesis. We also found that transglutaminases catalyze the incorporation of primary amines into Lp(a). Studies in cell culture systems have found that Lp(a) stimulates endothelial cells to synthesize and release plasminogen activator inhibitor-1. Further, Lp(a) inhibits the activation of transforming growth factor-beta in a coculture of bovine endothelial and smooth muscle cells.

Maximal portion of the young pig's sulfur amino acid requirement that can be furnished by cystine.

Three pig experiments were conducted using a chemically defined, amino acid diet under conditions in which all nutrients were 100% bioavailable to assess the maximal portion of the sulfur amino acid (SAA) requirement that could be furnished by cystine (Cys). In Exp. 1, a methionine (Met)-deficient diet containing .12% L-Met and .40% L-Cys was supplemented with graded levels of L-Met. Pigs weighing 10 kg initially responded quadratically (P less than .05) to Met supplementation. A two-slope, broken-line regression model (weight gain regressed on percentage of dietary Met) estimated an infection point at .23% dietary Met. A constant level of .46% dietary SAA with differing Met:Cys weight (wt:wt) ratios was used in Exp. 2. Pigs fed Met:Cys ratios of 60:40 and 50:50 had similar (P greater than .05) weight gains, but pigs fed a 40:60 Met:Cys ratio gained less (P less than .05) than those fed the other diets. Maintaining dietary sulfur at .111% in Exp. 3, pigs fed Met:Cys ratios (wt:wt) of 100:0, 55:45, 50:50, and 45:55 gained weight at similar (P greater than .05) rates, but pigs fed the 45:55 Met:Cys ratio had a tendency to produce lower weight gains. Regardless of whether a constant dietary SAA or sulfur level was maintained, no more than 50% of the young pig's total SAA requirement (wt:wt) could be furnished by Cys.

Sulfur-containing cyclic ketimines and imino acids. A novel family of endogenous products in the search for a role.

Aminoethylcysteine, lanthionine, cystathionine and cystine are mono-deaminated either by L-amino-acid oxidase or by a transaminase exhibiting the properties described for glutamine transaminase. The deaminated products cyclize producing the respective ketimines. Authentic samples of each ketimine were prepared by reacting the appropriate aminothiol compound with bromopyruvate, except cystine ketimine which required the interaction of thiopyruvate with cystine sulfoxide. Reduction of the first three mentioned ketimines with NaBH4 yields the respective derivatives with the saturated rings of thiomorpholine and hexahydrothiazepine. The same reduction is carried out enzymically by a reductase extracted from mammalian tissues. Properties of the members of this family of compounds are described. Gas chromatography followed by mass spectrometry permits the identification of most of these products. HPLC is very useful for the determination of the ketimines by taking advantage of specific absorbance at 380 nm obtained by prior derivatization with phenylisothiocyanate. Adaptation of these and other analytical procedures to biological samples disclosed the presence of most of these compounds in bovine brain and in human urine. By using [35S]lanthionine ketimine as a representative member of the ketimine group, the specific, high-affinity, saturable and reversible binding to bovine brain membranes has been demonstrated. The binding is removed by aminoethylcysteine ketimine and by cystathionine ketimine indicating the occurrence in bovine brain of a common binding site for ketimines. The reduced ketimines are totally ineffective in competing with [35S]lanthionine ketimine. Alltogether these findings are highly indicative for the existence in mammals of a novel class of endogenous sulfur-containing cyclic products provided with a possible neurochemical function to be investigated further.

The control of oxidant stress at fertilization.

Metazoan eggs alter their coats after fertilization to protect the early embryo. In sea urchins, this modification consists of a rapid, coordinated set of noncovalent macromolecular assembly steps that are stabilized by protein cross-linking. The sea urchin egg uses an oxidative cross-linking reaction that requires hydrogen peroxide and a secreted peroxidase and thus faces the challenge of oxidant stress at the beginning of its development. Protection from the deleterious effects of this oxidative mechanism is afforded by regulation of the production and utilization of oxidizing species. This regulation requires a specific protein kinase C-activated oxidase and ovothiol, an intracellular antioxidant.

Cobalt-induced inhibition of growth in cyanobacteria Anabaena doliolum and Anacystis nidulans: interaction with sulphur containing amino acids.

Cobalt, above 1 microM concentration was growth inhibitory for both A. doliolum and A. nidulans. Its toxicity was mitigated by sulphur containing amino acids (cystine and cysteine), however, methionine could not mitigate the cobalt toxicity at all.

Inhibition of receptor-coupled phosphoinositide hydrolysis by sulfur-containing amino acids in rat brain slices.

Sulfur-containing amino acids were found to inhibit norepinephrine-stimulated [3H]phosphoinositide hydrolysis in rat cortical slices. Of the amino acids tested, L-cysteine was the most potent, inhibiting the response by 42 and 85% at concentrations of 50 and 500 microM respectively. L-Cystine and L-serine-O-sulfate also inhibited the response to norepinephrine, but to a lesser degree than did L-cysteine. L-Homocysteic acid slightly potentiated phosphoinositide hydrolysis at a concentration of 100 microM, but caused inhibition at 500 microM. L-Cysteine sulfinate produced effects intermediate to those of L-cysteine and L-homocysteic acid, having no effect on the response to norepinephrine at 50 microM, but causing 84% inhibition at 500 microM. The D-isomers of cysteine and homocysteic acid were much less potent than were the L-isomers. Examination of the time course of the inhibition of norepinephrine-stimulated [3H]phosphoinositide hydrolysis by L-cysteine showed that it was inhibited almost completely after 15, 30, 45 and 60 min of incubation. L-Cysteine and L-homocysteic acid caused similarly strong inhibitions of the production of [3H]inositol monophosphate, [3H]inositol bisphosphate and [3H]inositol trisphosphate. The hydrolysis of [3H]phosphoinositides stimulated by norepinephrine in slices from rat hippocampus and striatum were inhibited by L-cysteine to an extent similar to that occurring in cortical slices. These results demonstrate that several sulfur-containing amino acids, some of which have been proposed to be endogenous excitatory amino acid neurotransmitters, effectively modulate the response to norepinephrine of the phosphoinositide second messenger system in rat brain.

Oxidative stress and the role of novel thiol compounds at fertilization.

A new class of thiols, the 1-methyl-4-mercaptohistidines, has been found in high concentrations in invertebrate eggs. This family, called the ovothiols, has unusual redox properties, including the ability to confer a CN- -resistant NAD(P)H oxidase activity on ovoperoxidase, the enzyme that catalyzes the physiological crosslinking of the fertilization envelope with dityrosine residues. Ovothiol has a redox potential of 44 mV positive to glutathione and thus is maintained in the reduced state in eggs by reduced glutathione, without the need for an ovothiol reductase. We propose that high concentrations of reduced ovothiol are present in eggs to protect them from the oxidative stress caused by the respiratory burst of fertilization.