Uptake Of Free Amino Acids Research Articles

Peptide hydrolysis and uptake of peptide hydrolysis products in the James River estuary and lower Chesapeake Bay

Abstract The lability of small peptides was investigated along the salinity gradient of the James River estuary and lower Chesapeake Bay using two fluorescent analogs, Lucifer Yellow Anhydride-alanine-valine-phenylalanine-alanine (LYA-AVFA) and LYA-tetraalanine (LYA-AlA4). Hydrolysis rates of these compounds were compared with each other, and with uptake rates of potential hydrolysis products and other smaller derivatives (e.g., glutamic acid and dialanine). Results suggest that rates of peptide hydrolysis and uptake of hydrolysis products were not always correlated with each other along the salinity gradient, or to other environmental parameters measured in the James River and lower Chesapeake Bay. This may be because diverse input and removal processes can influence both peptide hydrolysis and uptake, but not necessarily simultaneously. Rates of both peptide hydrolysis and free amino acid uptake were strongly associated with particles, particularly those freshly produced. This suggests more rapid turnover of enzymatically available organic nitrogen in regions with elevated phytoplankton biomass. Changes in the abundance and composition of dissolved amino acids in these waters were also examined. Dissolved amino acid compositions, but not concentrations, varied with salinity along a gradient from the tidal fresh James River to the mouth of Chesapeake Bay. These compositional variations demonstrate the mixing of terrestrial organic nitrogen and in-situ production along the salinity transect.

Horizontally acquired oligopeptide transporters favour adaptation of Saccharomyces cerevisiae wine yeast to oenological environment.

In the past decade, horizontal gene transfer (HGT) has emerged as a major evolutionary process that has shaped the genome of Saccharomyces cerevisiae wine yeasts. We recently showed that a large Torulaspora microellipsoides genomic island carrying two oligopeptide transporters encoded by FOT genes increases the fitness of wine yeast during fermentation of grape must. However, the impact of these genes on the metabolic network of S. cerevisiae remained uncharacterized. Here we show that Fot-mediated peptide uptake substantially affects the glutamate node and the NADPH/NADP(+) balance, resulting in the delayed uptake of free amino acids and altered profiles of metabolites and volatile compounds. Transcriptome analysis revealed that cells using a higher amount of oligopeptides from grape must are less stressed and display substantial variation in the expression of genes in the central pathways of carbon and nitrogen metabolism, amino acid and protein biosynthesis, and the oxidative stress response. These regulations shed light on the molecular and metabolic mechanisms involved in the higher performance and fitness conferred by the HGT-acquired FOT genes, pinpointing metabolic effects that can positively affect the organoleptic balance of wines.

Ocean acidification reduces feeding rates in the scleractinian coral Stylophora pistillata

A general decline in coral calcification has been observed in response to ocean acidification. It has recently been shown that heterotrophic feeding (the acquisition of organic nutrients) enables one massive coral species to maintain its calcification rates while exposed to ocean acidification but the consequences of higher seawater partial pressure of CO2 ( p CO 2 ) on coral feeding capacities themselves have never been investigated. Exposing long-term acclimated starved and fed colonies of the major reef builder Stylophora pistillata to either ambient pHT (8.06 ± 0.05; 417 Pa p CO 2 ) or low pHT (7.51 ± 0.06; 1697 Pa p CO 2 ) showed that higher feeding rates can counterbalance the negative effects of seawater acidification on coral calcification. Indeed, relative to unfed corals under ambient pHT conditions, calcification rates in unfed corals decreased by 53.6% but remained constant in fed corals under high p CO 2 conditions. Results also clearly show that acidification affects the feeding capacity and feeding effort of the branching coral S. pistillata. Organic nutrient acquisition was severely reduced under high p CO 2 , with a simultaneous decrease in the dissolved free amino acid uptake rates, the alkaline phosphatase activities, and the feeding rates on Artemia salina nauplii. Considering that all these processes are needed to fulfill the nutrient needs of the symbiotic association, on a long-term basis, lower feeding rates will likely aggravate the already fragile physiological state of many corals under warmer, more acidic conditions.

Cysteinyl‐glycine reduces mucosal pro‐inflammatory cytokine response to fMLP in a piglet model of intestinal atrophy

PepT1 is a small intestinal (SI) di/tripeptide transporter also capable of transporting bacterial peptides. We measured the mucosal cytokine response to a pro‐inflammatory peptide (fMLP) when delivered with cysteinyl‐glycine (CG); CG competes for PepT1‐mediated uptake and has anti‐inflammatory potential. Pigs (n=6, 10 d) received parenteral nutrition (PN) for 4 d to induce SI atrophy; littermates (n=6) remained with the sow (S). Subsequently, five 10 cm loops of the distal SI were isolated and perfused for 3 h with one of: 1) 5 mM cysteine + 5 mM glycine (5 mM cys + gly) 2) 5 mM CG 3) 10 μM fMLP 4) 5 mM cys + gly + 10 μM fMLP 5) 5 mM CG + 10 μM fMLP. In both S and PN, SI exposed to fMLP had higher mucosal TNF‐α and IFN‐γ compared to unexposed loops (p<0.001), while IFN‐γ was higher in PN compared to S pigs (p < 0.01). CG + fMLP treatment lowered the IFN‐γ response in both S and PN (p <0.05), but neither group responded significantly to free cys + gly. Interestingly, free cys + gly reduced the TNF‐α response in S pigs (p<0.001), but not in the PN group. CG + fMLP lowered TNF‐α compared to fMLP alone in both groups (p<0.001) and in S pigs the response was significantly more abated than with cys + gly (p < 0.001). Thus, CG was more effective than free cys + gly at attenuating a bacterial peptide‐induced cytokine response particularly in the injured SI; this may be due to impaired free amino acid uptake or effective competitive inhibition of fMLP uptake. (NSERC)

β-N-Methylamino- l-alanine (BMAA) uptake by the aquatic macrophyte Ceratophyllum demersum

Free-living freshwater cyanobacteria contain BMAA in both free cellular and protein-associated forms. Free BMAA released on bloom collapse or during cellular turnover creates a potential source of the non-proteinogenic amino acid for bioaccumulation and biomagnification in aquatic ecosystems. Uptake of free amino acids is well documented in macrophytes and the potential for aquatic macrophytes to bioaccumulate BMAA therefore poses a potential threat where such macrophytes constitute a food source in an ecosystem. BMAA uptake and accumulation by the aquatic macrophyte Ceratophyllum demersum was therefore investigated. Rapid uptake of significant amounts of BMAA was observed in C. demersum. Both free and protein-associated BMAA were observed with protein association following accumulation of free BMAA. The protein association suggests potential biomaccumulation by aquatic macrophytes and offers a possibility of phytoremediation for BMAA removal.

Microbial degradation rates of small peptides and amino acids in the oxygen minimum zone of Chilean coastal waters

Abstract We found similar microbial degradation rates of labile dissolved organic matter in oxic and suboxic waters off northern Chile. Rates of peptide hydrolysis and amino acid uptake in unconcentrated water samples were not low in the water column where oxygen concentration was depleted. Hydrolysis rates ranged from 65 to 160 nmol peptide L −1 h −1 in the top 20 m, 8–28 nmol peptide L −1 h −1 between 100 and 300 m (O 2 -depleted zone), and 14–19 nmol peptide L −1 h −1 between 600 and 800 m. Dissolved free amino acid uptake rates were 9–26, 3–17, and 6 nmol L −1 h −1 at similar depth intervals. Since these findings are consistent with a model of comparable potential activity of microbes in degrading labile substrates of planktonic origin, we suggest, as do other authors, that differences in decomposition rates with high and low oxygen concentrations may be a matter of substrate lability. The comparison between hydrolysis and uptake rates indicates that microbial peptide hydrolysis occurs at similar or faster rates than amino acid uptake in the water column, and that the hydrolysis of peptides is not a rate-limiting step for the complete remineralization of labile macromolecules. Low O 2 waters process about 10 tons of peptide carbon per h, double the amount processed in surface-oxygenated water. In the oxygen minimum zone, we suggest that the C balance may be affected by the low lability of the dissolved organic matter when this is upwelled to the surface. An important fraction of dissolved organic matter is processed in the oxygen minimum layer, a prominent feature of the coastal ocean in the highly productive Humboldt Current System.

Plant amino acid uptake, soluble N turnover and microbial N capture in soils of a grazed Arctic salt marsh

Summary The uptake of free amino acids by the grass Puccinellia phryganodes was investigated in soils of an Arctic coastal salt marsh, where low temperatures and high salinity limit inorganic nitrogen (N) availability, and the availability of soluble organic N relative to inorganic N is often high. Following the injection of 13C15N‐amino acid, 15N‐ammonium and 15N‐nitrate tracers into soils, rates of soluble nitrogen turnover and the incorporation of 13C and 15N into plant roots and shoots were assessed. Chloroform fumigation‐extraction was used to estimate the partitioning of labelled substrates into microbial biomass. Free amino acids turned over rapidly in the soil, with half‐lives ranging from 8.2 to 22.8 h for glycine and 8.9 to 25.2 h for leucine, compared with 5.6 to 14.7 h and 5.6 to 15.6 h for ammonium and nitrate, respectively. 15N from both organic and inorganic substrates was incorporated rapidly into plant tissue and the ratio of 13C/15N incorporation into plant tissue indicated that at least 5–11% of 13C15N‐glycine was absorbed intact. Microbial C and N per unit soil volume were 1.7 and 5.4 times higher, respectively, than corresponding values for plant C and N. Plant incorporation of 15N tracer was 56%, 83% and 68% of the comparable incorporation by soil microorganisms of glycine, ammonium and nitrate ions, respectively. These results indicate that P. phryganodes can absorb amino acids intact from the soil despite competition from soil microorganisms, and that free amino acids may contribute substantially to N uptake in this important forage grass utilized by lesser snow geese in the coastal marsh.

Extracellular enzyme activity and uptake of carbon and nitrogen along an estuarine salinity and nutrient gradient

Amino acid oxidation (AAO) and peptide hydrolysis (PH) are processes affecting the recycling of organic material and nutrients. We compared extracellular AAO and PH rates to C and N uptake rates along estuarine gradients of salinity, nutrients and productivity in the Pocomoke River, a subestuary of the Chesapeake Bay. This estuary is seasonally depleted in inorganic N, and rich in dissolved organic material (DOM) throughout the year. AAO, PH, and N uptake rates measured in 1999 and 2000 were not limited to particular size fractions measured, or to auto- or heterotrophic groups of organisms. At a station near the turbidity maximum, where chlorophyll a biomass was highest, smaller ( 1.2 μm) size-fraction, except at the least saline station in August of both years. Rates of AAO and PH were not linearly correlated with each other seasonally or spatially. Uptake of NH 4 + dominated total N uptake (>50%) at all but the freshwater station, although uptake of organic compounds was measurable at all sites. Rates of dissolved free amino acid uptake, measured using dually labeled compounds, were substantial (up to 11% of the total N uptake) and contributed both C and N for growth. Dual labels unambiguously demonstrated that uptake rates of amino acid C and N were uncoupled; amino acid N was taken up preferentially to amino acid C even when rates were corrected for N uptake from AAO. Conceptual models of DOM cycling should include the realization that enzymatic processes and uptake of DOM occur in both 'microbial' and larger size fractions. Thus, competition between bacteria and phytoplankton mixotrophs may be an important factor determining the relative uptake of C and N from amino acids and other organic substrates.

Amino Acid Catabolism and Generation of Volatiles by Lactic Acid Bacteria

Twelve isolates of lactic acid bacteria, belonging to the Lactobacillus, Lactococcus, Leuconostoc, and Enterococcus genera, were previously isolated from 180-d-old Serra da Estrela cheese, a traditional Portuguese cheese manufactured from raw milk and coagulated with a plant rennet. These isolates were subsequently tested for their ability to catabolize free amino acids, when incubated independently with each amino acid in free form or with a mixture thereof. Attempts were made in both situations to correlate the rates of free amino acid uptake with the numbers of viable cells. When incubated individually, leucine, valine, glycine, aspartic acid, serine, threonine, lysine, glutamic acid, and alanine were degraded by all strains considered; arginine tended to build up, probably because of transamination of other amino acids. When incubated together, the degradation of free amino acids by each strain was dependent on pH (with an optimum pH around 6.0). The volatiles detected in ripened Serra da Estrela cheese originated mainly from leucine, phenylalanine, alanine, and valine, whereas in vitro they originated mainly from valine, phenylalanine, serine, leucine, alanine, and threonine. The wild strains tested offer a great potential for flavor generation, which might justify their inclusion in a tentative starter/nonstarter culture for that and similar cheeses.

Peptide hydrolysis, amino acid oxidation, and nitrogen uptake in communities seasonally dominated by Aureococcus anophagefferens

Elevated levels of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) are among the factors implicated in the initiation of algal blooms. However, the degree to which phytoplankton augment their autotrophic metabolism with heterotrophic uptake of organic carbon that is associated with DON is unknown. We evaluated the relative importance of peptide hydrolysis, amino acid oxidation, and amino acid uptake over a seasonal cycle in an embayment on Long Island, New York, that had high concentrations of dissolved organic matter (DOM) and a bloom of the brown tide pelagophyte, Aureococcus anophagefferens. Amino acids were a significant component (up to 50%) of the total N uptake, particularly during the late summer. About half of the associated amino acid C was also taken up. Amino acid oxidation rates were an order of magnitude lower than free amino acid uptake rates, but still supplied up to 32.5% of the NH taken up. Up to 75% of the amino acid oxidation was in the bacterial size fraction (‹1.2 μm), and rates were significantly correlated with bacterial densities. Peptide hydrolysis rates were high, and most (up to 72%) occurred in the brown tide size fraction (1.2–5 μm). The high rates of peptide hydrolysis and amino acid uptake measured in cultures of A. anophagefferens confirm that this species can readily hydrolyze peptides and take up N and C from amino acids. Laboratory findings and size‐fractionation studies in the field suggest that A. anophagefferens plays a major role in consumption of both C and N from DOM.

Effect of glucose and glycine addition to cocoa mass before roasting on Maillard precursor consumption and pyrazine formation

AbstractCocoa mass was treated with glucose and/or glycine before roasting. For glucose‐treated samples the reductions in reducing sugars and free amino acids were 41.1 and 56.9% respectively, whereas for glycine‐treated samples the respective reductions were 16.2 and 37.4%. The combination of glucose plus glycine treatment led to 39.0 and 48.9% reductions in reducing sugars and free amino acids respectively. For free amino acids the reductions in control and glucose‐treated samples were similar, 56.2 and 56.9% respectively, indicating that the added glucose did not significantly (P < 0.05) influence the free amino acid uptake. When glycine was added, the consumption of free amino acids (37.4%) was lower than for the control and glucose treatments. When glucose and glycine were added, the reduction in free amino acids (48.9%) was lower than for the control and glucose treatments but higher than for the glycine treatment. Apparently, glycine addition inhibited the reducing sugar and free amino acid consumption, whereas glucose addition enhanced the reducing sugar and free amino acid consumption. The glucose plus glycine treatment was similar to the glycine treatment in the contents of 2,5‐dimethyl‐ and trimethylpyrazines. For 2‐methylpyrazine the value for glucose‐treated samples was lower than for control and glycine‐treated samples but almost equal to the value for the glucose plus glycine treatment. It is concluded that the addition of glucose and/or glycine does not affect pyrazine production.© 2002 Society of Chemical Industry

Some physiological studies on the effect of onion and garlic juices on the fish, Clarias lazera

Fish, Clarias Lazera were orally administered onion (Allium cepa) or garlic (Allium sativum) juices. Contents of glucose, free amino acids, total lipids, triglycerides, cholesterol, albumin, total protein, urea, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in serum as well as glycogen, free amino acids, total lipids, triglycerides and cholesterol contents in liver, white and red muscle were determined 5 h and 24 h after a single dose or after repeated doses (1 dose every 24 h for 5 days). Both onion and garlic treatments caused a temporary hypoglycemia as well as liver glycogen muscle depletion accompanied with a significant elevation in white muscle glycogen suggesting that dietary onion or garlic may enhance the process of glycogenesis in white muscle. Onion feeding caused hypolipidemia accompanied with significant decreases in liver content of total lipids in response to the repeated doses for 5 days and a severe rise in the lipid content of red muscle at all periods. Such results suggest that dietary onion may enhance the process of lipogenesis in red muscle of Clarias lazera. Probably, the main precursor of such lipogenesis in the red muscle was the serum fatty acids. Moreover, dietary onion led to hypocholesterolemia accompanied with a significant rise in cholesterol level of both liver and red muscle. On the other hand, dietary garlic significantly decreased serum free amino acids after 5 h and 5 days. Furthermore, significant elevations of liver free amino acids were dedected in the onion-treated fish group (24 h and 5 days) and in the garlic-treated fish group (5 h), probably due to oxidation of thiol components (sulfur attached to the proteins). This result was accompanied with either a marked increase in white muscle free amino acids at all periods of garlic fish group, or a noticeable drop in the onion group (repeated doses). Also, dietary onion and garlic treatment led to an elevation in hepatic ureagenesis in spite of the unchanged serum urea level. Significant decreases of both serum AST and ALT in garlic groups (all periods) and in onion groups (repeated doses) were observed. In liver, AST and ALT showed temporary rises after onion or garlic supplementation. In general, it could be concluded that both dietary onion and garlic have a transitory hypoglycemic action as well as a promoting effect on the process of glycogenesis in white muscle whereas dietary onion promotes lipogenesis in red muscle of fish. Also, dietary garlic may enhance the white muscle uptake of free amino acids which, in turn, may promote protein synthesis.

Kinetics of soil microbial uptake of free amino acids

Amino acids and proteins typically form the biggest input of organic-N into most soils and provide a readily available source of C and N for soil microorganisms. Amino acids can also be taken up directly by plant roots, providing an alternative source of available soil N. However, the degree to which plants can compete against the soil microbial population for amino acids in soil solution remains poorly understood. The aim of this study was to measure the rate of microbial uptake of three contrastingly charged 14C-labelled amino acids (glutamate1–, glycine0, lysine0.9+) over a wide concentration range (0.1–5 mM) and in two contrastingly managed soils varying in their degree of erosion, organic-C content and microbial biomass. Amino acid uptake was concentration dependent and conformed to a single Michaelis-Menten equation. The mean maximum amino acid uptake rate (Vmax) for the non-eroded (control) soil (high organic-C, high biomass) was 0.13±0.02 mmol kg–1 h–1, while half maximal uptake occurred at a concentration (Km) of 2.63±0.07 mM. Typically, Vmax was fourfold lower and Km twofold lower in the eroded soil (low available organic-C, low biomass) compared to the non-eroded (control) soil. Amino acid substrate concentration had little effect on the proportion of amino acid utilized in catabolic versus anabolic metabolism and was similar for both. While the results obtained here represent the summation of kinetics for a mixed soil population, they indicate that amino acid uptake is saturated at concentrations within the millimolar range. Because the affinity constants also were similar to those described for plant roots, we hypothesized that competition for amino acids between plants and microbes will be strong in soil but highly dependent upon the spatial distribution of roots and microbes in soil.

Vascular sources of phenylalanine, tyrosine, lysine, and methionine for casein synthesis in lactating goats.

The contribution to casein biosynthesis of peptides derived from blood was examined in late lactation goats (254 to 295 d in milk). Ratios of mammary uptake of free amino acids (AA) in blood to output of AA in milk protein and ratios of the enrichments of Phe, Tyr, Met, and Lys at isotopic plateau in secreted milk casein to the free AA in arterial and mammary vein blood were monitored during the last 5 h of a 30-h continuous i.v. infusion of [1-13C]Phe, [2H4]Tyr, [5-13CH3]Met, and [2-15N]Lys on two occasions: before (control) and on d 6 of an i.v. infusion of Phe (6 g/d). During the control, uptakes of free Phe and Met were less than their output in milk. This result was comparable with the labeling kinetic results, suggesting that vascular peptides contributed 5 to 11% of Phe and 8 to 18% of Met. Free Tyr and Lys uptakes during the control were sufficient for milk output; however, the labeling kinetics indicated that 13 to 25% of the Tyr and 4 to 13% of the Lys were derived from peptides. Infusion of Phe increased the uptake of free AA but reduced the contribution of peptides toward Phe (0 to 3%) and Tyr (8 to 14%) supply for casein synthesis. Whole body hydroxylation of Phe to Tyr increased from 10 to 18% with the infusion of Phe; within the mammary gland, this conversion was lower (3 to 5%). Results suggest that the mammary utilization of peptides containing Phe and Tyr appears to depend on the supply of free AA in blood.

Nutritional value of protein hydrolysis products (oligopeptides and free amino acids) as a consequence of absorption and metabolism kinetics.

When pigs were submitted to duodenal infusion of solutions containing a large percentage of small peptides (PEP) or free amino acids with the same pattern (AAL) amino acids appear in the portal blood more rapidly and more uniformly after infusion of PEP then after infusion of AAL, with the notable exception of methionine for which the opposite was true. These differences were lowered when a carbohydrate (maltose dextrin) was present in the solution, but nevertheless remained significant for the first hour after the infusion. The long-term (8-hour) uptake of free amino acids into the liver and the peripheral tissues differed in profile according to the nature of the duodenal infusion. Peripheral uptake was appreciably less well balanced after infusion of free amino acids (deficiency of threonine and phenylalanine) than after infusion of small peptides (deficiency of methionine). Accordingly, in the rat, under conditions of discontinuous enteral nutrition the mixture of small peptides was of greater nutritive value than the mixture of free amino acids. It thus appears that the absorption kinetics which results in important variations in the temporal distribution of free amino acids in the tissues may be at the origin of transitory imbalances in tissue amino acid uptake, and as a result of a lower nutritive value.

Biomass and nitrogen uptake by heterotrophic bacteria during the spring phytoplankton bloom in the North Atlantic Ocean

N biomass and N uptake by heterotrophic bacteria and phytoplankton were examined during the North Atlantic spring bloom (May 1989). Phytoplankton N had reached its zenith by the time the authors arrived at the station and increased only slightly during the first week of the study, while particulate nitrogen (PN) nearly doubled (increase of 450 mg N m −2), apparently due to the growth of heterotrophic organisms other than bacteria. During the second week, bacterial N doubled, increasing by 280 mg N m −2. These increases in biomass N and total PN were large compared to NO 3 − uptake and to N export from the upper 100 m. To examine the role of bacteria in greater detail, we estimated total N uptake and the uptake of free amino acids (DFAA), NH 4 +, and NO 3 −. The ratio of total N uptake by bacteria to total N uptake by phytoplankton varied greatly and was often high (0.2–0.9). When uptake was corrected for phytoplankton activity, heterotrophic bacteria accounted for 22–39% and 4–14% of NH 4 + and NO 3 − uptake. DFAA, NH 4 + and NO 3 − supplied 12–34%, 19–29% and 2–8% respectively, of the nitrogen needed for bacterial production. Preference indices further indicated that NO 3 − was the N source least preferred by heterotrophic bacteria. Nitrogen uptake fuelled a buildup of bacterial biomass and other suspended material that needs to be considered in reconciling new production and N export from the upper layers of the ocean during the spring bloom.

Free Amino Acids and Cell Volume Regulation in the SipunculidPhascolosoma arcuatum

The ability of Phascolosoma arcuatum to use free amino acids (FAAs) to regulate cell volume was studied After a downward transfer from 50% to 30% seawater (SW), there waspossibly a release of FAAs from the body tissues of P. arcuatum into the coelomic plasma, as indicated by the increase in the total FAA concentration in the latter compartment. When being confronted with an upward transfer from 50% to 100% SW P. arcuatum exhibited a small uptake of FAAs from the coelomic plasma by the body tissues during the initial phase of the transfer The significant increase in the total FAA content in the tissues of the worms exposed to 100% SW was mainly due to an increase in protein catabolism as reflected by a significantly smaller water-soluble protein content in the tissues of these worms compared to that of the worms exposed to 30% SW. Some of the FAAs released through increased proteolysis in P. arcuatum exposed to 100% SW might undergo partial catabolism and be converted into alanine and glycine, leading to a...

Uptake of free amino acids by the ophiuroid Microphiopholis gracillima (Stimpson) (Echinodermata) during disc regeneration

1. 1. Intact and regenerating brittlestars ( Microphiopholis gracillima) take up several amino acids from artificial seawater, and show significant increases in uptake with food deprivation. 2. 2. There were no differences in total amino acid uptake rates (determined by high performance liquid chromatography) between intact and regenerating animals. 3. 3. No increases in the gross rate of leucine uptake were apparent in either group over time; however, glycine uptake increased significantly. 4. 4. Catabolism of leucine and glycine, as evidenced by labeled CO 2 release, was highest among intact animals that had been held for several days. 5. 5. Dissolved leucine appears to be rapidly catabolized, while glycine tends to remain in tissues.

Ammonium and free amino acid uptake by a deep-sea mussel (Bathymodiolus sp., undescribed) containing methanotrophic bacterial symbionts

Mussels (Bathymodiolus sp., undescribed) that contain endosymbiotic methanotrophic bacteria were collected from the vicinity of hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico during September 1989 and 1991. In the presence of methane, these mussels took up ammonium and free amino acids (FAA) but not nitrate. Rates of ammonium uptake ranged from 0.11 to 0.78 μmolg-1 h-1 following initial concentrations of 3.7 to 140.0 μM. The relationship between uptake rate and ammonium concentration exhibited by these mussels appears to be non-linear, apparently differing from the linear kinetics exhibited by algal-invertebrate symbioses. Alanine and glycine were depleted from the medium by symbiotic mussels at rates of 0.10 and 0.07 μmolg-1 h-1 following initial concentrations of 1 μM. Given estimates of rates of carbon fixation and ammonium and FAA levels encountered by the mussels in situ, it is likely that ammonium uptake can meet the nitrogen needs of the association, and that acquisition of carbon and nitrogen from FAA uptake may be an important supplement.

Fiber-type discrimination in disuse and glucocorticoid-induced atrophy

Data are presented which demonstrate that phasic/glycolytic muscles atrophy more than tonic/oxidative muscles in response to exogenously introduced glucocorticoids. Data are also presented demonstrating that immobilization makes a muscle unusually sensitive to glucocorticoid-induced atrophy and that remobilization of a previously immobilized muscle protects a muscle from glucocorticoid-induced atrophy. These observations are discussed within the context of the role of mechanical activity in the acquisition and maintenance of fiber-type characteristics. In addition, the available data on the glucocorticoid receptor population in skeletal muscles of various types and circumstance are reviewed within the context of the recent observations concerning the glucocorticoid induction of the enzyme glutamine synthetase in skeletal muscle. It is proposed here that atrophy is not necessarily the response of skeletal muscle to glucocorticoids. Rather, atrophy is a possible consequence of the glucocorticoid-induced increase in export of amino acid carbon from the muscle. Whether such export causes a muscle to atrophy or perhaps even hypertrophy will depend on the capacity of the muscle to sustain its free amino acid pools. Mechanical activity greatly promotes the uptake of free amino acids in skeletal muscle. Such promotion takes the form of both contraction-induced uptake and increased insulin sensitivity. Within this perspective, it is suggested that tonic muscles and remobilized muscles are protected from atrophy by exogenous glucocorticoids because their high level of mechanical activity allows them to maintain their free amino acid pools.